ROMULUS, MICH.,NOV. 17 -- A doomed Northwest Airlines airplane had an alarm system to warn the crew of improper flap settings on takeoff, but it may not have worked if power to the system was interrupted, a witness in a federal investigation of the nation's second-deadliest air disaster testified today.

Testimony at the National Transportation Safety Board hearing into the Aug. 16 crash of Northwest Flight 255 has focused on the setting of the plane's wing flaps during its 22-second flight. The crash outside Detroit Metropolitan Airport killed 156 people.

Jack McDonnell, director of flight guidance and control at the airplane's maker, McDonnell Douglas Corp., said the takeoff warning system aboard the MD80 plane was designed to warn the crew of improper settings for slats and flaps -- panels on the leading and trailing edges of airplane wings that assist in providing lift.

NTSB reports have said the plane's flaps were retracted during takeoff. Witnesses have given conflicting testimony during the hearing on whether the flaps were extended, as they should have been, or retracted.

McDonnell said the takeoff warning system, linked to the throttle control, would activate if slats and flaps were not in proper position as the plane accelerated.

However, he said, "If there is no power on the flap warning system . . . and there was power to the throttle, there would be no warning." He said such a power interruption could be caused by a circuit breaker or a broken wire.

In preliminary reports, the NTSB said electric circuits to the system appeared to be functioning properly. But the agency also suggested that the system's circuit breaker may have been disconnected.