Your analysis of the US National Transportation Board papers on the American Airlines AA587 accident (Flight International, 9-15 March) fails to make clear increasingly muddy waters. For clarity it is best to compare the Airbus A300-600 rudder design with that of a Boeing equivalent.
Airbus uses a "fixed ratio" system, which means that full pedal travel equates to full rudder deflection available, at specific speeds. Further, the pedal breakout force (which is the amount of force that must be applied before the control moves) remains constant in the face of reducing pedal travel. Logical, assuming that the rudder will not be used at high speed.
Constant breakout force becoming more dominant with reduced travel and therefore a small incremental force for full stop travel is a system tailor made for overcontrol. Once the rudder has "broke out" you have uninhibited stop-to-stop travel and thereafter an engine pylon and fin breaking pilot induced oscillation on your feet. A part of the anatomy not noted for precision control, an achilles heel, tendon and tail in one.
Now consider the logic of the "ratio changer", applicable to the A300B2/B4 and all Boeing designs from the 747 Classic and subsequent aircraft. The rudder's travel is limited to ever-decreasing amplitudes at higher speeds at the rudder. This means that full pedal travel is needed to reach the rudder limit and therefore the breakout force does not dominate control force. Most importantly, if you have to stretch your leg that far, you will be less likely to perform a lethal "rudder reversal", as in AA587.
No winners? With the most seminal accident since the Comet era, it is challenging the assumption that we are all winners that mitigates this human tragedy.
David Connolly Brussels, Belgium
An American Airlines captain has been quoted as saying that the pilot who was at the controls of AA587 had a history of excessive rudder usage (rudder pulses) during wake turbulence encounters.
The fact that this unlucky pilot referred to the American Airlines Advanced Aircraft Manoeuvring Programme (AAMP) to justify his unexpected flight control inputs does not mean the AAMP was leading into the wrong direction. It just shows that however cautious you are while training people about rudder inputs there is always one getting it wrong.
The lesson for us can only be:
All pilots must be aware that no aircraft is built for quick flight control reversal inputs; Airbus cannot be held responsible for the abuse of flight controls. There is no training manual requiring any comparable input as it has been used on AA587. There is also no sense in discussing pilot-induced oscillation (PIO) as long as absolutely no such input was necessary; the rudder of the Airbus A300 is very effective, so use it with sense and you keep control;As the old instructor says: "Blend in rudder gently."
Georg Spieth Ober-Hilbersheim, Germany
Source: Flight International
