When an airplane approaches the runway at an angle, flying slightly sideways before straightening out for landing, this is a calculated maneuver. It is a deliberate technique pilots use to safely land an aircraft when winds blow across the runway, demonstrating pilot skill and ensuring a stable touchdown.
The Crosswind Challenge
A crosswind is a wind blowing perpendicular to an aircraft’s intended flight path, particularly a runway. During landing, this means the wind is not coming directly from the front (headwind) or the back (tailwind), but from the side. This sideways force can push an aircraft off the runway centerline, making it difficult to maintain alignment for a safe touchdown.
An aircraft in flight moves within a mass of air. If that air mass moves sideways, the aircraft drifts with it. To counteract this drift and keep the aircraft on its intended path, pilots apply specific control inputs. Without these adjustments, the aircraft would be carried away from the runway.
Pilot Techniques for Crosswind Landings
Pilots primarily use two techniques, often in combination, to manage crosswinds during landing: the “crab” method and the “wing-low” or “sideslip” method. Both address the lateral force of the crosswind to maintain the aircraft’s ground track aligned with the runway. The choice depends on factors like aircraft type and pilot preference.
In the “crab” method, the pilot points the aircraft’s nose into the wind, much like a crab walks sideways, to counteract the lateral drift. This means the aircraft’s fuselage is angled relative to the runway, but its actual path over the ground remains straight towards the centerline. Just before touchdown, the pilot “de-crabs” by using the rudder to align the aircraft’s nose with the runway, while simultaneously using ailerons to maintain wings level and counteract any remaining drift.
The “wing-low” or “sideslip” method involves lowering the wing into the wind and applying opposite rudder to keep the aircraft’s nose aligned with the runway centerline throughout the approach. This creates a controlled “slip” through the air, where the aircraft is flying slightly sideways relative to the air mass, but its path over the ground remains aligned with the runway. The upwind main landing gear touches down first, followed by the other main gear, and then the nose wheel. This method is often preferred for lighter aircraft and can be maintained through touchdown.
Ensuring Safety and Control
These specialized landing techniques ensure aircraft safety and control during crosswind conditions. Without them, the lateral force of the wind could push the aircraft off the runway, leading to a runway excursion. This could damage the landing gear and tires, or result in loss of directional control.
The aircraft’s sideways appearance during a crosswind landing demonstrates pilot skill in managing aerodynamic forces. By coordinating rudder and aileron inputs, pilots maintain stability and align the aircraft with the runway at touchdown. This prevents structural stress on the landing gear from sideways motion.
Factors Limiting Crosswind Operations
Aircraft have maximum permissible crosswind components, limits beyond which a landing cannot be performed. These limits are determined during certification and documented in the flight manual. For commercial jetliners, limits typically range from 25 to 40 knots (approximately 29 to 46 mph), varying by aircraft type. For example, a Boeing 737-800 often has a maximum crosswind component of around 33 knots for a dry runway.
Several factors influence crosswind limits and a pilot’s decision to land. Runway conditions, such as dry, wet, or icy surfaces, can reduce the allowable crosswind component. Strong gusts or wind shear, sudden changes in wind speed and direction, also make crosswind landings more challenging. Pilot experience and proficiency are factors, as individual pilots may have personal crosswind limits more conservative than the aircraft’s demonstrated capability. If conditions exceed these limitations, pilots may choose to divert to an alternate airport.