The AA587 inquiry has not finished, but some enlightening analyses have been published. The information should be not be used for blame
The US National Transportation Safety Board (NTSB) publication of learned papers about pilot-induced oscillation (PIO) as it relates to the November 2001 American Airlines 587 disaster could shed a great deal of light on the event for those who really want to understand what happened.
But if anybody was expecting the revelations at this stage of the ongoing investigation to clear matters up, they will be disappointed. The tentative conclusions suggested in the US-based studies clash totally with the extensively argued answering submission by aircraft manufacturer Airbus. The problem is that both parties are correct in what they say, yet they come to different conclusions.
The NTSB has already stated the simple facts of the case. AA587, an Airbus A300-600R took off from New York Kennedy airport with the first officer flying it. During its climb away it travelled through the wingtip vortices of a preceding Boeing 747. As it passed through the first vortex the pilot used quite considerable aileron input to steady the aircraft, but very little rudder. The captain commented on the vortex, and 15s later the aircraft hit the second one. The NTSB says of the co-pilot's reaction: "Both the wheel and the [rudder] pedal were moved repeatedly to their maximum positions." The PIOs caused by this control movement resulted in high yaw and loads on the fin well beyond its design loading, the fin separated and the aircraft went out of control.
Tests on an A300-600 by the NTSB have established that the aircraft definitely has a rudder control system much more sensitive than that of other large aircraft, both in terms of the amount of foot pressure needed on the pedals to gain a given amount of rudder deflection (less pressure in the case of the A300-600), and in the distance of pedal travel needed to take the rudder to full deflection (less travel in the -600). Using ratios derived from these findings for comparison with other types, the NTSB found that the A300-600 had a rudder 10 times as sensitive as that of its immediate forbears the A300B2 and B4, and twice as sensitive as that on any other transport aircraft. A study of PIO incidents in many different types by Dr Ronald Hess, professor and vice-chairman of the department of Mechanical and Aeronautical Engineering at the University of California, concluded that dangerous PIO is unlikely to occur in aircraft whose control design reflects the fact that PIO is the natural result of certain types of pilot- control input, combined with slight delays in control and aircraft reaction. Pilots, if they perceive their control input is not having the desired effect, increase the input. Then, when they get a dramatic result, they overreact the other way, setting up a cycle of pilot overreactions often so fast that control servo lag can neuter the control reaction the pilot wants.
Airbus claims that, if the AA587 pilot had made no control inputs at all on the second vortex encounter, the aircraft's bank would have increased from 20° to 30° in the left turn it was carrying out at the time. Airbus claims the pilot's heavy use of rudder to counter such a disturbance followed what he had been taught under American's advanced aircraft manoeuvring programme (AAMP). The manufacturer insists that teaching such practices in a simulator is dangerous, partly because they make high rudder input feel relatively innocuous.
American claims the Hess "criticism" of the -600's rudder sensitivity is a vindication of its training programme, and it slams Airbus for failing to warn -600 operators of the type's rudder characteristics. Airbus counters by saying American was warned of rudder input risks in a joint letter from the three major transport aircraft manufacturers, and was warned after previous serious crew-induced incidents in its A300-600 fleet. And so on it goes - accusation and counter accusation.
Naturally, many people interested in the accident will be severely tempted to cherry-pick the various submissions now available. It would be better for those who really want "the truth" to read all the papers with an open mind. This is essential for those - particularly the NTSB - entrusted with ensuring that the aviation industry learns from this complex event. This accident, as is true of most aviation disasters, had many contributory causes.
In these excellent studies, together with Airbus's response, the lessons are there to be learned. The industry, represented in this case by the investigators, the manufacturer, the airline - and airline pilots everywhere - must act on all the factors revealed, not each choose those that could be argued to exonerate them. No one is exonerated here.
Source: Flight International
