Density altitude calculator

1. Pressure altitude. The subscale is 1004 hPa and standard is 1013.25 hPa, so the standard datum is 9 hPa below the one set. Pressure altitude = 1054 ft.
2. The exam shortcut, 27 ft per hPa, gives 1050 ft. The figure above uses the full standard atmosphere relation, which is why the two differ slightly.
3. ISA temperature at 1054 ft = 15 - 1.98 x 1.1 = 12.9 degrees.
4. ISA deviation = 43 - 12.9 = 30.1 degrees.
5. Density altitude = pressure altitude + 120 x ISA deviation = 1054 + 120 x 30.1 = 4664 ft.

Density altitude is the single number that says how the aeroplane will perform, because thin air takes something from every part of the takeoff at once. The engine draws in less mass, so it makes less power. The propeller pushes less mass, so it makes less thrust. The wing meets less mass, so it needs a higher true airspeed to produce the same lift, while the airspeed indicator still reads the same number.

Exam trap: density altitude is not a height you fly at, and it is not read off an instrument. Nothing in the cockpit displays it. A question that offers you an altimeter reading as the density altitude is offering you the wrong answer.

This tool takes density altitude as pressure altitude plus 120 feet per degree of ISA deviation, which is the rule the DGCA answer keys are built on. An ASA CX-3 device uses the exact ICAO relation instead and will differ from this by a couple of hundred feet at height. Both are defensible, and the exam wants the one above.