Many of the aircraft accidents occur during the takeoff and landing phase of flight. One of the reasons of this accident is also the crosswind. Statistical data show that adverse wind conditions are involved in 33% of approach and landing accidents. Among them, 85% of incidents and accidents at landing is due to crosswind.
Everything that can be measured takes a reference point to calculate its value. Same is with the relative velocity, which is measured for one object with reference to another. Airspeed and groundspeed both can be defined as the speed of the aircraft, one relative to air and another with respect to ground. The speed of the wind flow towards the aircraft is simply called as wind speed.
Now this wind speed is cracked down into perpendicular components of crosswind and headwind or crosswind and tailwind. We can define this component of wind flow according to its direction of flow with respect to the direction of travel. If the direction of wind flow is same as the direction of travel (airspeed) then it is the tailwind.
It is known to be the best condition as it saves the time of travel by increasing the resultant airspeed. If the direction of wind flow is opposite to the direction of travel then it is the headwind and is known to be the best condition for landing and takeoff procedures.
Crosswind flows perpendicular to the direction of travel either from right to left or from left to right. It will make the landing and take-off of the plane challenging if the crosswind is sufficiently high. It is measured as the component of the wind speed and is abbreviated as X/WIND and are reported in Knots (Kt). It is used in expressions as “65Kt crosswinds” often in plural form.
The main problem encountered during the crosswind flow is that it disbalances the lift of the both wing and also creates a trajectory slip. So, how does it disbalance the lift of the wings? When crosswind hits an airplane, it affects the windward side more as fuselage shadows the leeward side.
Due to the flow of wind, more on one wing than other it sure creates some unbalance in the lift of the wings which causes the airplane to roll. So, a pilot uses the aileron to mitigate the problem. Now, how actually trajectory slip occurs. As in the windward side large area is provided by the fuselage to create the pressure which pushes the airplane away from the desired track. So, this trajectory slip is the observable effect caused by the crosswind.
So, the techniques pilot uses for the cruise, landing, and take-off during the crosswind are;
1. Landing techniques.
i. Crab method (zero sideslip)
ii. Sideslip / Wingdown method
iii. Forward slip
- Crab method:
In this method, the nose points towards the wind and aircraft approach the runway with the skewed position.
Some also call it as a crosswind drift. This position is maintained through the controls and just before the
landing flare rudder is applied to eliminate the crab and aileron to maintain wing level. It aligns the aircraft
at the center of the runway at touchdown. It is a safe and preferred method among others.
- Sideslip method:
Initially, the crab position is maintained then the airplane is banked into the wind and the centerline of the runway is maintained by decreasing. Landing procedure is same as above. Due to the slight residual bank angle, the touchdown is accomplished with upwind main wheels just before the downwind wheels.
The reasons this method is not commonly used is that it may cause possible spin off the airplane. As the sideslip increases stall speed of the airplane also increases, which may cause a possible spin. To recover it from 400 ft AGL on final approach is very difficult. The other reason is that if the turbulence is encountered during maintaining slip and controls fail to counter tackle then it may cause a possible spin.
- Forward slip :
In a strong crosswind, both crab and sideslip technique are combined to perform safe landing
2. Take-off technique
Same as the landing, but in reverse order. Align the aircraft in the centerline with rudder as needed to maintain in the centerline of the runway and aileron to bank the wing into the wind. As the plane takes off, reposition it into crab altitude and climb out.
3. Cruise condition
Just crab technique is used in the direction of wind along the desired travel path.