SNOWTAM decoder

series and number

SWEF0149

SNOWTAM number 149 in series EF.

Every SNOWTAM carries a serial number in a national series. A new SNOWTAM for the same runway replaces the earlier one outright, so the number is how you tell which assessment you are holding.

Exam trap. A SNOWTAM is not amended, it is replaced. Holding an older number for the same runway means holding a superseded assessment.

item A, aerodrome

EFHK

The aerodrome is EFHK.

Item A is the four letter ICAO location indicator of the aerodrome the assessment was made at. A SNOWTAM covers one aerodrome, and inside it one runway per assessment line.

item B, time of assessment

01150140

Assessed on the 15th of January at 01:40 UTC.

Item B is when the assessment was made, written as month, day, hour and minute in UTC. It is the observation time, not the time the message was sent, and a runway state is only as good as this time is recent.

Exam trap. The first pair is the MONTH, not the day. A SNOWTAM time group is eight digits where a METAR time group is six, and reading it as a METAR time puts the date out by a month.

item C, runway

04R

Runway 04R.

Item C is the runway the assessment is for, written as the designator alone. Every per-third reading that follows is ordered in the direction of THIS runway, so the same physical strip assessed from the other end is a separate report with the reciprocal designator.

Exam trap. A runway assessed from both ends produces two SNOWTAM runway lines, and they are not reverses of each other, because the touchdown third of one is the rollout third of the other.

item D, runway condition codes

5/5/5

Condition codes 5, 5, 5: touchdown good, midpoint good, rollout good.

Item D is the runway condition code for each third of the runway, from 6 for a dry runway down to 0 for the worst. The code is what the crew computes landing performance against, and the braking action word beside it is what the code reports.

Exam trap. The three readings run touchdown, midpoint, rollout in the direction of the runway in item C, not left to right along the ground and not lowest first. Read them backwards on runway 27 and you have described runway 09. The operating decision is made on the WORST of the three, not on an average of them.

item E, contaminant coverage

25/25/25

Coverage: touchdown 25% covered, midpoint 25% covered, rollout 25% covered.

Item E is the percentage of each third that is covered. It is only reported when more than 10% of a third is contaminated, so a third left as NR here is not a third nobody looked at, it is a third with almost nothing on it.

Exam trap. The three readings run touchdown, midpoint, rollout in the direction of the runway in item C, not left to right along the ground and not lowest first. Read them backwards on runway 27 and you have described runway 09. And a low percentage does not raise the condition code: the code in item D already accounts for it.

item F, contaminant depth

03/03/03

Depth: touchdown 3 mm deep, midpoint 3 mm deep, rollout 3 mm deep.

Item F is the depth of the contaminant on each third, in millimetres. Depth is reported for standing water, slush, wet snow and dry snow, and it is what separates a code 5 from a code 2 on the same contaminant.

Exam trap. The three readings run touchdown, midpoint, rollout in the direction of the runway in item C, not left to right along the ground and not lowest first. Read them backwards on runway 27 and you have described runway 09. The number is millimetres, never centimetres, and the 3 mm line is the one the matrix turns on.

item G, contaminant

WET_SNOW/WET_SNOW/WET_SNOW

Contaminant: touchdown wet snow, midpoint wet snow, rollout wet snow.

Item G names what is actually on each third: dry snow, wet snow, slush, ice, compacted snow, standing water, frost, or NO when the third is clear. It is the row of the matrix, and item D is the code that row produced.

Exam trap. The three readings run touchdown, midpoint, rollout in the direction of the runway in item C, not left to right along the ground and not lowest first. Read them backwards on runway 27 and you have described runway 09. Item G and item D have to agree: a third reporting ICE cannot also report a condition code of 5.

code 6, dry

A dry runway.

Nothing on the surface. Code 6 is the only code that is not reported as a braking action, because a dry runway is the performance case the aeroplane was certificated on.

code 4, good to medium

Compacted snow at an outside air temperature of minus 15 degrees or colder.

Cold compacted snow grips better than warm compacted snow. Minus 15 is the temperature the code turns on, and above that temperature the same surface is code 3.

code 3, medium

A slippery wet runway; dry or wet snow over 3 mm deep; compacted snow warmer than minus 15 degrees; dry or wet snow on top of compacted snow.

Braking is noticeably reduced and directional control needs care. This is where a landing distance assessment stops being a formality.

code 2, medium to poor

Standing water or slush more than 3 mm deep.

Depth of water is now the problem rather than friction alone: aquaplaning and drag from the displaced water both matter, and the takeoff case can be limiting as well as the landing case.

code 1, poor

Ice.

Braking is poor and directional control is poor with it. Crosswind limits published for a contaminated runway are what govern here.

code 0, less than poor

Wet ice; water on top of compacted snow; dry or wet snow on top of ice.

The worst code there is. A code 0 third is a runway an operator's own procedures will normally forbid, and it is reported so that the crew does not have to work out how bad it is.