A crosswind component is the part of the wind blowing straight across a runway; the headwind component is the part blowing along it. For a wind of speed V at an angle θ to the runway, they are V·sin θ and V·cos θ. We measured the clock rule, a mental shortcut for the first, against the sine. In a 20 kt wind, the version taught by CAA New Zealand and in AOPA's training tips reads high by up to 2.68 kt, at exactly 60°. It never reads low by more than 0.18 kt. Of the 91 whole degrees from 0 to 90, 25 are over 1 kt off. Every error grows with the wind. Ignoring magnetic variation at Singapore Changi costs at most 0.13 kt in that wind.
Instructors teach the clock method and the rule of sixths as the same fraction
CAA New Zealand's flight instructor guide gives the formula, "XWC = V × Sineθ", then the watch face. Thirty degrees is thirty minutes, half way round, so the crosswind is half the wind; 45° is three-quarters; and from 60°, "consider the crosswind component to be the full strength of the wind".
AOPA's tip of 1 September 2026 reads the face continuously, so 20° is a third of the way round. A 2016 AOPA Pilot article quotes the instructor Albrecht teaching sixths: each 10° off the runway puts one-sixth of the wind across it, and at 60° "all of the wind". The two wordings agree at every multiple of ten degrees. Sixths stops at the tens; the clock, read as minutes over 60, fills the gaps.
Sine values correct the memorised fractions most at sixty degrees
We compared the rule with the sine at every whole degree in a 20 kt wind, about 37 km/h on our conversion. The rule is exact at 0°, 30° and 90°. From 1° to 29° it reads slightly low, worst at 17°. From 31° to 89° it reads high: 15.0 kt against 14.14 at 45°, and 20 kt against 17.32 at 60°, an excess of 15.5% of the true value. The mean error is 0.67 kt.
The excess at 60° is 0.134 of the wind speed, so a 30 kt wind makes it 4.0 kt. Pilot Institute's calculator page says "The clock face method is accurate within 1 to 2 knots across most angles." At 20 kt, 82 of our 91 angles fall within 2 kt, but no fixed figure in knots holds at every speed.
A divide-by-ten shortcut fails its own accuracy test with the wind straight down the runway
Pilot Institute's crosswind landings page calls a second method "the most accurate": the angle in tens of degrees, plus two, times a tenth of the wind. Its example, 30° in 15 kt, gives 7.5 kt, exactly the sine. Applied as written to a wind straight down the runway, it gives 4 kt of crosswind in a 20 kt wind, where the truth is zero: a fifth of the wind at any speed.
In our sweep it is exact at 30° and near 77.7°, low from 31° to 77°, and 22 kt at 90°. It is off by more than 2 kt at 14 angles, and its mean error, 1.26 kt, is nearly twice the clock rule's.
Training guides drill a magnetic correction worth about a tenth of a knot at Changi
Magnetic variation is the angle between true and compass north. The US Aeronautical Information Manual says METAR, the routine airport weather report, gives direction "referenced to true north", while ATIS, the recorded airport broadcast, gives wind relative to magnetic north. CAA New Zealand says METAR winds "must first be converted to magnetic". We found no official statement of which north ATIS uses in Singapore or Malaysia.
Changi's obstacle chart in Singapore's AIP, the state's aeronautical information publication, gives variation of 23 minutes of arc east (about 0.4°) at its 2020 epoch. Skipping it changes a 20 kt crosswind by at most 0.13 kt. The METAR's rounding can put the reported direction up to 5° from the mean, worth up to 1.74 kt, thirteen times more. At Keflavík, with 12° west (2023) as published in Iceland's AIP, the slip is worth 4.18 kt. These worst cases fall near the runway line and the clock rule's at 60°, so they do not add.
Parallel runways at KLIA test the nearest-ten-degree numbering rule
The FAA manual defines a runway number as "the whole number nearest one-tenth the magnetic azimuth". Malaysia's AIP, effective 11 August 2026, puts KLIA's runways 14L, 14R and 15 all on 146.01° true, with variation of 0° (2024). So 14L and 14R point 6° from 140°, and 15 sits 4° from 150°. Using the number times ten instead of the published heading costs up to 2.09 kt on 14L or 14R in a 20 kt wind.
Until 16 October 2026 our crosswind and headwind component calculator read runway "9" as 9°, so wind from 120° at 20 kt gave 18.7 kt across and a 7.2 kt tailwind. It computed a gust tailwind without showing it, flagged a 0.07 kt tailwind in red with runway advice, and said runway 09 lies within about 5° of 090°. The revision that day reads 1–36 as a runway number with a note, shows the gust tailwind, calls anything under 0.5 kt negligible and drops the advice. Its defaults, runway 090 and wind 120° at 20 kt, give 10.0 kt from the right and 17.3 kt of headwind, as "9" now does.
Regulators explain the demonstrated crosswind as a guide from flight testing
The old US rule for small aeroplanes, 14 CFR 23.233(a), required a demonstrated 90° crosswind component of "not less than" 0.2 VSO, where VSO is the stalling speed with flaps and gear down: a floor. The section was removed effective 30 August 2017, and the current Part 23 text in the eCFR contains no crosswind figure. Transport aeroplanes fall under 14 CFR 25.237(a)(1): "at least 20 knots or 0.2 V SR0, whichever is greater, except that it need not exceed 25 knots".
CAA New Zealand calls the demonstrated figure "not a legal limitation". A 2016 FAA safety leaflet calls it "the maximum cross wind experienced in the course of flight testing for certification".
WMO coding instructs observers to round the wind to the nearest ten degrees
WMO-No. 782, the World Meteorological Organization's aerodrome-report handbook (2020, in a copy hosted by Poland's IMGW), reports wind "in steps of 10° true and 1 kt" as a 10-minute mean. A gust is coded only when the strongest 3-second gust exceeds the mean by 10 kt (5 m/s) or more. Why the two differ is covered in our guide to wind averaging time.
A wind reported as 20 kt from 120° therefore spans a mean direction of roughly 115° to 125°. On a runway of 090°, that span alone moves the crosswind from 8.5 to 11.5 kt, on our arithmetic. Under the same rule, a 20 kt mean gusting to 29 kt carries no gust group.
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