Investigators probing the crash that claimed 132 lives will turn to computer models to test any possible theories they develop from the wreckage of USAir Flight 427.
Federal Aviation Administration officials said Monday night that they would examine several possible scenarios, including the failure of an engine mount, at FAA headquarters in Washington, D.C.
Officials said they have no proof that the mount failed during the flight, but would examine the possibility because a broken engine mount was found at the crash site Monday.
Investigators also will use computers to examine their first theory: the possible partial deployment of the thrust reverser on the right engine.
But for such a problem to have arisen with the thrust reversers, both a primary safety system and its backup would have had to fail, experts said Monday.
The two systems are designed to prevent activation of an engine’s thrust reverser while the plane is in flight, thereby forestalling catastrophe.
Federal safety investigators have found evidence in the wreckage of the USAir Boeing 737 that indicates that half of the reverser mechanisms on the plane’s right engine may have been deployed before the jet plunged to the ground last Thursday as it approached Pittsburgh International Airport. All 132 passengers and crew members aboard died in the crash.
Thrust reversers act as giant brakes, redirecting engine airflow to help a plane that has just landed stop more quickly than if it had to rely only on brakes, wing slats and spoilers.
Even before the Flight 427 crash, the FAA was concerned enough about reversers to propose that a third safety system be installed on all Boeing 737 model planes and to require its installation on the Boeing 767 models. The two models of planes have different reverser systems.
Under the proposal, which the FAA suggested last October, airlines would be required to retrofit existing 737s with “synch locks.” The proposed regulation, which could become final in a few months, came after an Austrian 767 crashed over Thailand in 1991. The Air Lauda Boeing 767-300 broke into pieces and plunged into a jungle, killing 223 in a crash that was later blamed on faulty thrust reversers.
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After that accident, Boeing voluntarily began building new 737s with this third safety system, called a “synch lock,” to prevent premature deployment of the thrust reversers.
In the United States, most thrust reverser problems have been minor, and have been noted before or during takeoff, according to records from the FAA.
Under the primary thrust-reverser safety system on the 737, sensors activate if they detect as little as one-eighth of an inch of movement toward deployment, sending hydraulic fluid to the mechanism to force the thrust reversers back to the retracted position, experts said.
This system “overrides all other systems except a correct command from the cockpit,” said Steve Thieme, a spokesman for the Boeing Commercial Airplane Group. He defined such a command as a reverser activation by the pilot after landing.
The backup system features a locking device designed to hold the reverser assembly in the retracted position until it is released from the cockpit.
Boeing has manufactured 2,641 model 737 planes, among them 852 in the 737-300 series, including the USAir jet involved in Thursday’s crash.
“A 737 has never had a thrust reverser deploy in flight,” Thieme said.
Officials of the National Transportation Safety Board emphasized that it is far too early to conclude that a partial reverser activation caused last week’s crash. However, evidence suggesting such a deployment is seen as an important clue so far in the investigation.
Deployment of the reversers while the plane is in flight is prevented in one of two ways, experts said. In some 737 models, the wheels must be on the ground before the reversers will go into position, even if the pilot pulls the levers. On other 737s, two altimeters-one in front of the pilot, one in front of the co-pilot-must show no more than 10 feet between the plane and the ground to permit deployment.
Once on the ground, activation of thrust reversers from the cockpit requires a conscious effort: The pilot must reach over the throttle and pull two levers up and then back.
A plane that experienced a partial reverser activation in the right engine probably would swing violently to the right and then bank right, said Chuck Moren, associate professor of flight technology at Embry-Riddle Aeronautical University in Daytona Beach, Fla.
The pilot instinctively would hit the left rudder and aileron (French for “little wings”) controls to counteract the sharp movement, said Moren, who is certified to fly 737-300s.
An overcompensation conceivably could send the plane in that direction, Moren said, though he regards that as unlikely.
Why the USAir plane went left, a path described by eyewitnesses and confirmed by the flight data recorder, is not yet known.
Once Flight 427 headed nose first toward the ground, Moren said, the cockpit crew may not have been able even to attempt to regain control.
Meanwhile, the FAA said Monday that it has no plans now to require special inspections of 737 thrust reversers.