The National Transportation Safety Board said yesterday the McDonnell Douglas MD-11's design can lead to violent in-flight upsets in the hands of pilots not trained in its peculiar control characteristics.
And, in a report that began as an investigation of the danger to smaller aircraft by turbulence trailing the Boeing 757, the board urgently recommended that the Federal Aviation Administration immediately require smaller airliners and light planes to hold farther back in landing behind large jetliners.
If the FAA follows the safety board's recommendation on aircraft separation, it effectively could limit the capacity of the nation's strained air traffic control system.
The tri-jet MD-11, one of the world's newest jetliners, is McDonnell Douglas's bid to retain the widebody market it had captured with the older DC-10. So far, 112 have been built.
The safety board has investigated several sudden high-altitude incidents in which MD-11s pitched violently, including an incident last April involving a China Eastern MD-11 that killed two passengers and forced the plane to make an emergency landing at Shemya, Alaska.
The incident that led to yesterday's report occurred on Dec. 7, 1992, when a China Airlines MD-11 pitched violently and stalled. The crew regained control, leaving 246 passengers and 19 crew uninjured, but the aircraft was damaged.
The MD-11's problem stems from its fuel-efficient design, which involves placing some fuel tanks in the tail section. This shifts the center of gravity toward the rear, leading to less drag on tail and wing surfaces. However, it also leads to what McDonnell Douglas calls "relaxed stability."
The safety board said that a pilot not specifically trained to handle the plane under adverse circumstances such as moderate-to-severe turbulence may over-control the plane, leading to significant pitch, airspeed deviations and stalls.
The board recommended that the FAA establish new requirements for MD-11 flight control handling characteristics and require flight demonstrations to ensure that pilots can safely recover from high-altitude upsets.
McDonnell Douglas declined to comment.
Potentially more important economically to the airline industry are the recommendations growing from the investigation of 757 wake turbulence, which has led to three crashes of small planes landing behind 757s and to abrupt rolls by two smaller jetliners since December 1992. Thirteen people were killed in two of the crashes.
The board recommended the FAA increase the required separation behind landing 757s and all other heavier aircraft and that pilots be made more aware of the dangers. FAA spokesman Bob Buckhorn said the agency wrote to 700,000 pilots emphasizing the danger and is reviewing whether to change aircraft separation requirements.
Chris Chiames, a spokesman for the Air Transport Association, which represents major airlines, said, "If the FAA follows NTSB's guidelines, it's going to cause some delays at congested airports."