Aviation safety specialists scrambled yesterday to discover why a computerized voice did not announce "Flaps" to warn the pilot and copilot of Northwest Flight 255 that they had failed to set vital flight controls as they began their takeoff.

Flight 255 crashed in Detroit last Sunday, killing at least 156 people.

"It's mystifying . . . and very disturbing," a Federal Aviation Administration official said.

Senior FAA experts conferred here with representatives of the McDonnell Douglas Corp., manufacturer of the crashed two-engine MD80, in an attempt to understand the problem. Meanwhile, the National Transportation Safety Board's investigators continued to probe the wreckage for clues.

If preliminary findings hold up, the accident happened because of the convergence of two improbable events: the failure of the pilot and copilot to set the plane's flaps and the failure of the plane's warning systems to tell them of their omission, a failure that could also be caused by a human error as simple as not setting a circuit breaker.

"Should this be the finding of the safety board, that would be an extraordinary tragedy," FAA Administrator T. Allan McArtor said in an interview. "It's a tragedy for our whole aviation community. You work so hard to build into our system safety margins, procedures, mechanisms to deal with the human frailty. Checklists. Warning horns. When these safety mechanisms break down, it just stuns you."

Sam Clauzel, of the Douglas Aircraft Co.'s flight department in Long Beach, Calif., said the warning system in question is activated when throttles are advanced for takeoff. If flight controls -- flaps (trailing edge wing extenders), slats (leading edge wing extenders), horizontal stabilizer (the tail), ground spoilers (which help hold a plane on the ground after landing) and the parking brake -- are not in proper positions, the computerized voice is supposed to announce the problem loudly.

The warning is to come in plenty of time for a prudent pilot to abort the takeoff.

The cockpit voice recorder, recovered from the wreckage, shows that the flight crew did not call out a flap setting when going through the preflight checklist; it tells that the computerized voice never sounded "Flaps."

A similar voice, however, did warn of doom after the plane left the ground. "Stall," it said. When a plane is stalling, it is no longer flying.

Officials said yesterday they did not know why the warning did not work and that there many possible answers. The system is connected to a circuit breaker that could have been turned off; malfunctioning sensors, faulty wiring or a balky computer could also explain the mystery.

The flight data recorder recovered from the wreckage shows that the plane stalled because it was not configured to take off. Neither the flaps nor the slats were extended, according to the preliminary readout of the recorder.

One of the 160 eyewitnesses interviewed by the NTSB contradicted the data recorder evidence.

A Northwest copilot, awaiting takeoff in another jet, told federal investigators that the wing flaps on Flight 255 were extended at takeoff. The copilot, sitting in a cockpit about 300 feet from where the doomed jet left the ground, was in a "good" viewing position and was "very positive" about what he saw, the safety board's John K. Lauber said.

Another pilot awaiting takeoff directly behind Flight 255 told investigators he did not "notice anything unusual about the configuration of the aircraft," Lauber said. A third pilot in a small, chartered plane did not notice the flaps at all.

As a plane moves forward, air passes over and under its wings. The air on top takes longer to reach the back of the wings than the air on the bottom. The result of that difference is what aeronautical engineers call lift. The bigger the difference, the better the lift.

When extended, flaps and slats make the wing of a modern jet look from the side like a crescent and the wings have much greater lift than with flaps and slats retracted. Even a fully loaded jet can take off safely on a relatively short runway with flaps and slats extended.

Once an MD80 is airborne and gaining speed, slats and flaps are retracted to make the plane move more efficiently through the air. Flaps and slats are extended for almost all MD80 takeoffs, according to Douglas officials. In rare instances, the MD80 takes off with zero flaps but minimum slats, they said.

Flight control settings, including those for flaps and slats, are computed before takeoff. Considerations include the plane's weight and balance, weather, wind and altitude. Based on that information, the flight crew determines the appropriate speed at which to lift the plane's nose wheel off the ground.

In this case, according to the investigators' working theory, the computed correct airspeed for takeoff was reached, but the "clean" wings did not provide the expected lift because no flaps or slats were extended. Under those circumstances, the crash was inevitable.

Investigators yesterday were searching for physical evidence in the wreckage that would confirm the zero-flaps and slats reports from the flight data recorder.

It was a particularly difficult chore because the damage from impact was so great, according to an airline industry source who is closely monitoring the investigation.

Aviation experts discounted the importance of a report yesterday that a flap indicator switch on the crashed plane had failed in January and had been changed.

Douglas Aircraft's Clauzel said that "there is plenty of redundancy in there" and that the failure of one switch would not cause the warning system to fail.

The Air Line Pilots Association, while not contesting the early findings of the safety board, suggested it is possible that the crew had properly selected slat and flap settings, but that for a mechanical reason the flaps and slats failed to deploy although the cockpit warning device "was fooled into thinking that they were deployed."

The pilots' association also suggested that both the crew and the warning system could have failed, but complained that "incomplete information . . . has resulted {in} the misleading impression that the accident may have been caused solely by human error."

There are almost 500 MD80s in service worldwide. It is a highly computerized, fuel-efficient, quiet aircraft that has been praised by pilots and airlines alike. Staff writer Bill Peterson in Michigan contributed to this report.