OPERATIONS IN STRONG, GUSTY WINDS
Tail clearance is reduced during takeoffs performed in strong gusty winds and crosswinds because of the lift loss incurred by flight control inputs, primarily spoilers. With very large inputs, this loss can be significant (see figs. 3 and 4).
Approximately two years ago, Boeing revised wording in all production model flight crew training manuals (FCTM) to incorporate input from industry and safety professionals regarding tail strikes during strong and gusty winds. The Boeing FCTM recommends that crews use thrust settings higher than the minimum required. The use of a higher takeoff thrust setting reduces the required runway length and minimizes the airplane exposure to gusty conditions during takeoff roll, rotation, liftoff, and initial climb.
Pilots can take a number of steps to reduce the possibility of tail strikes during takeoff in gusty winds or strong crosswinds, including:
- Momentarily delaying rotation during the gust. As airspeed fluctuates back and forth (what is sometimes referred to as “bounce”), ensure that the airplane starts rotation at a speed that averages above rotate speed.
- Using a normal rate of rotation, but not a greater rate of rotation than normal. This faster rate may be a tendency if the airplane is slow to liftoff due to airspeed stagnation.
- Limiting wheel input to that necessary to maintain wings level. Pre-setting too much aileron increases drag and reduces lift with higher probability of cross control and reduced tail clearance margins. When safely airborne, smoothly transition from the slip by slowly releasing the rudder while maintaining desired track.
- Avoiding the tendency to quickly rotate the airplane off the ground during rotation in these wind conditions. Gusts up to 20 knots have been noted in the review of tail strike incidents.
- Rotating on the conservative side of gusts. Use normal rate of rotation a bit on the side of a slower versus faster rotation, similar to the engine-out case noted earlier.
If, after reaching the normal takeoff attitude, the airplane is not airborne, avoid the tendency to increase rotation rate. Either slow or momentarily stop rotation rate. Many tail strikes on takeoff occur when or just after the main gear is airborne.
TAIL STRIKE COMPARISON
This is a comparison of a normal takeoff and a tail strike takeoff in gusty wind conditions. Note that takeoff #2 suffers a 9- to 10-knot airspeed loss during the rotation. The pitch attitude increases at an increasing rate until the tail strike. This is primarily due to the continued elevator increased deflection during rotation.
AFT BODY CLEARANCE BREAKDOWN
Guidelines that relate to Boeing airplanes show that airspeed loss, lateral control deflection, a greater than average pitch rate, and a maximum pitch rate in excess of 4 degrees per second all contribute to reduced tail clearance margins. The numbers change, but the concepts hold true for other models.
|Factor||Incremental Difference from Nominal||Reduction in
Aft Body Clearance
|Airspeed loss||Each 1 knot below the nominal liftoff speed||=2.8 inches †|
|-ΔCL from lateral controls||Each 0.1 of (-ΔCL) from lateral controls||=14 inches|
|Either/Or||Average pitch rate to 10 degrees pitch attitude||Each 0.1 deg/sec in the average pitch rate above 2.5 deg/sec||=2.8 inches †|
|Maximum pitch rate||Each 0.1 deg/sec above 4.0 deg/sec||=1.3 inches|