The European Aviation Safety Agency should take note of the US National Transportation Safety Board's recommendation that it should review rudder control system design criteria to reduce the risk of vertical stabiliser overload when the rudder is used at high true airspeeds.

The first hint that this might be wise was the fatal American Airlines Airbus A300-600 accident in 2001. The pilot flying reacted to a moderate wake turbulence upset with a succession of opposite rudder inputs that induced increasing sideslip angles as the aircraft fishtailed. This eventually exceeded the fin's structural limits by a large amount and broke it off. At the time, although the NTSB hinted that the A300-600's rudder pedals produced too much rudder deflection for too little pilot input, the consensus was that the pilots of large airliners should be trained not to overreact in the way the American first officer did that day, which was thought to be the result of what he was taught in the carrier's upset recovery training programme. The report concluded that if the pilot had taken his hands and feet off the controls at the wake vortex encounter, the aircraft would have been safe.

Now, however, there is more evidence that, despite pilot awareness of the risk of fin overload, some pilots are still liable to use a sequence of opposing rudder inputs following a wake vortex encounter. Study of such an event in January 2008 involving an Air Canada A319 showed that the pilot used three or four opposite inputs in sequence, and in doing so overloaded the fin by 29% beyond the design maximum, but it remained intact.

The fact that some pilots still react like this is sufficient reason for EASA to review the case for rudder system redesign. EASA should also give serious consideration to the NTSB's opinion that varying the rudder pedal ratio and increasing the amount of pressure required with increased speed is more effective than progressively reducing rudder pedal travel. Rate of application matters, not just the amount of rudder deflection. And the more intuitive feedback that pilots get from heavier pedal pressure is important.

Meanwhile, since this will take a while to achieve, pilots should be reminded periodically of their duty to be gentle with the controls. In a conventionally controlled aeroplane no pilot would pull back roughly on the control column at cruising speed, so it should not be too much to ask that he/she apply the same restraint to rudder use.


Source: Flight International