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'Sensible Risk' David Evans- Avionics Magazine
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Safety In Avionics: Flying in the Face of a �Sensible Risk�
David Evans

It is significant when pilots express their lack of confidence in the safety of an airplane. Yet some pilots have done so. In the wake of the fatal Nov. 12, 2001, crash of an American Airlines A300 into a residential area of Belle Harbor, N.Y., less than two minutes after takeoff from John F. Kennedy International Airport, a number of American�s A300 pilots have suggested in a letter that the remaining fleet of 34 aircraft be grounded until the cause of the crash is determined. The composite tailfin snapped off, and the airplane has a history of uncommanded rudder movements.

All 260 aboard Flight 587 were killed, plus another five persons on the ground. It was the first fatal crash of an Airbus aircraft in North America, and the first involving a composite tailfin. Some sort of dynamically divergent oscillation of the rudder appears to have overloaded the tailfin�s strength-bearing capability.

The crash was the second worst in the history of U.S. commercial aviation�the worst being an American Airlines DC-10 that crashed May 25, 1979, also on takeoff, from Chicago�s O�Hare International Airport. The DC-10 accident, too, involved a structural failure: the left engine and pylon separated, causing the leading edge slats on the wing to retract when the hydraulic fluid bled out. All 273 aboard were killed.

The A300 pilots note that the DC-10 fleet was immediately grounded "until we were able to determine that engine pylon bolts were to blame." In contrast, the cause of the Flight 587 crash seems to be a deepening mystery. "Why are American Airlines� A300s still flying when they are unsure whether the same structural failure that occurred on Nov. 12 will occur again?" a letter to "Fellow Airbus Pilots" asks.

"What we are asking and advocating is not any type of job action," the letter adds. "We would like each of you to evaluate your feelings on the issue of whether or not the fleet should be grounded until a definitive cause for this accident can be determined, along with ways to prevent a similar occurrence�"

Reaction to the letter could be likened to a match falling on dry tinder. Even though it was signed, "Fraternally," the Allied Pilots Association (APA), the union of American Airlines pilots, distanced itself, asserting that the letter is "not a union-led initiative." The union stated publicly that it did not support grounding. American Airlines rejected the call for grounding, saying, "Nothing in the examination of the Airbus fleet�suggests there is a need to ground this fleet."

Manufacturer Airbus declared, "If Airbus had any indication whatsoever of a safety issue, we would notify the authorities and airlines immediately. The letter from this small group of pilots is based on opinion."

The National Transportation Safety Board (NTSB), which is investigating the Flight 587 crash, believes the evidence thus far reveals a safety concern regarding rudder inputs. "We have calculated that certain rudder movement inputs by the pilots could cause a catastrophic failure of an airliner�s vertical tailfin," NTSB Chair Marion Blakey said at a Feb. 8 press conference, in which she announced an interim safety recommendation on the matter. "This concern is not limited to the A300 or even to Airbus models. Our concern is industry-wide," she added.

More specifically, dangerous combinations of sideslip angle and rudder position can cause aerodynamic forces sufficient to snap off the tail. In the more direct terms of a Safety Board official, "A nearly full rudder deflection in one direction and then a full deflection in another direction can cause loss of the tailfin."

Investigators know from the flight data recorder (FDR) that a series of left/right rudder movements occurred in the moments before rudder and stabilizer departed the airplane. However, the FDR did not record pilot inputs on the rudder pedals. Blakey says, "We do not know [if those rudder movements] were caused by the pilots."

Reminded that some A300 pilots suggest grounding the fleet, Blakey says, "We would not recommend grounding the fleet, based on what we do not know." This argument, it seems, could be turned around to argue for grounding, as unknown circumstances could combine to kill again.

In the interim, the Safety Board recommends that pilots be educated about the potentially lethal consequences of full slam-bang rudder deflections to opposite limits. If excessive rudder deflections are the problem, expanding the authority of the rudder limiter might be a more straightforward and standardized means of addressing the problem, rather than enjoining pilots to make more prudent rudder inputs. An NTSB official reports that the rudder movements captured on the FDR "were at a speed that can occur in either case," by the machine or by the man. "We can change the pilot inputs quicker," he says. It would take years to isolate any problem in the rudder control system, design a fix and retrofit the fleet.

The entire article:

http://www.aviationtoday.com/reports/avionics/0402safety.htm
 
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