The Boeing aircraft company has known for close to a decade that Boeing 737s can pitch up or roll without warning during icy flying and has worked closely with airlines to counter the problem, a company official yesterday told a federal panel investigating the crash of Air Florida Flight 90.
The Air Florida 737 crashed while taking off in a snowstorm on Jan. 13, killing 78 people. Air Florida officials have said they believe the plane's control characteristics played a role in the accident and suggest that Boeing has not properly publicized those problems.
Last week, the National Transportation Safety Board, which is leading the investigation, released a document showing that 23 control incidents have been reported to Boeing or appeared in the media since 1971. One occurred the day of the Washington crash, when a 737 operated by the West German carrier Lufthansa rolled and pitched before being brought back under control.
Robert R. Larson, technology chief for Boeing, testified yesterday that the company has issued three bulletins on 737 pitch-up tendencies to airlines since 1974, conducted extensive testing on the problem and published the results in its technical magazine. Though the pitch-up and roll are considered a hazard, it is believed to be controllable, Larson said.
Air Florida received the most recent of the bulletins last summer. Among the conditions listed as accompanying the incidents were temperatures near freezing, falling snow, and use of thrust reversers during taxiing. That bulletin gave specific instructions on ways to avoid the problem and recover control if the plane suddenly pitched.
Last week, witnesses testified that Flight 90 used thrust reversers when it had trouble pulling away from the gate. Some witnesses have said that the plane had ice or snow on it as it began its take-off, leading to suspicion that the plane may have encountered the same rolling and pitch-up after take-off that other 737 pilots have reported.
Taking off with ice on the wings violates basic principles of aviation safety whatever the type of plane, pilots say. But in the case of 737s, ice on leading edge "slats," flap-like devices that are extended during take-off and landing to give extra lift, has been found to be particularly dangerous.
"We have done everything possible to tell people to keep the slats clean," Larson testified yesterday. Boeing's tests included simulating ice on the leading edges of a 737's wings, then flying the plane to determine its control characteristics, he said.
While acknowledging special pitch-up problems in the 737, Larson said that all swept-wing jets tend to pitch up and then down as they go into a stall, an aerodynamic state in which air ceases to move over a plane's wings fast enough to keep it in the air.
Ice on top of a 737's wing is very dangerous, Larson said, but Boeing has found that up to 1/8 inch can accumulate on its wings' undersurfaces without affecting performance. Two weeks after the crash, Larson said, the company inserted new language into its 737 manual to clarify that leading edge slats are not to be considered part of the wing's lower side and must be clear of ice.
Resolution of the pitch-up issue will be crucial in assigning blame for the accident and determining who will be liable to pay millions of dollars in damages that survivors and relatives of the 78 victims have already begun seeking in the courts.
Larson also said Boeing would reevaluate techniques for de-icing planes on the ground. Another witness at yesterday's hearings, Federal Aviation Administration technician Richard Adams, criticized the way Flight 90 was de-iced. The antifreeze solution used was too weak, he said, different concentrations were applied to the plane's two sides, and there were poor procedures concerning who should inspect the plane afterwards.
On Jan. 28, the board issued preliminary recommendations calling on the FAA to direct airlines to underline to their employes the dangers of icing. Three weeks later, the FAA responded that it had done so the day the recommendations were issued. It also agreed to stress icing in controller training and operations and conduct studies on icing.
But the FAA disputed another board recommendation, that visual inspection of wings be required if more than 20 minutes pass between de-icing and takeoff. The FAA said 20 minutes is not a valid figure, as weather conditions affect anti-icing fluid's effectiveness and said pilots should make sure their planes are clear regardless of how much time has passed.
The board also sought procedures to increase separation on the ground when airplanes taxi in snowy conditions. The FAA ruled that current procedures are adequate.
In another development yesterday, the Air Line Pilots Association submitted a study to the board concluding that the curved approach from the north to National Airport's main runway "is the most difficult approach procedure to a primary runway of any air carrier airport in the continental U.S."