A crosswind landing
is a landing
thumb|A [[Mute Swan]] alighting. Note the ruffled feathers on top of the wings indicate that the swan is flying at the [[Stall |stall]]ing speed...
maneuver in which a significant component of the prevailing wind is perpendicular to the runway
According to ICAO a runway is a "defined rectangular area on a land aerodrome prepared for the landing and take-off of aircraft." Runways may be a man-made surface or a natural surface .- Orientation and dimensions :Runways are named by a number between 01 and 36, which is generally one tenth...
An aircraft is a vehicle that is able to fly by gaining support from the air, or, in general, the atmosphere of a planet. An aircraft counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines.Although...
in flight are subject to the direction of the winds in which the aircraft is operating. For example, an aircraft in flight that is pointed directly north along its longitudinal axis will, generally, fly in that northerly direction. However, if there is a west wind in the air in which the aircraft is flying, then the actual trajectory of the aircraft will be slightly to the east of north. If the aircraft was landing on a northbound runway, it would need to compensate for this easterly component of velocity caused by the west crosswind.
In situations where a crosswind
A crosswind is any wind that has a perpendicular component to the line or direction of travel. In aviation, a crosswind is the component of wind that is blowing across the runway making landings and take-offs more difficult than if the wind were blowing straight down the runway...
is present, the aircraft will adopt a yaw orientation with respect to the runway and will drift laterally as it approaches the runway. These pose significant safety issues when safe operation of the undercarriage
The undercarriage or landing gear in aviation, is the structure that supports an aircraft on the ground and allows it to taxi, takeoff and land...
requires the body and the velocity of the aircraft to be aligned with the runway at touch down. The landing gear designs of the "pioneer era" 1909 Bleriot XI
The Blériot XI is the aircraft in which, on 25 July 1909, Louis Blériot made the first flight across the English Channel made in a heavier-than-air aircraft . This achievement is one of the most famous accomplishments of the early years of aviation, and not only won Blériot a lasting place in...
, and the much later Cold War B-52 strategic jet heavy bomber, were designed and each built with an unusual feature to counteract the problem: with the B-52, all four of its landing gear bogies could be steered, allowing the aircraft to land with the wheels facing the direction of travel even if the nose was not pointed in the same direction. The Bleriot XI had pivoting main gear legs, which passively allowed the main gear wheels to castor together about each of their vertical axes as a unit to allow small-angle crosswind landings, with bungee-cord loaded rigging members between the lower ends of the main wheel forks, to bring the wheels back to a "directly-ahead" orientation after touchdown.
To meet these conflicting requirements, three standard procedures are used for executing a safe landing in a cross wind situation. These landings are called the Crab, De-Crab and Sideslip techniques.
If the crosswind landing is not executed safely, the aircraft may experience wingstrike
Wingstrike is when an aircraft's wing hits the ground during takeoff or landing, most often as a complication of a crosswind landing.Unexpected gusts of wind may cause an aircraft to roll to one side or the other during landing, whether they are performing a crosswind landing or not.However,...
, where a wing hits the runway.
The following guidelines are advised by Boeing for a crosswind landing. These guidelines assume steady wind (no gusting). These winds are measured at 10 m (33 feet) tower height for a runway 45 m (148 feet) in width. Basically, there are 3 landing techniques which may be used to correct for cross winds: De-Crab, Crab, and Sideslip.
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 centerline (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
Thrust is a reaction force described quantitatively by Newton's second and third laws. When a system expels or accelerates mass in one direction the accelerated mass will cause a force of equal magnitude but opposite direction on that system....
. Wings are maintained level throughout the approach. Just before the flare, opposite rudder
A rudder is a device used to steer a ship, boat, submarine, hovercraft, aircraft or other conveyance that moves through a medium . On an aircraft the rudder is used primarily to counter adverse yaw and p-factor and is not the primary control used to turn the airplane...
(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 center.
The airplane can land using crab only (zero side slip) up to the landing crosswind guideline speeds.
On dry runways, upon touchdown the airplane tracks towards the upwind edge of the runway while de-crabbing to align with the runway. Immediate upwind aileron is needed to ensure the wings remain level while rudder is needed to track center line. The greater the amount of crab at touchdown, the larger the lateral deviation from the point of touchdown. For this reason, touchdown in a crab only condition is not recommended when landing on a dry runway.
On very slippery runways, landing the airplane using crab only reduces drift towards the downwind side of a touchdown, and may reduce pilot workload since the airplane does not have to be de-crabbed before touchdown. However, proper rudder and upwind aileron must be applied after touchdown to ensure directional control is maintained.
This sideslip crosswind technique is to maintain the aircraft's heading aligned with the runway centerline. The initial phase of the approach is flown using the Crab technique to correct for drift. The aircraft heading is adjusted using rudder and aileron
Ailerons are hinged flight control surfaces attached to the trailing edge of the wing of a fixed-wing aircraft. The ailerons are used to control the aircraft in roll, which results in a change in heading due to the tilting of the lift vector...
s 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 with the upwind main wheels touching down just before the downwind 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.