Crosswind landing
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A crosswind landing is a landing manoeuvre in which a significant component of the prevailing wind is perpendicular to the runway centreline.
Aircraft are generally directionally stable and have a tendency to point into the wind direction, so that in a crosswind the aircraft will adopt a yaw orientation with respect to the runway. Also, the cross wind causes the aircraft to drift laterally on approach. Safe operation of the undercarriage, however, requires the body and the velocity of the aircraft to be aligned with the runway at touch down, with the aircraft positioned near the centreline.
In order to meet these conflicting requirements, three standard procedures are recommended for executing a safe landing in a cross wind, these are called the Crab, De-Crab and Sideslip techniques.
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[edit] Crosswind
A crosswind is a wind condition when the prevailing wind is blowing across the runway. Definition: “a wind that blows across a particular route, flight path, or direction of travel” (source: Encarta).
[edit] Crosswind Landing
Because the wind vector shown below has a significant component perpendicular to the runway, the aircraft will drift laterally away from the runway centerline. Therefore, it is necessary to correct the aircraft heading to remain on the centreline.
Example:
An aircraft is approaching Runway 09, i.e. on a runway oriented at a bearing 090 degrees and the wind is blowing 40 knots from 150 degrees. The aircraft's heading will initially be 090 degrees, in order to line up with the runway, but if it maintains that heading, it will drift laterally and end up landing downwind of the runway. The wind direction relative to the runway heading is 150 - 090 = 60 degrees. The crosswind is the wind component perpendicular to the runway heading i.e. sin60degrees x 40kts = 35kts.
[edit] Techniques
The following guidelines are advised by Boeing for a crosswind landing. These guidelines assume steady wind (no gusting). These winds are measured at 33ft Tower height for a runway 148 feet in width. Basically, there are 3 landing techniques which may be used to correct for cross winds: Crab, De-Crab, and Sideslip.
[edit] De-Crab
The objective of this technique is to maintain wings level and the aircraft position near the runway centerline during approach. The nose points into the wind so that the aircraft approaches the runway slightly skewed with respect to the runway centreline (crabbing). This gives the impression of approaching the runway flying sideways, which can be disorienting for the pilot. Position is maintained by balancing the crosswind component, or more accurately the drag force arising from it, with engine thrust. Wings are maintained level throughout the approach. Right before the flare, opposite rudder (downwind rudder)is applied to eliminate the crab, with a simultaneous application of opposite aileron to maintain a wings-level attitude, so that at touch down, the body, velocity vector, and bank angle are all aligned with the runway, and the aircraft is positioned near the centre.
[edit] Crab
This is similar to the De-Crab technique. The principal difference is the aircraft touches down whilst still crabbing. The position on the runway is corrected after touch down. This applies significant slip angle to the tyres, and increases the lateral loads on the undercarriage, so this technique is bounded by speed restrictions and is not generally recommended.
[edit] Sideslip
This requires a higher level of skill. The purpose of this technique is to maintain heading aligned with the centreline. The initial phase of the approach is flown using the Crab technique to correct for drift. The aircraft heading is adjusted using rudder and ailerons to align with the runway. This places the aircraft at a constant sideslip angle, which its natural stability will tend to correct. Sufficient rudder and aileron must be applied continuously to maintain the sideslip at this value. The dihedral action of the wings has a tendency to cause the aircraft to roll, so aileron must be applied to check the bank angle.
With a slight residual bank angle, a touchdown is typically accomplished, for the cross wind direction shown, with the right main wheels touching down just before the left wheels. Excessive control must be avoided because over-banking could cause the engine nacelle or outboard wing flap to contact the runway/ground.
In strong crosswind conditions, it is sometimes necessary to combine the crab technique with the sideslip technique.
[edit] See also
[edit] Reference
- Boeing Flight Crew Training Manual
[edit] External links
- Videos can be found here .
- Crosswind landing test of A380
