Pilot error becomes focus in probe. Preflight check system apparently failed to catch error
Boston — The pilot of Northwest Flight 255 may have made a fatal error by not extending the wing flaps of his plane. A checklist system that might have caught a potentially fatal mistake apparently didn't work, and investigators say crash data from the plane's black box show its flaps were not extended.
Tapes of the cabin conversation do not indicate the pilots checked the flaps. A warning horn apparently did not sound.
``[Investigators] could hear the verbal checking off of some things,'' National Transportation Safety Board spokesman Alan Pollock said Wednesday. ``So far they have not heard anything that indicates [the pilots] checked the flaps.''
The jet's captain, John Maus, was reportedly very experienced.
An FAA spokesman said Captain Maus had 22,000 hours of flying experience. Co-pilot and first officer David Dodds reportedly had 7,400 hours of flying experience.
New information continues to surface from Flight 255's black box, and investigators refuse to say that the flap position, or any other single element, was the cause of the crash Sunday.
Extending flaps increases the surface area of an aircraft's wings and increases lift. Flaps are extended in most takeoff situations. If flaps were not extended, it would be a very hazardous mistake although not necessarily a fatal one, experts say. While unusual, takeoffs with flaps retracted are possible when the pilot does not want added lift because of a strong head wind.
But such was apparently not the case for Flight 255, which is reported to have had a 10-mile-per-hour tail wind. The tail wind decreased the jet's air speed and consequently the amount of lift. Weighin in at 144,000 pounds at takeoff, just 3,000 pounds less than its 147,000 maximum, the twin-engine MD-82 needed all the lift it could get.
``These factors always act in combination,'' says Charles O. Miller, a former test-pilot, safety investigator, and now air-safety consultant. ``You don't usually find that any one of these things by themselves is going to cause an accident.''
While flaps may not have been extended, a more troubling question is why the pilots apparently failed to check the flaps. According to the National Transportation Safety Board, a check of flap position is a federal requirement.
Still, Northwest Airlines reportedly eliminated the flaps from one pre-flight checklist, according to the Wall Street Journal yesterday. Northwest officials did not return phone calls from the Monitor seeking to confirm that report.
The airline did apparently continue to require a flap check during the ``taxi-checklist'' prior to takeoff. But this apparently didn't happen, investigators say.
No one is sure why the pilots, who checked some items, didn't check the flaps. Certainly they were busy, having just been shifted to a new runway to avoid possibly dangerous downward gusts called ``wind shear.'' The pilots were also busy answering repeated radio transmissions from air traffic controllers.
In addition, wind speed and direction, air temperature, weight of the aircraft, flap configuration, the plane's center of gravity, the length of runway, and the airspeed required for takeoff are interrelated.
Pilots watching Flight 255 takeoff said it took off at a very steep angle. An air traffic controller estimated it reached only 150 feet in altitude. Black box data released yesterday indicated the plane only reached 48 feet above the ground.
Though he refuses to speculate, Miller describes a possible scenario: ``You're getting into the air at too low an airspeed with the nose cocked up too high. You don't have enough power to just fly out of it and overcome the drag. The only way to get more airspeed is to point the nose down. But if you're too close to the ground, you're in a trap.''