RTR(A) phraseology

Emergency

MAYDAY and PAN PAN, engine and system failures, TCAS RA, airprox and medical priority.

Airport is closed

Used when ATC informs a flight in progress that its destination airport is closed (here, due to a major fire at Mumbai with an indeterminate delay) and asks the pilot to state intended alternative action. Since this is a diversion within the same control area, Ahmadabad Control is providing service to both the original routing and the new destination (Ahmadabad/AAE), no handoff to a different ATC unit is needed; the controller can issue the diversion clearance directly.

Pilot

[Station] [Callsign] [response/report]

ATC

[Callsign] [instruction/information/clearance]

ATC

IFly 430, Ahmedabad Control

Pilot

Ahmedabad Control, IFly 430, go ahead.

ATC

IFly 430, Mumbai airfield close due major fire, delay not determined, request your intentions.

Pilot

Would like to land at Ahmedabad, IFLY 430

ATC

IFly 430, Ahmedabad control, cleared to Ahmedabad, report position.

Pilot

Cleared to Ahmedabad. Position Pratapgarh 0600, FL320, setting course for Ahmedabad, estimating Ahmedabad 0700.

"Go ahead" by Ahmadabad Control is simply an invitation/acknowledgment that ATC is ready to pass information, used here because ATC is initiating the exchange (informing the pilot of the airfield closure), rather than the pilot calling ATC with a request. This is the reverse of most exchanges so far, where the pilot initiates.
"Delay not determined" signals to the pilot that holding is not a viable option (no estimate for when Mumbai might reopen), this is the trigger for the pilot to choose an alternate rather than holding and waiting.
"Request your intentions" places the decision with the pilot, ATC doesn't direct where to divert; the pilot, considering fuel, weather, company alternates, etc., decides and states the choice (Ahmadabad).
The clearance "cleared to AAE" is comparatively brief compared to the earlier departure clearance, since the aircraft is already airborne and en route, a full departure-style clearance isn't needed; only the new destination/clearance limit is required, with the aircraft continuing on its current level/routing until further instructions.
"Report position" is ATC's way of re-establishing a normal reporting sequence for the new routing, since the diversion changes the flight-planned route, ATC needs a fresh position report to begin tracking progress towards the new destination.
The pilot's position report ("Pratapgarh 0600 FL 320") follows the standard format: position, time, level, then adds the diversion-specific elements: "setting course for AAE" (confirming the turn towards the new destination has been made) and "estimating AAE 0700" (revised ETA for the new destination).
Note there's no requirement here for the pilot to restate persons on board, endurance, etc., as this is not a PAN PAN/MAYDAY, it's a routine operational diversion. The airfield closure is Mumbai's problem, not an emergency for this aircraft (assuming fuel/endurance comfortably supports the diversion, which isn't questioned).
If fuel were a concern for reaching the alternate, the pilot would need to declare PAN PAN (minimum fuel) or MAYDAY (fuel emergency) as appropriate, the calm, routine tone of this exchange implies fuel is not an issue.
This contrasts with the earlier "Co-pilot fainted" MAYDAY scenario, which was also effectively a diversion (return to Ahmadabad) but for an emergency reason, here the diversion is for an external operational reason (destination closed) with no emergency at the diverting aircraft itself.

Reference: ICAO Doc 9432

Airprox - No risk of collision

Used when a pilot reports an Airprox occurrence classified as "No Risk of Collision" (Category C), the encounter was reported (the pilot noticed another aircraft in proximity and considered it worth reporting at the time), but on the pilot's own assessment, the geometry, separation, and/or relative paths meant no genuine collision risk existed. This is the lowest-severity Airprox category that still constitutes a reportable occurrence, distinct from a routine traffic sighting that wouldn't be reported at all.

Pilot

AIRPROX [category]. [Station], [Callsign]. POSITION [position] [time], TRUE AIR SPEED [speed], [level], [weather conditions], TIME OF INCIDENT [time], [description of other traffic: type, action, estimated separation, direction]

ATC

[Callsign] [Station] ROGER AIRPROX

Pilot

Airprox, no risk of collision. Delhi Control, IFly 430. Position Jaipur 0500, true air speed 350, FL320, VMC, time of incident 0505. One Boeing 737 passed approximately 4 NM to our right, same level, TCAS traffic advisory received, direction 090. No avoiding action taken.

ATC

IFly 430, Delhi Control, roger airprox

"AIRPROX, no risk of collision" stated upfront tells ATC this is a report for the record, not an indication that ATC needs to take urgent action, though, as with the other categories, the final classification is determined by the investigating board, not the pilot's initial assessment.
A Category C report often arises from situations like: another aircraft sighted/detected (via TCAS Traffic Advisory, not RA) passing at a distance that, while perhaps closer than the pilot expected or was briefed for, still afforded adequate separation and/or time to react if needed, or an aircraft that was, on reflection, never actually on a converging path despite an initial impression of proximity.
Compared to Category B (5 NM, level bust through the reporting aircraft's level) and Category A (very close, TCAS RA), a Category C report might describe something like a TCAS Traffic Advisory (TA) only (no RA), with the other aircraft passing at a separation that, while triggering the TA, didn't represent an actual conflict, or an aircraft that was visually sighted and assessed as clear of conflict (in VMC, unlike the IMC scenario in Category B).
Pilots may file Category C reports out of caution, if an encounter felt close enough to warrant reporting at the time, but on reflection (or per TCAS/ATC data) turned out not to be a genuine risk, reporting it as Category C still feeds into the safety data system (helps identify patterns, e.g., a particular procedure or airspace design that generates frequent "felt close but actually fine" encounters, which might still warrant review even if no individual incident was dangerous).
The level of detail in the report may be similar to Categories A/B (position, time, other traffic description), but the tone/framing of the "intentions" or outcome would reflect that no avoiding action was necessary, or that any action taken was precautionary rather than urgent.
ATC's response remains the same minimal acknowledgment, "roger your airprox", since the formal classification process is identical regardless of the pilot's initial assessment of category.

Reference: ICAO Doc 9432

Airprox - Risk not determined

Used when a pilot reports an Airprox occurrence classified as "Risk Not Determined" (Category D), the pilot is reporting a proximity event but lacks sufficient information to assess whether a collision risk existed. This typically arises when the other aircraft was detected only briefly, ambiguously, or through limited means (e.g., a fleeting TCAS contact with no resolution, a visual sighting too brief to judge separation or closure, or a report based on a third party's information rather than the pilot's own direct observation), such that the pilot cannot honestly characterize the encounter as A, B, or C.

Pilot

ATC

[Callsign] [Station] ROGER AIRPROX

Pilot

Airprox, risk not determined. Delhi Control, IFly 430. Position Jaipur 0500, true air speed 350, FL320, IMC, time of incident 0505. Brief TCAS contact observed, traffic faded before resolution advisory, unable to determine type, separation, or heading of other aircraft.

ATC

IFly 430, Delhi Control, roger airprox.

"AIRPROX, risk not determined" stated upfront signals to ATC that this report is being filed out of caution/completeness, but the pilot is explicitly not asserting a risk level, distinguishing this from Categories A–C, where the pilot offers at least a provisional assessment.
This category often arises from incomplete information: a TCAS contact that appeared and disappeared without clear data, a momentary visual sighting where the pilot couldn't judge altitude/distance/heading of the other aircraft, or a report prompted by ATC/another party noting a conflict that the reporting pilot didn't fully perceive themselves.
Unlike Categories A–C, the "description of other traffic" element may be notably sparse or qualified with uncertainty, e.g., "believed to be," "unable to determine exact separation," "brief contact only", since the entire point of Category D is that key details are missing or unclear.
Despite the uncertainty, the position/time/level/speed information for the reporting aircraft itself should still be as complete as possible, this is information the pilot definitely knows, and it's essential for the investigating board to correlate with radar data, which may be able to fill in the gaps the pilot couldn't observe directly.
The investigating board, with access to radar/radar replay data from both aircraft (if identifiable), may be able to resolve a Category D report into A, B, or C after analysis, Category D often reflects the pilot's uncertainty at the time of reporting, not necessarily a permanent "unknowable" status for the final investigation.
As with all other categories, ATC's response remains the same minimal acknowledgment.

Reference: ICAO Doc 9432

Airprox - Risk of collision

Used when a pilot reports an Airprox occurrence classified as "Risk of Collision" (Category A), the most severe Airprox category, where, based on the pilot's immediate assessment, another aircraft came into such close proximity, with such relative geometry, speed, and lack of effective separation, that a collision was seriously possible without the avoiding action taken (or in some cases, despite avoiding action). This is reported promptly so ATC can begin immediate investigation (radar data preservation) and so any other aircraft involved can also be identified and contacted.

Pilot

ATC

[Callsign] [Station] ROGER AIRPROX

Pilot

Airprox, risk of collision. Delhi Control, IFly 430. Position Jaipur 0500, true air speed 350, FL320, IMC, time of incident 0505. One Boeing 737 passed directly below us at approximately 200 ft vertical separation, TCAS RA "climb" followed and complied with, direction 360.

ATC

IFly 430, Delhi Control, roger airprox

"AIRPROX, risk of collision" stated upfront signals the highest-severity category to ATC, though, as with Category B, the final classification is determined by an investigating board; this is the pilot's immediate assessment based on how close and threatening the encounter felt.
A Category A report often follows a TCAS Resolution Advisory (RA), an automated "climb" or "descend" instruction from the aircraft's collision avoidance system, since RAs are triggered specifically when projected flight paths indicate an imminent close encounter; mentioning whether an RA occurred and whether it was followed is highly relevant supporting detail for this category.
Compared to Category B ("safety not assured," 5 NM in the earlier example), a Category A report would typically describe a much smaller separation, potentially under 1 NM laterally and/or very little vertical separation (e.g., aircraft passing at the same level or within a few hundred feet), and/or a converging geometry where, absent the avoiding action taken, paths would have intersected.
The description of the other aircraft's behavior matters: "descended through our level" (as in Category B) describes a level bust that resolved before becoming critical; for Category A, language like "aircraft passed directly overhead/below at [very close separation], TCAS RA 'climb' followed" conveys that the encounter was actively unfolding as a collision risk, not just a level deviation that happened to occur nearby.
As with Category B, this is a report after the immediate event, by the time the pilot is transmitting, the conflicting aircraft has typically passed and avoiding action (if any) has been completed; this is not an ongoing MAYDAY/PAN PAN situation (though a pilot experiencing a genuine imminent collision risk during the event, before resolution, might use an urgency/distress call in the moment, the Airprox report is the after-the-fact notification regardless).
ATC's response remains minimal ("roger your airprox"), the formal investigation and risk-category confirmation happens off-frequency via the national Airprox board process, using radar data, both aircraft's reports, and ATC transcripts.

Reference: ICAO Doc 9432

Airprox - Safety not assured

Used when a pilot reports an Airprox occurrence to ATC, here, classified by the reporting pilot as "Safety Not Assured" (Category B), where another aircraft (a Boeing 737) descended through this aircraft's level at a distance considered too close to be comfortable, while operating in IMC (reducing the ability to visually acquire and avoid the other traffic). The report is made promptly after the event to ensure ATC has a real-time record while details are fresh, and so ATC can initiate its own investigation/data-pull (radar tapes, etc.) for the formal Airprox process.

Pilot

ATC

[Callsign] [Station] ROGER AIRPROX

Pilot

Airprox, safety not assured. Delhi Control, IFly 430. Position Jaipur 0500, true air speed 350, FL320, IMC, time of incident 0505. One Boeing 737 descended through our level at about 5 NM, in direction 360.

ATC

IFly 430, Delhi Control, roger airprox.

"AIRPROX, safety not assured" stated upfront immediately tells ATC the nature and (pilot's initial assessment of) severity of what follows, though as discussed, the final category is determined by an investigating board, not the pilot; this is the pilot's initial report, which triggers that process.
The position/time/speed/level block (Jaipur 0500, TAS 350, FL 320) establishes the reporting aircraft's own flight parameters at the time, essential for the investigation to correlate with radar data and the other aircraft's track.
"IMC" (Instrument Meteorological Conditions) is a critical detail, it explains why visual avoidance wasn't possible/reliable, which is directly relevant to the risk assessment (an encounter in VMC where both aircraft saw each other and maneuvered might be Category C; the same geometry in IMC with no visual contact could be Category B or A).
"Time of incident 0505", separate from the position report time (0500), pinpoints exactly when the conflict occurred, which may differ slightly from when the position report was taken, and is essential for ATC/investigators to pull the correct radar replay window.
"One Boeing 737 descended through our level at about 5 NM in direction 360" packs in: the other aircraft's type (737, useful for ATC to identify which flight from radar/flight data), the nature of the conflict (descending through this aircraft's level, a vertical conflict, possibly indicating a level bust by the other aircraft), the estimated distance (5 NM, the pilot's assessment of how close, used as supporting information for the risk classification but not the classification itoutselfself), and the direction (360, i.e., the other aircraft was to the north, or passed/approached from that bearing).
"Roger your airprox" is a minimal acknowledgment, ATC doesn't typically debate or immediately respond with analysis on-frequency; the report is logged, and ATC's investigation (and the pilot's own written Airprox report, normally filed after landing) proceed separately.
This radio report is the immediate notification, standard procedure also requires the pilot to file a formal written Airprox report (e.g., to the relevant national Airprox board) after landing, with more detailed information than can be efficiently passed by radio.
Note the distinction from the earlier "red flare" PAN PAN, that was reporting someone else's distress (maritime); this is reporting a safety event involving this aircraft itself (a near-conflict with another aircraft), but it's still not a MAYDAY/PAN PAN, since by the time it's reported, the immediate conflict has passed (the other aircraft already descended through and presumably continued away), this is a report, not an ongoing emergency requiring priority handling.

Reference: ICAO Doc 9432

Altimeter System Unreliable

Used when an aircraft becomes RVSM non-compliant in RVSM airspace due to either a transponder Mode C failure or an unreliable altimeter system. The pilot reports position, current level, the specific failure, declares RVSM non-compliance, and requests descent out of RVSM airspace. ATC instructs the aircraft to leave the ATS route, descend to FL270, and resume normal navigation passing FL290. The ATC response and pilot readback are identical in both scenarios, only the stated reason in the pilot's initial report differs. No Pan Pan declaration is required as the aircraft remains in controlled flight and the crew can safely manage the situation.

Pilot

[STATION] [CALLSIGN] [POSITION], [LEVEL], ALTIMETER SYSTEM UNRELIABLE, AIRCRAFT RVSM NON-COMPLIANT, REQUEST DESCENT [LEVEL]

ATC

LEAVING [CURRENT LEVEL] FOR [CLEARED LEVEL], TURNING [LEFT/RIGHT] TO LEAVE ATS ROUTE, DESCEND [LEVEL], PASSING [LEVEL] WILL RESUME NORMAL NAVIGATION [CALLSIGN]

Pilot

Varanasi Control, IFly 430, 40 nautical miles south of BBN, FL350, altimeter system unreliable, aircraft RVSM non-compliant, request descent FL270.

ATC

IFly 430, Varanasi Control, leave ATS route, descend FL270, passing FL290 resume normal navigation.

Pilot

Leaving FL350 for FL270, turning left to leave ATS route, descend FL270, passing FL290 will resume normal navigation. IFly 430.

RVSM (Reduced Vertical Separation Minima) airspace applies between FL290 and FL410, where vertical separation between aircraft is reduced to 1000 ft. To operate in RVSM airspace, aircraft must have functioning altimetry systems and a serviceable transponder Mode C, failure of either makes the aircraft RVSM non-compliant and it must immediately vacate RVSM airspace.
The pilot includes current level (FL350) in the initial report, giving ATC immediate awareness of the aircraft's vertical position and how far it needs to descend to exit RVSM airspace.
FL270 is specifically requested because it is 2000 ft below FL290, the lower boundary of RVSM airspace. Descending to FL270 ensures the aircraft exits RVSM airspace completely with an adequate buffer, where standard 2000 ft vertical separation minima apply instead of the reduced 1000 ft RVSM standard.
"Transponder Mode C unserviceable", Mode C provides automatic altitude reporting to ATC radar. Without it, ATC cannot independently verify the aircraft's level, making RVSM separation impossible to guarantee for surrounding traffic.
"Altimeter system unreliable", RVSM requires two independent altimeter systems to be functioning. If either becomes unreliable, the crew cannot guarantee accurate level-keeping to the 1000 ft separation standard required in RVSM airspace.
Both scenarios produce identical ATC responses and pilot readbacks, from ATC's perspective the required action is the same regardless of which system has failed. The distinction matters only for maintenance and technical records.
"Leave ATS route", the aircraft must deviate laterally from the ATS route before descending, ensuring separation from other aircraft on the same route at adjacent levels during the descent.
"Passing FL290 resume normal navigation", FL290 is the lower boundary of RVSM airspace. Once below FL290 the aircraft is clear of RVSM airspace and can rejoin the ATS route under standard separation minima.
No Pan Pan declaration is required, the aircraft remains in controlled flight and the crew can safely manage the descent. Pan Pan would only be appropriate if the failure escalated to a point where the crew lost confidence in their altitude awareness entirely.
"Turning left to leave ATS route", the pilot specifies the direction of turn in the readback, confirming the lateral deviation to ATC. The direction depends on the specific route and traffic situation at the time.

Reference: ICAO Doc 9432

Bad Weather at Destination

Used when a pilot, having received information about adverse weather at the destination (via company dispatch, ATIS, weather radar, or other off-frequency sources), proactively decides to divert to an alternate before the situation deteriorates further. Unlike a diversion initiated by ATC informing the pilot of a problem, here the pilot already has the information and has already made the decision, the transmission to ATC combines reporting position, stating the reason and decision, and requesting the descent needed to begin the diversion profile, all in a single call.

Pilot

[Station] [Callsign], INFORMED [reason for diversion]. [Position] [time] [level], WILL DIVERT [alternate], ESTIMATING [alternate] [time]. REQUEST [requirement].

ATC

[Callsign], CLEARED TO [alternate], [instruction], REPORT [condition].

Pilot

Trivandrum Control, IFly 430. Informed bad weather at destination. Overhead Cochin 0600, FL250. Will divert Coimbatore, estimating Coimbatore 0630. Request descend.

ATC

IFly 430, Trivandrum Control, cleared to Coimbatore, descend to FL60, report passing FL100.

Pilot

Leaving FL250 for FL60, will report passing FL100. Request weather Coimbatore, IFLY 430

Stating "informed bad weather at destination" gives ATC the reason for the diversion upfront, this is useful context even though ATC doesn't need to approve the decision itself (the pilot has command authority to divert), as it helps ATC understand why this aircraft's routing is changing and may prompt ATC to pass on any further weather information they hold.
The position report (Cochin, 0600, FL 250) anchors the request, ATC needs to know where the aircraft currently is before issuing a descent clearance towards the new destination.
"Will divert Coimbatore, estimating Coimbatore 0630" combines the decision (where) with the consequence (revised ETA), giving ATC both pieces of information needed to update separation/sequencing for Coimbatore arrivals.
"Request descend" is the operational need driving this call, the pilot wants to begin the descent profile towards Coimbatore promptly, since delaying descent could mean a steeper/later descent or holding near the new destination.
ATC's clearance, "cleared to Coimbatore, descend to FL 60, report passing FL 100", follows the same pattern as the earlier Mumbai diversion: new clearance limit, descent instruction, and an intermediate reporting point (FL 100) to monitor progress through the descent.
The pilot's readback ("leaving FL 250 for FL 60, will report passing FL 100") confirms the descent clearance, then immediately follows with a new request ("request weather Coimbatore"), practical, since the pilot will want current weather at the new destination to plan the approach, especially given the diversion was weather-driven in the first place.
"Standby for weather Coimbatore", ATC acknowledges the request but doesn't have the information immediately to hand; this is a normal holding response while ATC retrieves the METAR/ATIS for Coimbatore, with the expectation of a follow-up transmission once available.
This scenario, compared to the Mumbai one, demonstrates that diversions can originate from either side, ATC-initiated (when ATC has information the pilot lacks) or pilot-initiated (when the pilot has information ATC may not immediately have), and the call structure adapts accordingly: ATC-initiated diversions begin with "request your intentions," while pilot-initiated diversions begin with the pilot stating the reason and decision together.

Reference: ICAO Doc 9432

Brakes Hot Indication

Used when an aircraft's brakes overheat during taxi (often after a heavy landing, prolonged braking, or rejected takeoff) to the point of producing abnormal noise, smoke, or a loss of braking effectiveness, creating a risk of brake fire or tyre/wheel failure if the aircraft continues to move or stops without intervention. This does not threaten the aircraft in the immediate "grave and imminent" sense of a MAYDAY, but requires urgent ground support, fire crews standing by and possibly towing, hence PAN PAN rather than MAYDAY.

Pilot

PAN PAN PAN PAN PAN PAN [Station] [Callsign] [aircraft type] UNDERCARRIAGE BRAKES OVERHEATED [consequences] REQUEST [requirement]

ATC

[Callsign] [Station] ROGER PAN, [hold/stop instruction] [services dispatched] [towing on standby]

Pilot

Pan pan, pan pan, pan pan. Delhi Ground, IFly 430. Undercarriage brakes overheated, loud sound heard, aircraft stopping slowly on taxiway Alpha, short of link 2. Request fire fighting services.

ATC

IFly 430, Delhi Ground, roger pan. Remain on taxiway Alpha, stop aircraft. Fire fighting services dispatched, towing on standby.

Stop the aircraft as soon as practicable and avoid using the brakes further once overheating is suspected, additional braking can convert hot brakes into a brake fire. Communicate this intention to ATC as part of the call.
Fire services will want to know the aircraft's exact position (taxiway/stand and any reference point) since they need to drive a vehicle to it, be precise, as in this scenario's "short of link 2."
POB is relevant here because if a brake fire develops, an evacuation may become necessary, ATC and fire crews plan on this basis even if no fire is currently visible.
Towing is requested because once brakes have been overheated, the aircraft may be unable to move under its own power for a cooling period (sometimes 30–60+ minutes), and blocking a taxiway has knock-on effects for other traffic.
Do not shut down engines or evacuate unless fire/smoke is confirmed or instructed, premature evacuation on a hot ramp carries its own injury risk (slides, proximity to hot brakes/wheels). This decision rests with the commander based on visual indications.
If smoke, fire, or a tyre burst is observed or the brakes temperature continues climbing uncontrollably, this can be upgraded to MAYDAY.
Once fire services confirm brakes are cooling normally and no fire risk exists, the PAN PAN can be cancelled with "[callsign], cancel PAN PAN, brakes cooling normally."

Reference: ICAO Doc 9432

Co-pilot fainted

Used when an aircraft experiences a medical emergency on board, here, a flight crew member (co-pilot) has lost consciousness and cannot be revived, constituting "grave and imminent danger" both to the affected person and potentially to flight safety (single-pilot operation, especially if the captain becomes incapacitated too). MAYDAY is appropriate over PAN PAN because an unconscious crew member is a serious medical emergency requiring priority handling, immediate descent/diversion, and ground medical assistance on arrival, not merely an "urgency" situation.

Pilot

MAYDAY MAYDAY MAYDAY [nature of emergency] [Station] [Callsign] [aircraft type] [nature of emergency] [intentions] POSITION [position] [level] ETA [destination] [time] ENDURANCE [time] PERSONS ON BOARD [number]

ATC

[Callsign] [Station] ROGER YOUR MAYDAY, DESCEND TO [level] [condition/instruction]

Pilot

Mayday, mayday, mayday. Ahmadabad Control, IFly 430, Airbus. Co-pilot fainted, unable to revive him, landing at Ahmadabad. Position 90 DME AAE radial 045, FL 320. ETA Ahmadabad 0500. Endurance 0400. Persons on board 70.

ATC

IFly 430, Ahmadabad Control, roger mayday. Descend to FL50 when runway in sight.

Pilot

IFly 430, leaving FL320 for FL50, will report runway in sight.

The five mandatory MAYDAY elements are present: (1) callsign, IFly 430, (2) type, Airbus, (3) nature, co-pilot fainted, unable to revive, (4) position and level, 90 DME AAE radial 045, FL 320, (5) intentions, landing at Ahmadabad. POB (70) and endurance (0400) follow as required additional information.
Squawk 7700 should be set simultaneously with this call.
"Unable to revive him" signals this is not a brief fainting spell but a sustained loss of consciousness, likely prompting ATC to alert medical/ambulance services at the destination in addition to standard emergency response.
Position is given as a DME/radial fix from a navaid (AAE) rather than a named waypoint, common when the aircraft's position doesn't correspond to a charted reporting point, giving ATC precise lateral and distance information to plot the aircraft.
ATC's immediate priority response is to clear airspace and altitude for an expedited descent, "descend to FL 50 when runway in sight" gives the pilot discretion on timing the descent profile while pre-clearing the level needed for approach, reducing further radio exchanges during a high-workload single-pilot phase.
"When runway in sight" links the descent clearance to a visual condition rather than a fixed point/time, appropriate if this is expected to be a visual approach, and reduces coordination calls needed from the remaining pilot.
"Will report when runway in sight" is the pilot's commitment to the next update, giving ATC a defined trigger for the next exchange and confirming the instruction was understood.
With POB 70, ATC will coordinate medical personnel, a stretcher/ambulance, and likely priority gate/stand allocation and ground handling on arrival, none of this needs to be requested explicitly; it follows from the MAYDAY declaration and stated nature.
Endurance (0400 = 4 hours fuel remaining) being well in excess of the short ETA to Ahmadabad indicates fuel is not a constraint, included per the standard MAYDAY format regardless, as ATC may ask for it if not given.

Reference: ICAO Doc 9432

Diversion

Used when continuing to the planned destination is no longer feasible, weather below minima, runway closed, fuel state, technical reason, medical priority, and the pilot elects to land at an alternate. The pilot states the new destination, the reason, and the routing request. ATC re-clears, hands off, and (if the diversion is consequent to a MAYDAY or PAN) carries forward the priority.

Pilot

[Station] [Callsign] DIVERTING TO [alternate] DUE [reason] REQUEST [routing] [level] [next ATIS]

ATC

[Callsign] ROGER DIVERTING [alternate], CLEARED [routing] [level], CONTACT [next frequency]

Pilot

Mumbai Approach, Akasa Air 1532, diverting to Ahmedabad due weather below minima Mumbai, request direct AHM, descent and routing

ATC

Akasa Air 1532, Mumbai Approach, roger diverting Ahmedabad, cleared direct AHM via flight planned route, maintain FL three five zero, contact Mumbai Control 132.05, advise fuel state and persons on board

Pilot

Cleared direct AHM via flight planned route, maintain FL three five zero, Mumbai Control 132.05, fuel state one hour five zero minutes, persons on board one six two, Akasa Air 1532

Pilot

Mumbai Control, Akasa Air 1532, FL three five zero, direct Ahmedabad, diversion due weather, information Delta Ahmedabad

ATC

Akasa Air 1532, Mumbai Control, identified, descend FL two five zero, expect Ahmedabad ILS runway two three, contact Ahmedabad Approach 119.1 at one zero zero DME AHM

Pilot

Descend FL two five zero, expect ILS runway two three, Ahmedabad Approach 119.1 at one zero zero DME, Akasa Air 1532

State the reason on the initial diversion call, weather, fuel, technical, medical, so ATC knows whether to involve fire and rescue or simply re-clear. "Weather Mumbai" is treated as a routine re-route; "technical" prompts a query about RFFS posture.
If the diversion follows a MAYDAY or PAN, the original priority is retained until you cancel it explicitly. "Diverting due engine failure" is still under the PAN declared earlier, the priority does not need to be re-declared.
On first contact with the diversion airfield's facility, lead with the alternate's ATIS letter so ATC knows you are current on the runway in use, QNH, and any temporary restrictions. Examiners often ask this in viva, "what's the first thing you give Ahmedabad Approach?", and the answer is the ATIS letter.
Update fuel calculations against the new ETA the moment the diversion is committed. If the new arrival fuel will be below final reserve, declare MAYDAY FUEL immediately, the diversion call alone does not flag a fuel emergency.
Operator OPS must be advised in parallel (typically via ACARS / SATCOM / company frequency) so ground handling, customs, and pax services are pre-positioned at the alternate.

Reference: Doc 9432 §11.8 / DGCA CAR Section 8 Series O Part II

Emergency Descent

Initiated for rapid loss of cabin pressure, smoke or fire requiring immediate descent to a breathable / safer level. Descent rates may exceed 6,000 ft/min and a track offset is taken to avoid traffic at intermediate levels. The MAYDAY call goes out simultaneously with donning oxygen and starting the descent, voice may not be available later if the cabin altitude is still climbing.

Pilot

MAYDAY MAYDAY MAYDAY [Station] [Callsign] [aircraft type] EMERGENCY DESCENT [reason] LEAVING [level] FOR [target level] OFFSETTING [direction] [intentions]

ATC

[Callsign] [Station] ROGER MAYDAY, ALL TRAFFIC IN AREA STAND BY, [clearance to target level] [vectors as required]

Pilot

MAYDAY MAYDAY MAYDAY, Mumbai Control, Akasa Air 1532, Airbus A320, emergency descent due rapid depressurisation, over IVURA, leaving FL three seven zero for FL one zero zero, offsetting one five miles right of airway, diverting Ahmedabad

ATC

Akasa Air 1532, Mumbai Control, roger MAYDAY, all traffic in area stand by, descend pilot's discretion FL one zero zero, cleared direct Ahmedabad when ready, fire services standby Ahmedabad

Pilot

Descend pilot's discretion FL one zero zero, direct Ahmedabad when ready, Akasa Air 1532

Pilot

Mumbai Control, Akasa Air 1532, emergency descent complete, level FL one zero zero, persons on board one six two, endurance one hour four zero minutes, request continue Ahmedabad for landing

ATC

Akasa Air 1532, roger, emergency descent complete acknowledged, cleared direct Ahmedabad, descend FL eight zero, contact Ahmedabad Approach 119.1 at 5 zero DME

Squawk 7700 alongside the MAYDAY call, emergency descent is a defined contingency on every Indian Centre's coordination checklist and traffic below is cleared the instant the squawk is seen.
Take a 15-NM track offset right of the airway during the descent (left in left-hand traffic regions; the Indian FIR standard is right). This places the aircraft clear of opposite-direction traffic at intermediate FLs on the same airway.
Target FL100 unless terrain forces a higher MSA, FL100 is the highest level at which supplemental oxygen is not required for the cabin. State the target level explicitly on the MAYDAY call so ATC can clear the descent in one exchange.
Once level at the safe altitude, transmit "emergency descent complete, level [FL]" so adjacent traffic can resume normal flow.
If the cause is fire or smoke rather than depressurisation, the priority is the nearest suitable airport (not just a breathable level), divert immediately once stable.

Reference: Doc 9432 §11.6 / Airbus/Boeing AFM emergency-descent procedures

Engine Failure (Enroute Drift Down)

A single-engine failure on a twin in cruise is reported as PAN PAN when the aircraft can continue safely at a lower level (drift-down) and divert to the nearest suitable; it escalates to MAYDAY if the second engine is affected or terrain/weather makes the drift-down level untenable. The structured PAN report passes nature, position, level, drift-down target, and the diversion intention so Centre can clear the descent and re-route without further negotiation.

Pilot

PAN PAN PAN PAN PAN PAN [Station] [Callsign] [aircraft type] ENGINE FAILURE NUMBER [n] [position] [level] DRIFT DOWN TO [level] DIVERTING TO [alternate] REQUEST [clearance]

ATC

[Callsign] [Station] ROGER PAN, DESCEND [level] CLEARED [routing] [RFFS posture as applicable]

Pilot

PAN PAN, PAN PAN, PAN PAN, Mumbai Control, Akasa Air 1532, Airbus A320, engine number one failure shut down, over BBB, FL three six zero, drift down to FL two four zero, diverting to Hyderabad, request descent and direct routing

ATC

Akasa Air 1532, Mumbai Control, roger PAN, descend to FL two four zero, cleared direct Hyderabad, report level FL two four zero, advise persons on board, endurance, and any dangerous goods

Pilot

Descending FL two four zero, direct Hyderabad, persons on board one six two, endurance two hours one zero minutes, no dangerous goods, Akasa Air 1532

ATC

Akasa Air 1532, roger, fire services and ambulance standby at Hyderabad as a precaution, contact Hyderabad Approach 127.9 at one zero zero DME

Pilot

Standby acknowledged, Hyderabad Approach 127.9 at one zero zero DME, Akasa Air 1532

Drift-down level is computed by the FMS and is the highest altitude the aircraft can maintain on the remaining engine at the chosen Mach. State the target level on the initial PAN call so ATC clears traffic below in one exchange instead of two.
An engine shutdown is reported as "engine number [n] failure shut down" rather than "engine out", the verbatim phrase tells ATC the engine is secured (no relight in progress) and the aircraft is committed to single-engine flight.
ATC will usually pre-position RFFS at the alternate as a precaution even on a stable PAN PAN, particularly if the diversion is below RVSM or single-engine performance is marginal.
If the situation deteriorates, second engine indications, vibration, fire on the secured engine, upgrade to MAYDAY immediately. Don't continue under PAN.

Reference: ICAO Annex 6 / Doc 9432 §11.5

Fire / Smoke On Board

Declared as MAYDAY, fire on an aircraft is grave and imminent danger regardless of whether the source appears contained. The crew advises nature and location (cockpit, cabin, cargo, engine), intentions (nearest suitable, direct routing), POB and endurance. ATC clears the most direct profile to land and pre-positions full crash-category RFFS at the runway.

Pilot

MAYDAY MAYDAY MAYDAY [Station] [Callsign] [aircraft type] [TYPE AND LOCATION OF FIRE / SMOKE] [position] [level] [intentions] PERSONS ON BOARD [POB] ENDURANCE [hours minutes]

ATC

[Callsign] [Station] ROGER MAYDAY, [most direct profile] [RFFS on standby] [all stations standby]

Pilot

MAYDAY MAYDAY MAYDAY, Mumbai Control, Akasa Air 1532, Airbus A320, forward cargo smoke warning, fire bottles discharged smoke persisting, eight zero DME northeast of Mumbai, FL three six zero descending, request immediate landing Mumbai any runway, persons on board one six two, endurance two hours one zero minutes

ATC

Akasa Air 1532, Mumbai Control, roger MAYDAY, cleared direct Mumbai, descend pilot's discretion three thousand feet, expect ILS runway two seven, fire services full standby, all stations standby

Pilot

Direct Mumbai, descend pilot's discretion three thousand feet, expect ILS two seven, Akasa Air 1532

Pilot

Mumbai Approach, Akasa Air 1532, MAYDAY in progress, four zero DME inbound, leaving FL one five zero for three thousand feet, requesting straight-in ILS two seven

ATC

Akasa Air 1532, Mumbai Approach, identified, descend three thousand feet, fly heading two five zero, cleared ILS runway two seven, fire services in position, advise the captain crash crew on frequency one two one decimal six after landing

Pilot

Descend three thousand feet, heading two five zero, cleared ILS runway two seven, crash crew 121.6 after landing, Akasa Air 1532

POB and fuel endurance (hours and minutes, never weight) are the two pieces of information ATC will always ask for. Pass both on the initial MAYDAY call so a follow-up exchange isn't needed.
Name the location, cockpit / cabin / forward cargo / aft cargo / engine / wheel well. Each triggers a different ground-response posture and a different crew checklist; ATC's RFFS briefing depends on it.
Land at the nearest suitable airport, not necessarily the planned destination, time is the dominant factor in fire scenarios. The Air India 182 and Swissair 111 case studies are both standard DGCA exam references for the "nearest suitable" decision.
Squawk 7700 immediately. Fire and rescue services need the runway visual on the ground equipment from the moment the alert is raised, every second of preparation matters when the aircraft arrives.
After landing, expect to stop on the runway for an immediate evacuation if smoke or fire persists. ATC will pass the airport's tactical fire frequency (usually a 121.x discrete) so the captain can coordinate with the on-scene commander.

Reference: ICAO Annex 6 Part I §4.5 / Doc 9432 §11.5

Flaps stuck, Overweight landing

Used when an aircraft develops a flap malfunction (flaps stuck at a position unsuitable for landing, often retracted or at an intermediate setting) shortly after departure, while the aircraft is still significantly above its maximum landing weight. Landing immediately would be "not advisable" due to the combination of higher approach/landing speed (from reduced flap setting) and excess weight, both increase landing distance and stress on the airframe/gear beyond certified limits. PAN PAN is appropriate because the situation is abnormal and requires priority handling (holding airspace, fuel-burn time) but does not constitute grave and imminent danger, the aircraft can fly normally, it simply cannot land safely yet.

Pilot

PAN PAN PAN PAN PAN PAN [Station] [Callsign] [malfunction] PASSING [level], INTEND TO [intention], AIRCRAFT OVERWEIGHT FOR LANDING, REQUEST [level] AND [airspace/sector requirement]

ATC

[Callsign] [Station] ROGER PAN PAN, CLEARED TO [level], PROCEED TO FLY BETWEEN RADIAL [xxx] AND RADIAL [xxx] AT DME [xx] AND MORE

Pilot

Pan pan, pan pan, pan pan. Chennai Approach, IFly 430. Flaps stuck, passing FL 60. Intend to land back. Aircraft overweight for landing. Request FL 50 and sector to exhaust fuel.

ATC

IFly 430, Chennai Approach, roger pan pan. Cleared to FL 50, proceed to fly between radial 050 and radial 075 at DME 45 and more.

Pilot

Cleared to FL 50, proceed to fly between radial 050 and radial 075 at DME 45 and more, IFLY 430

ATC

IFly 430, Chennai Approach, duration to fly for fuel exhaustion, four zero minutes.

Pilot

Chennai Approach, IFly 430. Cancel pan pan. Flaps operating normal, proceeding to Mumbai as per flight plan. Request permission to set course and climb to FL 300.

ATC

IFly 430, Chennai Approach, climb to FL 100 in holding area and set course.

The core issue is two compounding factors: flaps stuck (affecting landing speed/distance) AND overweight (affecting landing distance/structural limits), either alone might be manageable, but together they make an immediate landing inadvisable. The pilot's stated intention is therefore not "land now" but "burn fuel to reduce weight, then land."
"Request FL 50 and sector to exhaust fuel" combines a level request with an airspace request, the pilot needs both vertical clearance and a defined lateral area to hold/fly racetrack patterns while burning fuel, without conflicting with other traffic or terrain.
ATC's response defines a holding sector by radial and DME ("between radial 050 and radial 075 at DME 45 and more"), this is a geographically bounded block of airspace the aircraft can maneuver within freely, rather than a fixed holding pattern at one point, giving the pilot flexibility for fuel-burn maneuvering.
ATC proactively provides "duration to fly for fuel exhaustion: 40 minutes", this is useful planning information for the pilot (how long the overweight condition will persist) even though the pilot didn't explicitly ask; it reflects ATC's awareness that this affects the pilot's workload planning and any contingency decisions.
The PAN PAN is cancelled once the underlying cause resolves ("flaps operating normal"), note that cancellation addresses the flap issue; the overweight condition was a consequence of the flap problem (need to burn fuel before landing), so once flaps are normal, the overweight issue is now also moot (the aircraft can simply continue as planned without needing to land overweight at all).
"Cancel PAN PAN, flaps operating normal, proceeding to Mumbai as per flight plan", the pilot reverts entirely to the original flight plan, abandoning the diversion/return-to-land intention, since the problem that necessitated it no longer exists.
"Request permission to set course and climb to FL 300", this is essentially requesting a new departure-style clearance from the holding area, since the aircraft's lateral position is now in the holding sector rather than on the original departure track.
ATC's final instruction, "climb to FL 100 in holding area and set course", is a two-part instruction: climb to an intermediate level while still within the holding sector (for separation/sequencing reasons, perhaps other traffic still needs FL 50–100 in that area), and only then set course (i.e., turn towards Mumbai/resume the flight-planned route). This is more conservative than immediately granting FL 300, likely because Chennai Approach needs to coordinate the climb through other levels/traffic before releasing the aircraft on its way.
This is a good example of PAN PAN being used for an operational/performance constraint rather than a mechanical failure threatening the aircraft's structural integrity directly, the aircraft is flying normally throughout; the constraint is about the landing phase only.

Reference: ICAO Doc 9432

Hydraulic Failure (Single System)

Reported as PAN PAN on a multi-redundant aircraft when one hydraulic system fails but the others remain serviceable. The call communicates which system is lost, the operational consequences (gear free-fall, no anti-skid, no autobrake, no nosewheel steering), and the runway/landing-distance requirement so ATC can sequence to the longest available runway and pre-position towing equipment for the rollout.

Pilot

PAN PAN PAN PAN PAN PAN [Station] [Callsign] [aircraft type] HYDRAULIC SYSTEM [green/blue/yellow] FAILURE [consequences] REQUEST [requirement]

ATC

[Callsign] [Station] ROGER PAN, [runway and length] [vectors / track miles] [towing on standby]

Pilot

PAN PAN, PAN PAN, PAN PAN, Mumbai Approach, Akasa Air 1532, Airbus A320, hydraulic system green failure, gear gravity extension required, no nosewheel steering, longer landing distance, request longest available runway and one five mile final

ATC

Akasa Air 1532, Mumbai Approach, roger PAN, runway two seven available, length three four four five metres, vectors for one five mile final, advise persons on board, endurance, and request towing on standby

Pilot

Runway two seven, one five mile final, persons on board one six two, endurance one hour four zero minutes, request towing on stand-by for rollout, Akasa Air 1532

ATC

Akasa Air 1532, roger, towing arranged, expect to vacate via parallel taxiway, descend three thousand feet, reduce to one eight zero knots

Pilot

Descend three thousand feet, reduce one eight zero knots, Akasa Air 1532

Name the system the aircraft type uses (A320: green / blue / yellow; B737: A / B / standby; B777: left / centre / right). Examiners check for the correct system designator, "hydraulic failure" without a system name will be marked down.
Three pieces of information always go on the initial PAN: which system, the operational consequence, and the runway/distance need. That lets ATC clear the appropriate runway in one exchange.
On A320 GREEN failure, brief: gear gravity extension, slats only (no flaps), no normal brakes (alternate brakes with accumulator pressure, no anti-skid), no nosewheel steering. Vref is increased and landing distance roughly doubles.
Request towing because with no nosewheel steering you cannot taxi clear after landing. State this explicitly so ATC can sequence following traffic and hold them on long final.

Reference: Doc 9432 §11.5 / Airbus/Boeing AFM hydraulic-failure procedures

Inter-Aircraft Relay

Used when an aircraft (IFly 160) cannot establish direct communication with the controlling unit (Ahmedabad Control) due to range or terrain limitations, and requests another aircraft (IFly 430) in range to relay its position report and crossing clearance request. The relay aircraft passes the message to ATC, receives the clearance, and relays it back to the requesting aircraft. This is a standard procedure ensuring continuous communication coverage in areas where direct VHF contact is not possible.

Pilot

[STATION] [RELAY CALLSIGN] RELAYING FOR [CALLSIGN], POSITION [FIX] [TIME], [LEVEL], ESTIMATING [BOUNDARY] [TIME], ETA [DESTINATION] [TIME], TRUE AIRSPEED [SPEED], REQUESTING CROSSING CLEARANCE

ATC

[RELAY CALLSIGN] [STATION] ROGER, [CALLSIGN] CROSSING CLEARANCE GRANTED, CROSS [BOUNDARY] AT [LEVEL]

Pilot

Ahmedabad Control, IFly 430, relaying for IFly 160, position Indore 0500, FL220, estimating Nagpur-Ahmedabad control boundary 0515, ETA Ahmedabad 0530, true airspeed 250, requesting crossing clearance.

ATC

IFly 430, Ahmedabad Control, roger, IFly 160 crossing clearance granted, cross Nagpur-Ahmedabad control boundary at FL220.

Pilot

Ahmedabad Control, IFly 430, wilco, relaying to IFly 160.

Pilot

IFly 160, IFly 430, Ahmedabad Control clears you to cross Nagpur-Ahmedabad control boundary at FL220.

Pilot

IFly 430, IFly 160, roger, crossing Nagpur-Ahmedabad control boundary at FL220.

"Relaying for IFly 160", the relay aircraft identifies itself and clearly states it is acting as a relay, not transmitting on its own behalf. This ensures ATC knows the position report and request belongs to IFly 160, not IFly 430.
The relay aircraft transmits the full position report on behalf of IFly 160, position, level, estimates, true airspeed, and the crossing clearance request, exactly as IFly 160 would have transmitted directly if in range.
"Position Indore 0500, FL220", the position report elements follow standard format: fix, time, and level. True airspeed is included as it is a required element for position reports over non-radar airspace.
"Estimating Nagpur-Ahmedabad control boundary 0515", the control boundary estimate is given rather than a waypoint, since crossing the boundary is the point at which Ahmedabad Control takes responsibility for the flight.
"Crossing clearance granted", ATC's clearance is issued to IFly 430 for relay to IFly 160. The clearance authorises IFly 160 to cross into Ahmedabad airspace at the specified level.
IFly 430 acknowledges the clearance with "wilco" before relaying to IFly 160, confirming to ATC that the message has been received and will be passed on.
When relaying the clearance to IFly 160, IFly 430 attributes it correctly, "Ahmedabad Control clears you", making clear the clearance authority is ATC, not the relay aircraft.
IFly 160's final acknowledgement is addressed to IFly 430, not Ahmedabad Control, since IFly 160 is still out of direct communication range with ATC. The relay chain remains intact until IFly 160 is within direct VHF range of Ahmedabad Control.

Reference: ICAO Doc 9432

Inter-Unit Coordination Failure

Used when an aircraft is approaching or crossing a control boundary but the transferring unit (Delhi Control) has failed to pass flight details to the receiving unit (Lucknow Approach) and is not responding to coordination attempts. The pilot, unable to establish contact with Delhi Control, reports directly to Lucknow Approach with full position information. Lucknow takes control of the aircraft on the current frequency, issues a frequency retention instruction, and requests a position report at the next fix.

Pilot

[STATION] [CALLSIGN] NEGATIVE CONTACT WITH [UNIT], POSITION CROSSING CONTROL BOUNDARY [ROUTE], [LEVEL], ESTIMATING [FIX] [TIME]

ATC

[CALLSIGN] [STATION] ROGER, MAINTAIN THIS FREQUENCY, REPORT [FIX] [LEVEL]

Pilot

Lucknow Approach, IFly 430 Heavy, negative contact with Delhi Control, crossing Delhi-Kolkata control boundary, FL110, estimating LLK 0600

ATC

IFly 430, Lucknow Approach, roger, maintain this frequency, report LLK, FL110.

Pilot

maintain this frequency, will report crossing LLK, FL110. IFly 430.

"Negative contact with Delhi Control", the pilot uses "negative contact" rather than "lost contact" to indicate they were unable to establish contact at all, as opposed to a contact that was established and then lost. This distinction helps Lucknow understand the nature of the communication breakdown.
"Position crossing control boundary Delhi Kolkata ATS route", the pilot gives their position relative to the ATS route boundary rather than a VOR/DME fix, since they are mid-boundary crossing. This gives Lucknow immediate situational awareness of where the aircraft is in relation to the handoff point.
"Estimating LLK 0600", LLK is the ICAO identifier for Lucknow (Chaudhary Charan Singh International Airport). Giving the estimate allows Lucknow to plan sequencing even without the formal flight details from Delhi.
"Maintain this frequency", rather than issuing a new frequency, Lucknow instructs the aircraft to stay on the current frequency. This avoids any further frequency change risk when the handoff has already broken down once.
"Report LLK, FL110", Lucknow requests a position report at LLK at FL110, confirming the aircraft's level and confirming it has reached the fix. This serves as a substitute for the formal transfer of control that Delhi failed to execute.
The pilot's readback adds "crossing" before LLK, "will report crossing LLK", which is more precise than just "report LLK," confirming they understand the report is expected as they pass over the fix.
This scenario highlights the pilot's responsibility to self-initiate contact with the next unit when the transferring unit fails to provide a frequency change or handoff, rather than continuing on the old frequency indefinitely, the pilot proactively contacts Lucknow Approach.

Reference: ICAO Doc 9432

Lost contact during climb

Used during departure climb when a pilot loses contact with the controlling unit (here, Delhi Tower) and re-establishes contact with the next available unit (Mumbai Approach) to report the lost contact and current status. Mumbai Approach retains the aircraft on their frequency and provides traffic information about conflicting reciprocal traffic before issuing the climb clearance, similar to the earlier Delhi Tower/Approach lost contact scenario but here occurring during climb rather than descent, and with an additional traffic conflict element that must be resolved before the climb can continue.

Pilot

[Station] [Callsign] [aircraft type] NEGATIVE CONTACT WITH [unit], [current level] FOR [cleared level]

ATC

[Callsign] [Station] MAINTAIN THIS FREQUENCY, CLIMB AS CLEARED, RECIPROCAL TRAFFIC [callsign] [type] [level/trend], ESTIMATE TO CROSS AT [reference point]

Pilot

Mumbai Approach, IFly 430 Heavy, negative contact with Delhi Tower. Approach 3500 for FL70.

ATC

IFly 430, Mumbai Approach, maintain this frequency, climb as cleared. Reciprocal traffic IFly 655, Boeing 777, descending to FL70, estimate to cross at 37 DME.

Pilot

Climbing as cleared, reciprocal traffic IFly 655, Boeing 777, descending to FL70, estimate to cross at 37 DME, IFLY 430

Pilot

Mumbai Approach, IFly 430, DME 37. IFly 655 reported DME 26, request climb FL330.

"Negative contact with Delhi Tower" establishes why the aircraft is calling Mumbai Approach, the handoff from Tower failed, and rather than continuing to attempt contact on a non-responding frequency during a critical climb phase, the pilot correctly sought the next available unit.
"Approach 3500 for FL70" gives Mumbai Approach the aircraft's current cleared altitude (3500) and its cleared climb level (FL70) in a single compact statement, telling the controller both where the aircraft is now and where it's going, allowing immediate separation assessment without further questions.
"Maintain this frequency", Mumbai Approach retains the aircraft rather than sending it back to Delhi Tower or another unit, since the aircraft already has working contact here; this mirrors the earlier Tower/Approach lost contact resolution.
"Climb as cleared", rather than issuing a new clearance, Mumbai Approach ratifies the existing clearance (FL70 as the pilot stated), keeping the aircraft on its current profile while simultaneously flagging the traffic conflict. This is efficient, no clearance change is needed yet, just awareness of the conflict.
"Reciprocal traffic INDAIR 655, Boeing 777, descending to FL70, estimate to cross at 37 DME", this is a full traffic information call: callsign, type, level/trend (descending to FL70, exactly this aircraft's cleared level), and where the crossing is expected (37 DME). The combination of "descending to FL70" and this aircraft "climbing to FL70" means both aircraft are converging on the same level from opposite directions, a genuine vertical conflict requiring the pilot to be aware and ready.
The pilot's readback of the traffic information in full is essential here, this is safety-critical information (potential conflict), and a correct readback confirms the pilot has the details right (callsign, type, level, DME crossing point).
"DME 37", the pilot's report, mirroring the earlier reciprocal traffic scenario, marks the point at which the conflict should be resolved (INDAIR 655 has passed the crossing point and separation is restored).
"INDAIR 655 has reported DME 26, request climb FL330", ATC's response is unusual here: rather than immediately clearing the climb (as in the earlier reciprocal traffic scenario), ATC passes information about INDAIR 655's current position (DME 26) before the pilot makes the climb request. This suggests ATC is monitoring both aircraft and using INDAIR 655's reported position to confirm separation is now adequate, DME 26 vs DME 37 means INDAIR 655 has passed the crossing point and is now 11 DME beyond it, confirming the conflict is resolved. The pilot's climb request to FL330 follows immediately, reflecting awareness that the conflict condition is now resolved.
The climb request (FL330) is included in the same transmission as the DME report, efficient, avoiding a separate call once the conflict is confirmed clear, similar to the earlier "passing FL120 for FL150, request climb FL350" approach of combining a position update with a climb request.

Reference: ICAO Doc 9432

MAYDAY (Distress Call)

Used when the aircraft is in grave and imminent danger requiring immediate assistance, engine failure, fire, structural damage, fuel emergency. Has absolute priority on the frequency. The word "MAYDAY" is spoken three times so any partial reception is unambiguous, then the full five-element distress message follows: callsign, type, nature, position and level, intentions, persons on board, endurance.

Pilot

MAYDAY MAYDAY MAYDAY [Station] [Callsign] [aircraft type] [nature of emergency] [position] [level] [intentions] PERSONS ON BOARD [POB] ENDURANCE [hours minutes]

ATC

[Callsign] [Station] ROGER MAYDAY, [clearance / vectors] [RFFS arrangements] [all stations standby]

Pilot

MAYDAY MAYDAY MAYDAY, Mumbai Tower, Akasa Air 1532, Airbus A320, engine failure number two, five DME west of Mumbai, passing two thousand five hundred feet climbing four thousand, request immediate return for landing runway two seven, persons on board one six two, endurance two hours one zero minutes

ATC

Akasa Air 1532, Mumbai Tower, roger MAYDAY, climb maintain four thousand, fly heading two seven zero, expect vectors ILS runway two seven, fire services on standby, all stations standby

Pilot

Climb maintain four thousand, heading two seven zero, expect ILS two seven, Akasa Air 1532

ATC

Akasa Air 1532, report dangerous goods on board and any special handling required

Pilot

Negative dangerous goods, no special handling required, Akasa Air 1532

Squawk 7700 simultaneously with the MAYDAY call, every controller within radar coverage sees the alert tag instantly.
The five distress-message elements are mandatory and ordered: (1) callsign, (2) type of aircraft, (3) nature of emergency, (4) position and level, (5) pilot's intentions. POB and endurance are added on the back of the call. The viva often tests the order, "position before intentions" is the most common error.
ATC will routinely ask for dangerous goods, special handling (wheelchair pax, lap infants), and POB / endurance if you haven't already passed them. Have all four ready before keying the mic.
Once the immediate situation is stable, the MAYDAY can be downgraded with "Akasa Air 1532, cancel distress, [callsign] descending normally", that's a defined phrase. ATC will reply "distress cancelled, [time] UTC."
Emergency calls are normally retained on the originating frequency until the aircraft is on final or on the ground, Tower will not hand you off to Ground for taxi-in unless the aircraft is intact and stopped.

Reference: ICAO Annex 10 Vol II §5.3 / Doc 9432 §11

MAYDAY versus PAN PAN

MAYDAY for distress requiring immediate assistance. PAN PAN for urgency concerning safety when immediate assistance is not required.

Pilot

MAYDAY MAYDAY MAYDAY, {UNIT}, {CALLSIGN}, {PROBLEM}, {INTENTIONS}, {POSITION_LEVEL_HEADING}. PAN PAN PAN follows the same order for urgency.

ATC

{CALLSIGN}, MAYDAY received, {ASSISTANCE_AND_INSTRUCTIONS}.

Pilot

MAYDAY MAYDAY MAYDAY, Mumbai Control, Skyline 345, engine failure, diverting to Mumbai, two zero miles north, flight level one eight zero, heading one eight zero.

ATC

Skyline 345, Mumbai Control, MAYDAY received, runway two seven available, descend and maintain one two thousand feet, report heading.

Say the signal three times at the start of the first distress or urgency transmission.
Give the unit addressed if possible, then aircraft identification, nature of the problem, intentions, position, level and heading.
Declare early. You may cancel the state later if the situation improves.
Distress traffic has absolute priority. Urgency traffic has priority over all communications except distress traffic.

Reference: ICAO Doc 9432, 9.2 to 9.3

Medical Priority (HOSP / MEDEVAC / PAN PAN MEDICAL)

Used when an aircraft is carrying patients, donor organs, or medical personnel and requires priority handling. The level of priority depends on urgency: an air ambulance with a stable patient files STS/HOSP and uses the HOSPITAL prefix; a life-threatening case in flight is declared on RT as PAN PAN MEDICAL or, if grave and imminent, MAYDAY.

Pilot

PAN PAN PAN PAN PAN PAN MEDICAL [Station] [Callsign] [nature of medical condition] [position] [level] REQUEST [intentions]

ATC

[Callsign] [Station] ROGER PAN MEDICAL, [clearance, priority handling, advise POB and endurance]

Pilot

PAN PAN, PAN PAN, PAN PAN, MEDICAL, Delhi Control, Akasa Air 1532, passenger suffering suspected heart attack, unable to revive, over IVURA, FL one five zero, request immediate descent and priority landing at Delhi

ATC

Akasa Air 1532, Delhi Control, roger PAN MEDICAL, descend FL one zero zero, expect direct Delhi for ILS runway two eight, ambulance and paramedics arranged, report persons on board and fuel endurance

Pilot

Descend FL one zero zero, direct Delhi for ILS two eight, persons on board one six two, endurance two hours one zero minutes, Akasa Air 1532

Three ICAO status indicators in flight-plan field 18 cover patient-carrying flights, in increasing order of priority: STS/HUM (humanitarian, relief flights), STS/HOSP (hospital flight, e.g. routine air ambulance with a stable patient), and STS/MEDEVAC (life-critical medical evacuation, highest medical priority).
When STS/MEDEVAC is filed, the pilot prefixes the callsign with "MEDEVAC" on every transmission, "MEDEVAC Akasa Air 1532". For STS/HOSP the prefix is "HOSPITAL". This signals the priority continuously without re-declaring it on each call.
Two acceptable forms for the urgency prefix on a medical case: (a) "PAN PAN, PAN PAN, PAN PAN, MEDICAL", the urgency call "PAN PAN" repeated three times followed by "MEDICAL" once, which is the form most commonly tested in the DGCA viva, and (b) "PAN PAN MEDICAL" repeated three times. Both are recognised by ATC; the first is shorter on the frequency and is the form pilots typically use in practice.
After the prefix, transmit the elements in this order: station, callsign, nature of condition, position (named waypoint or DME/bearing), level (cruising or leaving), then the request. Position-before-intention matters, ATC cannot clear the descent without knowing where you currently are.
"Unable to revive" is the accepted RT phrase to indicate cabin crew CPR/defibrillator efforts have been attempted and the patient has not responded. It tells ATC the situation is time-critical without forcing a clinical declaration on the frequency.
If the patient's life is in grave and imminent danger and the priority needed exceeds PAN PAN, for example, the only suitable airport is far away and any delay would be fatal, upgrade to MAYDAY MAYDAY MAYDAY. The aircraft itself doesn't have to be in danger to use MAYDAY; the threat to a life on board is sufficient.
ATC will typically ask the same two follow-ups as for any priority call: persons on board and fuel endurance. Have them ready before the medical call goes out.

Reference: ICAO Doc 4444 §16.4 / Doc 9432 §11 / Annex 2

Minimum Fuel & MAYDAY FUEL

"Minimum fuel" is an advisory, the flight is committed to land at a specific aerodrome and any change of clearance may result in landing with less than final reserve. It is NOT a declaration of emergency. "MAYDAY FUEL" is the formal fuel emergency declaration to be made when the calculated landing fuel at the nearest aerodrome where a safe landing can be made will be below final reserve fuel (typically 30 minutes at 1,500 ft above the alternate in standard conditions).

Pilot

[Station] [Callsign] MINIMUM FUEL [or MAYDAY MAYDAY MAYDAY (Station) (Callsign) FUEL]

ATC

[Callsign] ROGER MINIMUM FUEL [or ROGER MAYDAY FUEL, REPORT FUEL REMAINING, NO DELAY EXPECTED]

Pilot

Mumbai Approach, Akasa Air 1532, minimum fuel

ATC

Akasa Air 1532, Mumbai Approach, roger minimum fuel, turn right heading two zero zero, cleared ILS runway two seven, no delay expected, report fuel remaining and persons on board

Pilot

Right heading two zero zero, cleared ILS two seven, fuel remaining four five minutes, persons on board one six two, Akasa Air 1532

Pilot

MAYDAY MAYDAY MAYDAY, Mumbai Approach, Akasa Air 1532, FUEL, fuel remaining three zero minutes, request immediate vectors for ILS runway two seven priority over all traffic

ATC

Akasa Air 1532, Mumbai Approach, roger MAYDAY FUEL, all traffic standby for Akasa Air 1532, turn left heading zero five zero, descend three thousand feet, cleared ILS runway two seven, fire services on standby

Pilot

Left heading zero five zero, descend three thousand feet, cleared ILS two seven, Akasa Air 1532

"Minimum fuel" is an advisory, not an emergency. ATC will avoid putting you into a hold or extending track miles but will not freeze the frequency or pre-position RFFS. The flight is still expected to land at the planned destination.
"MAYDAY FUEL" is the formal emergency. The phrase format is the standard MAYDAY call with "FUEL" added immediately after the callsign, e.g. "MAYDAY MAYDAY MAYDAY, Mumbai Approach, Akasa Air 1532, FUEL." Once declared, expect priority handling, fire and rescue standby, and clearance to the closest suitable runway over all other traffic.
Fuel remaining is reported in TIME (hours and minutes), never weight. State "fuel remaining three zero minutes", do not say "two thousand kilograms" on the RT.
The trigger for MAYDAY FUEL is the calculated landing fuel below FINAL RESERVE (typically 30 min in standard conditions). It is NOT triggered by landing below planned alternate fuel, that is a minimum fuel advisory case.
After landing on a MAYDAY FUEL, expect a regulatory report (CA-13 in India). The captain should preserve fuel-log data for the investigation.

Reference: ICAO Annex 6 Part I §4.3.7.2 / Doc 9432 §9.6 / DGCA CAR Section 8 Series O Part II

Negative Contact During Descend

Used when an aircraft, while descending and attempting to establish contact with the next ATC unit (typically Approach), is unable to do so, either due to a frequency change error, coverage gap, or the next unit not responding. Rather than continuing to attempt contact on a frequency that isn't working while descending through busy terminal airspace, the pilot reports back to the previous unit (here, Tower, who may still be receiving) to flag the failed handover. The previous unit, if able, retains the aircraft and issues clearance directly, bypassing the unresponsive intermediate unit, to ensure the descent and approach continue safely without a gap in ATC control.

Pilot

[Station] [Callsign] [aircraft type] NEGATIVE CONTACT [unit] PASSING [level] FOR [level]

ATC

[Callsign] [aircraft type] [Station], MAINTAIN THIS FREQUENCY, CLEARED TO [fix] FOR [approach type], DESCEND TO [altitude], REPORT REACHING [fix]

Pilot

Delhi Tower, IFly 430 Heavy, negative contact Delhi Approach, passing FL70 for FL50.

ATC

IFly 430 Heavy, Delhi Tower, maintain this frequency, cleared to DPN for ILS approach, descend to 3500, report reaching VOR.

This is not a lost-comms (7600) situation, the aircraft has full two-way contact with Tower; it's specifically Approach that isn't being reached. Squawk remains unchanged.
The report should include level passing and level cleared to, so the receiving unit can immediately assess separation before issuing further instructions, "passing FL 70 for FL 50" tells Tower exactly where the aircraft is in its descent.
Tower's response, "maintain this frequency", effectively means Tower is now providing the service Approach would normally provide for this aircraft, at least until contact is re-established or the aircraft is positioned for handoff to a unit that can hear it.
The clearance issued (route to a fix, approach type, altitude, report point) is a full approach clearance, Tower must have the coordination/authority to issue this, which in real operations usually means prior coordination between Tower and Approach, or Tower having approach control responsibilities at smaller/combined units.
"Report reaching [fix]" gives the controller a confirmation point, useful both for normal sequencing and as an indicator that, if no report is received, something is wrong (further escalating concern).
If contact with Approach is re-established later in the descent/approach, normal handoff procedures resume; if not, the aircraft may continue the entire approach on the frequency it has working contact with.

Reference: ICAO Doc 9432

Negative Contact with Tower After Departure

Used when an aircraft becomes airborne but is unable to establish VHF contact with the departure aerodrome tower. The pilot contacts Delhi Radio, an HF relay communication station, to report the communication failure, provide a position report, and confirm the climb profile. Delhi Radio acknowledges and issues a climb continuation instruction with a mandatory level report, which the pilot reads back. This is a standard procedure for relaying communications when direct VHF contact with a controlling unit is not possible.

Pilot

[RELAY STATION] [CALLSIGN] NEGATIVE CONTACT WITH [UNIT], POSITION [DISTANCE] NAUTICAL MILES [BEARING] OF [FIX], [TIME], PASSING [LEVEL], CLIMBING AS CLEARED

ATC

[CALLSIGN] [STATION] CONTINUE CLIMB AS CLEARED, REPORT PASSING [LEVEL]

Pilot

Delhi Radio, IFly 430, negative contact with Jodhpur Tower, position 30 nautical miles north of Jodhpur, 0500, passing FL70, climbing as cleared.

ATC

IFly 430, Delhi Radio, continue climb as cleared, report passing FL150.

Pilot

Will report passing FL150. IFly 430.

"Delhi Radio", Delhi Radio is an HF aeronautical communication station, not an ATC unit. It acts as a relay between the aircraft and ATC when direct VHF contact is not possible due to range or terrain. Contacting Delhi Radio is the correct procedure when VHF contact with the departure or controlling unit cannot be established.
"Negative contact with Jodhpur Tower", the pilot reports the specific unit they failed to contact, giving Delhi Radio the information needed to coordinate with Jodhpur ATC on the ground.
"Position 30 nautical miles north of Jodhpur", bearing and distance from the departure aerodrome is given as the position reference since no VOR/DME fix has been reached yet. North is correct as the flight is routing towards Delhi, which lies north of Jodhpur.
"0500", the time in the pilot's transmission refers to the current time or estimated time, giving Delhi Radio a reference for coordination. In full position report format this would be tied to a fix estimate, but in this context it serves as a time reference for the relay.
"Passing FL70, climbing as cleared", the pilot confirms both the current level and that they are continuing on the cleared climb profile, assuring Delhi Radio that no deviation has occurred despite the communication failure.
"Continue climb as cleared", Delhi Radio does not issue a new clearance but confirms the existing clearance remains valid, avoiding any confusion about whether the communication failure has affected the climb clearance.
"Report passing FL150", an intermediate level report is requested so Delhi Radio can monitor the aircraft's climb progress and coordinate with the relevant ATC unit as the aircraft climbs into controlled airspace.
The pilot's readback is brief and correct, only the report instruction is read back since "continue climb as cleared" requires no readback of a specific level or route.

Reference: ICAO Doc 9432

Series of Red Flares

Used when a pilot observes a distress signal from another party, here, a series of red flares fired from a vessel at sea, and reports this to ATC for onward relay to the appropriate maritime/coastguard rescue authorities. This is not the reporting aircraft's emergency; the aircraft is acting as a relay for someone else's distress signal, similar in principle to the earlier radio-relay scenario but for a visual distress indication from outside the aviation system entirely. PAN PAN is used because this is urgent information requiring priority handling and action by the receiving authority, even though the reporting aircraft itself faces no danger.

Pilot

PAN PAN PAN PAN PAN PAN [Station] [Callsign] POSITION [position], SEEN [description of distress signal] FROM [source], ON BEARING OF [bearing] DISTANCE [distance] NAUTICAL MILES

ATC

[Callsign] [Station] ROGER PAN PAN, TAKING ACTION

Pilot

Pan pan, pan pan, pan pan. Mumbai Control, IFly 430. Position Goa. Seen a red flare being fired from sea, from a ship in distress, on bearing of 120, distance 50 nautical miles.

ATC

IFly 430, Mumbai Control, roger pan pan, taking action.

Red flares (or red pyrotechnic signals generally) are a recognized international distress signal under maritime conventions (and also covered under ICAO Annex 2 as a signal observed by aircraft indicating distress), a pilot observing this is obligated to report it, as it may be the only indication of an emergency reaching anyone capable of summoning help.
The position information given, bearing and distance from the aircraft's own position (Goa), allows the receiving authority to localize the vessel even though the aircraft itself doesn't know the vessel's exact coordinates; ATC or subsequent rescue coordination can convert this bearing/distance into a lat/long fix.
Unlike the aircraft's own PAN PAN calls, there's no "intentions" element from the pilot regarding their own flight, the aircraft will simply continue its flight as planned (no diversion implied) unless ATC specifically requests assistance (e.g., orbiting the area, providing updates), though in practice ATC's "taking action" likely means notifying maritime rescue coordination centers, and the aircraft's job is largely done after this report.
"Taking action" is a deliberately brief acknowledgment, ATC doesn't need to detail to the pilot what follow-up steps will occur (alerting Coast Guard, MRCC, etc.); it simply confirms the report has been received and will be acted upon, allowing the pilot to return to normal operations.
If ATC needs more information (exact time of observation, number of flares, further descriptions, or for the aircraft to remain in the area/orbit to assist in relaying further information to a rescue vessel/aircraft), a follow-up exchange would occur, but absent that, this exchange is considered complete.
This is analogous to a "DETSEMAC" or similar marine distress relay procedure in some training syllabi, pilots are trained to recognize and report such signals as part of general airmanship/SAR awareness, independent of their own flight's normal operations.
Note this differs from the earlier "relay" scenario (INDAIR 430 relaying INDAIR 160's position report), that was relaying another aircraft's routine ATC communication; this is reporting an observed external event (maritime distress) that the aircraft happened to witness.

Reference: ICAO Doc 9432

Sinking Ship in Sight

Used when a flight crew sights a vessel in distress at sea and reports it to ATC using a PAN PAN urgency call. The aircraft is not itself in distress, the urgency relates to the sinking ship observed below. The crew provides the position, bearing, distance, sea state, vessel type, and condition to allow ATC to coordinate maritime rescue services. ATC acknowledges with a brief "roger PAN PAN" and initiates rescue coordination behind the scenes.

Pilot

PAN PAN PAN PAN PAN PAN, [STATION] [STATION] [STATION], [CALLSIGN] [CALLSIGN] [CALLSIGN], [NATURE OF DISTRESS], [LOCATION], BEARING [DEGREES], DISTANCE [NM] NAUTICAL MILES, [SEA STATE], [VESSEL TYPE], [CONDITION], REQUEST IMMEDIATE ASSISTANCE

ATC

[CALLSIGN] [STATION] ROGER PAN PAN

Pilot

PAN PAN PAN PAN PAN PAN, Mangalore Control Mangalore Control Mangalore Control, IFly 430 IFly 430 IFly 430, sinking ship in sight, off the coast of Mangalore, bearing 120 degrees, distance 45NM, sea rough, waves high, appears to be a passenger ship, tilted to port, request immediate assistance.

ATC

IFly 430, Mangalore Control, roger PAN PAN.

"PAN PAN" is the second highest level of urgency call in aviation after MAYDAY. It is used when there is an urgent situation that is serious but not immediately life-threatening to the aircraft itself. In this case the aircraft is safe, the urgency relates to the vessel in distress observed below.
"PAN PAN PAN PAN PAN PAN", repeated three times, as are the station name and callsign. This triple repetition ensures the transmission is heard clearly by all stations monitoring the frequency, even in poor radio conditions.
"Mangalore Control Mangalore Control Mangalore Control", the station is repeated three times, consistent with PAN PAN call format. This mirrors the MAYDAY format where both the station and callsign are each said three times.
"Sinking ship in sight", the nature of the emergency is stated immediately and clearly after the identification elements, telling ATC at once what type of rescue coordination is required, maritime, not aeronautical.
"Bearing 120 degrees, distance 45 nautical miles", position is given as a bearing and distance from the coast reference point, which is the most practical way to describe a position at sea from an aircraft. This information is passed directly to maritime rescue coordination centres.
"Sea rough, waves high", sea state is an important element of the report as it affects the ability of rescue vessels and helicopters to operate in the area and approach the stricken vessel.
"Appears to be a passenger ship", the crew qualifies their identification with "appears to be," which is correct since positive identification of vessel type from altitude is not always possible. This avoids stating as fact something that may be incorrect.
"Tilted to port", "tilted to left" in the original transcript is corrected to "tilted to port," which is the correct nautical term for the left side of a vessel. Using "port" rather than "left" is more precise and universally understood in a maritime rescue context.
"Roger PAN PAN", ATC's acknowledgement is deliberately brief. There is no readback of details required; ATC confirms receipt of the urgency call and immediately initiates coordination with maritime rescue services without prolonging the RT exchange.

Reference: ICAO Doc 9432

TCAS Resolution Advisory (RA)

Used the moment the TCAS issues a Resolution Advisory (RA), "CLIMB CLIMB", "DESCEND DESCEND", "MONITOR VERTICAL SPEED", "INCREASE CLIMB", etc. The flying pilot follows the RA immediately, even if it contradicts the current ATC clearance; ICAO Doc 4444 §15.7.4 specifically authorises a deviation from a clearance to comply with an RA. The pilot-monitoring (PM) then reports the deviation to ATC using the prefix "TCAS RA" so the controller stops issuing conflicting instructions and clears traffic at the intermediate levels. A second report ("CLEAR OF CONFLICT, RETURNING TO …") follows the moment TCAS announces "CLEAR OF CONFLICT" and the aircraft is on its way back to the previously assigned level.

Pilot

[Station] [Callsign] TCAS RA [climbing/descending/maintaining as commanded] → on "clear of conflict": [Station] [Callsign] CLEAR OF CONFLICT, RETURNING TO [previously assigned level]

ATC

[Callsign] ROGER, [no further instruction until RA is over] → on the pilot's recovery report: [Callsign] ROGER, [reissue or amend the previous clearance]

Pilot

Mumbai Control, Akasa Air 1532, TCAS RA, climbing

ATC

Akasa Air 1532, Mumbai Control, roger

Pilot

Mumbai Control, Akasa Air 1532, clear of conflict, returning to FL three six zero

ATC

Akasa Air 1532, roger, maintain FL three six zero, report level

Pilot

Maintain FL three six zero, wilco, Akasa Air 1532

Follow the RA IMMEDIATELY, even if it contradicts the current ATC clearance. ICAO Doc 4444 §15.7.4.1 explicitly authorises the deviation; Annex 2 §3.6.2.3 makes it mandatory. The viva penalises any answer that says "check with ATC first".
Two phrases, in this fixed order: (1) "TCAS RA" on the deviation, (2) "CLEAR OF CONFLICT, RETURNING TO [level]" on recovery. Do not say "TCAS climb" or "avoiding traffic", the prefix examiners mark for is the literal phrase "TCAS RA".
Once you have reported "TCAS RA", ATC must not issue you further instructions until you report "CLEAR OF CONFLICT". This is the only situation in controlled airspace where you legally fly a profile other than the cleared one without first asking permission.
If you receive an "INCREASE CLIMB" or "INCREASE DESCENT" (strengthened RA), apply it without a separate transmission, the original "TCAS RA, climbing/descending" call already covers the manoeuvre. A second report is only required on "CLEAR OF CONFLICT".
Disengage the autopilot for a manual RA response unless the aircraft has TCAS-coupled autoflight (A350, B787, some A320neo and 737 MAX builds). On non-coupled types, manual control is faster and avoids the autopilot snapping back to the cleared level mid-manoeuvre.
Do not manoeuvre opposite to an RA to clear traffic visually, the resolution is computed against the threat's reciprocal RA, and a contradictory manoeuvre converts a coordinated separation into a collision course. "Eyes on the traffic" is a verification, never a substitute for compliance.

Reference: ICAO Doc 4444 §15.7.4 / Annex 2 §3.6.2.3 / Doc 9432 §11.6

Transponder Codes (7500 / 7600 / 7700)

Three reserved Mode A codes signal specific emergencies on radar regardless of voice. Knowing the trio and the rules around setting them is a recurring DGCA Part 2 viva fundamental.

Pilot

[squawk action, no voice required, but voice should follow if possible]

ATC

[ATC alerts emergency services, prioritises the flight, and on 7500 launches the unlawful-interference response]

ATC

Examiner: What does 7500 indicate?

Pilot

Candidate: 7500, unlawful interference, hijacking, or seizure of the aircraft.

ATC

Examiner: What does 7600 indicate?

Pilot

Candidate: 7600, radio communication failure, loss of two-way contact.

ATC

Examiner: What does 7700 indicate?

Pilot

Candidate: 7700, general emergency, used in conjunction with any MAYDAY or PAN PAN declaration in addition to the assigned code if voice is available.

7700 is the general emergency code, squawk it whenever a MAYDAY or PAN is declared, in addition to your assigned code if voice is available. It lights every radar screen in range and triggers a different alarm tone in the ATC suite.
7500 must never be tested, selected casually, or set during normal flight, it triggers an armed response on the ground including airport lockdown procedures and law-enforcement deployment to the stand. Once selected it cannot be quietly cleared.
Common memory aid: 7-5 Taken Alive (hijack), 7-6 Talking Nix (lost comms), 7-7 Going to Heaven (emergency). Examiners accept the mnemonic but expect the formal phrase as the primary answer.
When changing transponder codes for any reason, avoid scrolling through the three reserved codes. Pass through 1000 or 1200 instead, e.g. to go from 7444 → 7500 you cycle 7444 → 1000 → 7500. Modern transponders have a guarded knob for exactly this reason.
On Mode S transponders the flight ID (aircraft identification) is also transmitted, verify it matches the ATC strip on first contact. A mismatch will be queried before any of the three reserved codes are accepted.

Reference: ICAO Annex 10 Vol IV §2.1.4 / Doc 9432 §11.4

Transponder Mode C Unserviceable

Pilot

[STATION] [CALLSIGN] [POSITION], [LEVEL], TRANSPONDER MODE C UNSERVICEABLE, AIRCRAFT RVSM NON-COMPLIANT, REQUEST DESCENT [LEVEL]

ATC

[CALLSIGN] [STATION] LEAVE ATS ROUTE, DESCEND [LEVEL], PASSING [LEVEL] RESUME NORMAL NAVIGATION

Pilot

Varanasi Control, IFly 430, 40 nautical miles south of BBN, FL350, transponder Mode C unserviceable, aircraft RVSM non-compliant, request descent FL270.

ATC

IFly 430, Varanasi Control, leave ATS route, descend FL270, passing FL290 resume normal navigation.

Pilot

Leaving FL350 for FL270, turning left to leave ATS route, descend FL270, passing FL290 will resume normal navigation. IFly 430.

RVSM (Reduced Vertical Separation Minima) airspace applies between FL290 and FL410, where vertical separation between aircraft is reduced to 1000 ft. To operate in RVSM airspace, aircraft must have functioning altimetry systems and a serviceable transponder Mode C, failure of either makes the aircraft RVSM non-compliant and it must immediately vacate RVSM airspace.
The pilot includes current level (FL350) in the initial report, giving ATC immediate awareness of the aircraft's vertical position and how far it needs to descend to exit RVSM airspace.
FL270 is specifically requested because it is 2000 ft below FL290, the lower boundary of RVSM airspace. Descending to FL270 ensures the aircraft exits RVSM airspace completely with an adequate buffer, where standard 2000 ft vertical separation minima apply instead of the reduced 1000 ft RVSM standard.
"Transponder Mode C unserviceable", Mode C provides automatic altitude reporting to ATC radar. Without it, ATC cannot independently verify the aircraft's level, making RVSM separation impossible to guarantee for surrounding traffic.
"Altimeter system unreliable", RVSM requires two independent altimeter systems to be functioning. If either becomes unreliable, the crew cannot guarantee accurate level-keeping to the 1000 ft separation standard required in RVSM airspace.
Both scenarios produce identical ATC responses and pilot readbacks, from ATC's perspective the required action is the same regardless of which system has failed. The distinction matters only for maintenance and technical records.
"Leave ATS route", the aircraft must deviate laterally from the ATS route before descending, ensuring separation from other aircraft on the same route at adjacent levels during the descent.
"Passing FL290 resume normal navigation", FL290 is the lower boundary of RVSM airspace. Once below FL290 the aircraft is clear of RVSM airspace and can rejoin the ATS route under standard separation minima.
No Pan Pan declaration is required, the aircraft remains in controlled flight and the crew can safely manage the descent. Pan Pan would only be appropriate if the failure escalated to a point where the crew lost confidence in their altitude awareness entirely.
"Turning left to leave ATS route", the pilot specifies the direction of turn in the readback, confirming the lateral deviation to ATC. The direction depends on the specific route and traffic situation at the time.

Reference: ICAO Doc 9432

Unlawful Interference (Hijack)

Used when the aircraft is subject to unlawful interference and covert communication is required. Squawk 7500 silently if voice cannot be used; otherwise transmit the standard distress phrase. Once 7500 is set, ATC will route to a designated isolated parking position (every major Indian airport has a pre-surveyed bay) and a security cordon is established before the aircraft is met.

Pilot

[squawk 7500] [voice if possible: "TRANSPONDER SEVEN FIVE ZERO ZERO" or pre-briefed covert phrase]

ATC

[Callsign] [Station] ROGER, [no further interrogation; ATC clears the aircraft direct, all routine queries suspended]

Pilot

Mumbai Control, Akasa Air 1532, transponder seven five zero zero

ATC

Akasa Air 1532, Mumbai Control, roger seven five zero zero, continue present route, contact me with intentions when able

Pilot

Continue present route, intentions when able, Akasa Air 1532

Pilot

Mumbai Control, Akasa Air 1532, request to divert to Mumbai for unscheduled maintenance, request priority handling and direct routing

ATC

Akasa Air 1532, Mumbai Control, roger, cleared direct Mumbai, descend FL one five zero, expect ILS runway two seven, taxi instructions on landing

Squawking 7500 alone is sufficient, every controller within radar coverage will see the alert and security forces will be alerted on the ground. The pilot is never required to transmit voice if it would compromise safety.
If ATC asks to verify the 7500 code (e.g. "confirm transponder set seven five zero zero"), do NOT scroll back to 7500. The accepted covert affirmations are "transponder squawking as set" or "affirm," depending on company SOP, never repeat the code.
Operator SOPs publish covert phrases that signal hijack while sounding routine to a non-pilot. Common examples: "requesting maintenance check on departure," "company VIP on board," "diversion for technical reasons." The phrase is briefed at conversion training and rehearsed each recurrent.
Once 7500 is observed, ATC routes the aircraft to the airport's pre-surveyed isolated parking position. At major Indian airports this is a bay on the airfield perimeter, away from terminals and fuel farms, with vehicle access for tactical response teams.
All routine queries from ATC are suspended once 7500 is set, no descent restrictions, no requests for SELCAL check, no level reports. The only acceptable transmissions to the aircraft are essential safety items.

Reference: ICAO Annex 17 / Doc 9432 §11.4 / AIP India ENR 1.13