Wind Components and Crosswind Limits

An aeroplane takes off and lands relative to the air, but it accelerates and stops relative to the runway. The wind makes the difference. Its component along the runway, a headwind or a tailwind, changes the groundspeed at which the aeroplane lifts off or touches down, and so the distance it needs. Its component across the runway, the crosswind, changes little in the distance but a great deal in the handling, and every aeroplane has a crosswind figure that its pilots respect.

Wind is therefore resolved into components for every take-off and landing calculation. The rules then treat the result conservatively: only half of a reported headwind may be credited, and one and a half times a reported tailwind must be assumed. Separately, the flight manual and the operator set the crosswind and tailwind limits within which the aeroplane may be operated.

Resolving wind components

The surface wind is reported as the direction it blows from and its speed. The first step is the angle between the wind direction and the runway direction; runway 27, for example, points about 270°. The components follow from simple trigonometry:

• headwind component = wind speed × cos (angle); it becomes a tailwind component when the angle exceeds 90°;
• crosswind component = wind speed × sin (angle).

Three examples:

• Runway 27, wind 240° at 20 kt: the angle is 30°, so the crosswind is 20 × 0.5 = 10 kt and the headwind 20 × 0.87, about 17 kt.
• Runway 26 (260°), wind 210° at 20 kt: the angle is 50°, so the crosswind is 20 × 0.77, about 15 kt, and the headwind 20 × 0.64, about 13 kt.
• Runway 27, wind 200° at 25 kt: the angle is 70°, so the crosswind is 25 × 0.94, about 23 kt, and the headwind only 25 × 0.34, about 9 kt.

Swapping sine and cosine is the classic error. A useful check is that at small angles the crosswind is the small component.

Use the same north for both. Runway designators, the surface wind passed by the tower and the ATIS are magnetic. METAR and TAF winds are true. Mixing them puts the angle out by the local magnetic variation, which is small in western Europe but large at high latitudes.

In the cockpit the clock code gives a quick crosswind estimate: read the angle as minutes past the hour and take that fraction of the wind. Fifteen degrees gives a quarter, 30° a half, 45° three quarters and 60° or more the whole wind. It is exact at 30° and 90° and very slightly low below 30°, but between 30° and 90° it overestimates: three quarters against a true 0.71 at 45°, and the whole wind against 0.87 at 60°. For a limit check, that error is on the safe side. It does not give the headwind, for which the cosines above are needed. Many handbooks also print a crosswind component chart that does the same resolution graphically.

Headwind and tailwind effects

The aeroplane lifts off at a fixed airspeed. With a headwind it reaches that airspeed at a lower groundspeed; with a tailwind, at a higher one. Since the distance needed to accelerate grows rapidly with the groundspeed to be reached, a headwind shortens the take-off and a tailwind lengthens it, and by more than the same headwind would save. As a rule of thumb, a tailwind of 10 per cent of the lift-off speed increases the take-off distance by about 20 per cent. A tailwind also lengthens both parts of an accelerate-stop: more distance to reach the decision speed and a higher groundspeed when braking begins.

On landing the effect is similar, because the energy to be dissipated depends on the square of the groundspeed. A 10 kt tailwind can add about 20 per cent or more to a transport aeroplane's landing distance, and runway overruns have repeatedly combined tailwind with a wet or contaminated runway and a long touchdown (see runway excursion).

In the climb, wind does not change the rate of climb, which depends on excess power, but it changes the gradient over the ground. A headwind steepens it and a tailwind flattens it, which matters when obstacles lie beyond the runway. The other factors that change the distances, such as density altitude, slope and surface, are covered in factors affecting take-off and landing performance.

Wind factoring: 50 per cent and 150 per cent

EASA's performance rules for commercial air transport require take-off and landing calculations to use not more than 50 per cent of a reported headwind component and not less than 150 per cent of a reported tailwind component. The factoring guards against a wind that turns out less helpful, or more harmful, than reported, and it deliberately treats a tailwind more harshly than a headwind.

Whether the pilot applies the factors depends on the data. For large aeroplanes the factoring is written into the certified flight manual data, for example the landing distances under CS 25.125 and 14 CFR 25.125, so the crew enters the reported wind. The Class B charts in CAP 698 also include it, which is visible in the steeper slope of their tailwind lines. Where a chart does not include it, the pilot factors the wind before entering it. Applying the factors twice, or not at all, is a common error.

For a Class B landing, the dispatch check is also made in still air on the most favourable runway, in case the forecast headwind does not materialise, as well as with the factored forecast wind on the runway most likely to be in use.

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Crosswind component and crosswind limits

A crosswind has little direct effect on take-off or landing distance, but it tests control. On the ground the aeroplane tends to weathercock into the wind and the upwind wing tends to rise; in the air it drifts downwind. The pilot holds aileron into the wind during the take-off roll and uses rudder to keep straight, and on landing either crabs into wind and aligns the aeroplane with the runway before touchdown or lands wing-low. These techniques are covered in landing technique and crosswind operations.

Crosswind limits are usually stated with gusts included, so the crosswind component of the gust value, not only of the steady wind, is compared with the limit. Strong or gusty crosswinds also change the approach: on the A320, with a crosswind above 20 kt, the approach speed should be at least VLS + 5 kt, and the increment may be raised up to 15 kt at the crew's discretion.

A contaminated runway reduces the crosswind that can be controlled, because the tyres can produce less side force. Airbus publishes maximum recommended crosswinds for the A320 by runway condition, the same for take-off and landing, including:

Maximum demonstrated crosswind

The maximum demonstrated crosswind component is the highest crosswind at which the aeroplane was shown to be controllable during certification, flown by a test pilot with normal skill. It records what was demonstrated, not what the aeroplane can theoretically withstand. For light aeroplanes, FAA certification has required satisfactory control in a 90° crosswind of at least 0.2 VS0, and the value demonstrated is published in the POH. For large aeroplanes, CS 25.237 and its FAA equivalent require a demonstrated 90° crosswind component to be established for dry runways.

The figure is not automatically a limitation. It becomes one only if the flight manual lists it in the limitations section, or if the operator adopts it as an operating limit in its operations manual. The A320's demonstrated figure is 38 kt, gusts included; Airbus states that it is not a flight manual limitation and recommends that operators do not intentionally exceed it. The Embraer E190 (first generation) has published limits of 38 kt on a dry runway and 31 kt on a wet one. For a light aeroplane pilot, the demonstrated figure is a sensible ceiling for a personal limit, to be approached only with experience and reduced for gusts and a slippery surface.

Tailwind limits

Many transport aeroplanes are limited to a tailwind component of 10 kt for take-off and landing, the value up to which their landing performance is normally certified. The A320 and the Boeing 737 NG are both limited to 10 kt. Higher figures exist: the EASA type certificate data sheet for the 737-8 records a 15 kt tailwind capability as demonstrated, but states that this is not an operational approval above 10 kt, while the operator limits quoted for the Embraer E190-E2 allow 15 kt. Operators often set lower tailwind limits on wet or contaminated runways, where the stopping margin is already small.

Two further rules shape which runway is used. Under ICAO PANS-OPS, noise abatement should not be the deciding factor in choosing a runway when the crosswind component, including gusts, exceeds 15 kt or the tailwind component, including gusts, exceeds 5 kt. And under ICAO Annex 14, the number and orientation of an aerodrome's runways should give a usability factor of at least 95 per cent: the crosswind component should stay below the value set for the aeroplanes the aerodrome serves at least 95 per cent of the time.

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.