You're joining the circuit, the ATIS has just given you the wind, and it's well off the runway. You want to know how much of it is coming across you, and you want to know before you turn final. You don't need a calculator for that. You need a clock face.
Take the angle between the wind and the runway and read it as minutes past the hour. If the wind is 15 degrees off, that's a quarter of an hour, so a quarter of the wind is blowing across you.
| Angle off the runway | Minutes on the clock | Crosswind |
|---|---|---|
| 15° | quarter past | a quarter of the wind |
| 30° | half past | half the wind |
| 45° | quarter to | three quarters of the wind |
| 60° or more | the full hour | all of it |
Angles in between work the same way. 20° is a third of the hour, so a third of the wind. A 20 kt wind 30° off the runway gives 10 kt of crosswind, and 45° off gives about 15 kt. It's how the Civil Aviation Authority of New Zealand teaches it in its flight instructor guide.
It's close enough for flying. And when it's out, it tends to give you a bit more crosswind than there really is, which is the safe way round. If it puts you right on your limit, check the exact figure in the crosswind calculator.
Getting the angle is simple subtraction. Take the smaller number from the larger, and if the answer is more than 180, take it from 360. Say you're on runway 34 with the wind from 020. 340 minus 20 is 320, and 360 minus 320 is 40, so the wind is 40° off.
One warning. Don't run the clock backwards to get the headwind. It gives you more headwind than you'll have, so your take-off and landing distances come out too short. For anything that goes into a performance figure, use the calculator.
A worked example from Sydney#
Here's a real report from Sydney, and it's a breezy one:
SPECI YSSY 082324Z AUTO 20018G28KT 9999 // BKN036 OVC044 16/10 Q1021
Wind from 200° at 18 kt, gusting 28 kt. Before you start subtracting, there's a catch. METAR winds are in degrees true and runway numbers are normally magnetic, so convert first. NOAA's World Magnetic Model puts the variation at Sydney at 13° east, which makes the wind 187° magnetic. (For real flying, use the variation printed on your chart or in the AIP.)
Runways 16L and 16R are published at 155° magnetic, so the wind is 32° off the nose. On the clock that's just over half past, so just over half the wind. Call it about 10 kt across in the mean wind and 15 kt in the gusts. The gust figure is the one to hold against your limit.
Now try runway 25, which sits at about 241° magnetic. The same wind is 54° off. That's nine tenths of the hour, about 16 kt across and 25 kt in the gusts. If you have a choice, we'd take 16.
What catches people out#
Gusts#
Always work out the crosswind from the gust figure. On Sydney's runway 16 that's 15 kt rather than 10 kt. The mean wind looks friendlier, but the gust is what you'll meet in the flare.
True and magnetic#
This is the one that trips up the most people. METARs and TAFs give the wind in degrees true. A tower or ATIS gives the take-off and landing wind in degrees magnetic, the same reference as the runway number. The UK CAA (CAP 746) and the FAA (Order JO 7900.5E) both say so, and the New Zealand CAA's instructor guide tells students to convert METAR and TAF winds before working out the angle.
Canada's Northern Domestic Airspace is the exception. Transport Canada's AIM says runway headings there are given in degrees true.
So before you subtract, either convert the METAR wind to magnetic or compare it with the runway's true heading. Just don't mix the two.
If you forget to convert, the answer can come out too high or too low. Boston is a good example of too low:
METAR KBOS 082254Z 26014G29KT 10SM FEW044 SCT200 12/04 A2968 RMK AO2 PK WND 28029/2245 SLP050 T01170039 $
Wind 260° true at 14 kt, gusting 29 kt. Boston's variation is 14° west, so the wind is from 274° magnetic. Runway 22 lies at about 214° magnetic (200° true), so the wind is 60° off and the clock calls the whole 29 kt gust crosswind.
Now do it the lazy way. Subtract 214 from the METAR's 260 without converting and you get 46°. The clock then gives you about 22 kt, which is well short, and that's an uncomfortable surprise to find in the flare. With west variation, as at Boston, the mistake makes the crosswind look smaller than it is. With east variation it does the opposite. In Sydney the same slip puts the wind 45° off runway 16 and gives about 21 kt in the gusts instead of 15.
The runway number has its own quirks too. It's rounded, and it can be out of date. The FAA defines it as the whole number nearest one tenth of the magnetic bearing of the centre line. Runways aren't renamed every time the variation drifts, so Sydney's 07/25 now lies at about 061° and 241° magnetic, 9° off its number. Take the heading from the AIP (the Chart Supplement in the US, the Canada Flight Supplement in Canada) rather than the paint.
Variable winds#
Auckland, on a damp morning:
METAR NZAA 090030Z AUTO 06009KT 020V080 9999 -RA SCT027/// SCT036/// BKN047/// 19/18 Q1020 RERA
Wind 060° at 9 kt, varying between 020° and 080°. The 020V080 group means the direction varied by 60° or more, and less than 180°, over the last ten minutes. UK and US coding both use that 60° threshold, with different minimum wind speeds; how to read a METAR sets them out. With about 20° of east variation, the mean is 040° magnetic and the range runs from 000° to 060° magnetic.
On runway 05R, at about 052° magnetic, the mean wind is only 12° off. That's under 2 kt across, and it looks like a lovely day for it. But at the 000° end of the range the wind is 52° off, and the clock rule gives you nearly 8 kt from the left. So work it out for the worst direction in the range, not the mean.
Maximum demonstrated crosswind and personal limits#
Most pilots compare their answer with the maximum demonstrated crosswind in the pilot's operating handbook. That number is a record of testing, not a hard limit. The New Zealand CAA calls it "not a legal limitation, but a guide". The FAA's Personal Minimums for Wind leaflet describes it as "the maximum cross wind experienced in the course of flight testing for certification".
The leaflet also notes that the US certification rule for light aeroplanes at the time (the former 14 CFR 23.233, replaced in 2017) required a 90° crosswind of at least 0.2 VS0 to be demonstrated safe for taxiing, take-off and landing. For an aeroplane that stalls at 49 kt, that's just under 10 kt. Not a lot.
The same leaflet suggests documenting your own crosswind performance with an instructor at least once a year and setting personal minimums from that. Many clubs also set a lower figure for students and newly checked-out members. On a gusty day, hold the gust component against the lowest of the three.
Getting the exact figure#
When the clock says you're close, open the crosswind calculator from your airfield and pick a runway. It fills in the runway's true heading and, where there's a recent METAR nearby, the current wind, so both sit on the same reference. If you type in a tower or ATIS wind instead, remember that wind is magnetic, so pair it with the runway's magnetic heading. You get headwind and crosswind for both runway ends, mean and gust, flagged against a limit you enter.
Cancelling bookings for wind#
Sometimes the answer is that you're not going. That's disappointing, especially on a day that looked fine from the car park. No booking system can check the wind for you. That call stays with the pilot or instructor, on the latest report.
When a booking is cancelled for weather, Volanti records the reason, and the aircraft or instructor can be held for everyone so nobody books straight back into the same wind. When the hold is released, the waiting list runs again on its own.
Before your next gusty slot, pull your airfield's report from the METAR and TAF tool and put the wind and gust into the crosswind calculator. If the codes are unfamiliar, how to read a METAR explains the wind group.