EGYPTAIR CRASH MYSTERY DEEPENS; BLACK BOX SUGGESTS `CONTROLLED DESCENT'.

Byline: Matthew L. Wald The New York Times

The first unusual event in the flight of EgyptAir 990 on Oct. 31 was the plane's autopilot disconnecting and then, about eight seconds later, what appeared to be ``a controlled descent'' that was smooth and straight, investigators at the National Transportation Safety Board said on Wednesday evening.

Aviation experts said such a descent would be very unlikely unless the pilots were still flying the plane.

James E. Hall, the chairman of the safety board, gave only a brief description of preliminary information from the flight data recorder, the one black box that the Navy has retrieved so far from the crash site, about 60 miles from Nantucket. However, the information did not solve the mystery of why the plane crashed.

The safety board badly wants to recover the cockpit voice recorder, which could have information such as the pilots talking to each other about the plane's problem.

But the new information appeared to rule out two possibilities: a malfunctioning autopilot throwing the plane into a dive, or thrust reversers deploying.

A thrust reverser, used to help a plane brake after landing, deployed in flight in 1991, causing the only crash of a Boeing 767 for mechanical reasons.

Aviation experts said that the sketchy information released so far painted a picture compatible with an emergency descent, although there is no way yet to judge why the crew would initiate one.

Reasons could range from a variety of mechanical problems to an explosion on board that could breach the plane's skin and cause a loss of air pressure.

But the flight data recorder and radar data combined show that even if the plane had some severe mechanical problem, it was still under control at the beginning of its dive, experts said.

Hall said that the data show an uneventful flight, with the plane cruising levelly at 33,000 feet. ``The first event we note is the autopilot disconnecting,'' Hallsaid in a briefing in Washington, flanked by officials of the Egyptian government and the airline.

Investigators are not sure yet whether someone in the cockpit flipped off the autopilot, which can be used to maintain altitude, heading and speed, or whether the autopilot disconnected itself in response to some problem on the plane that left it unable to follow instructions. Manual disconnection of the autopilot, however, is an early step in an emergency descent.

About eight seconds after the autopilot disconnection, he said, ``the airplane begins what appears to be a controlled descent.'' In cruise flight at authorized altitude, the plane would ordinarily not change altitude without asking permission from air traffic controllers, something that the crew did not do.

The flight data recorder quit capturing data late in the dive but not at the end, when investigators say the plane pitched back up, climbed more than a mile and a half, and then plunged again. The last six or seven seconds of data has not yet been read from the tape, according to people involved in the investigation, because the tape itself, which resembles what is used in a standard tape recorder, is contaminated by metals it picked up from the spindles and the recording heads while it was underwater. However, the investigators are confident that they can eventually decipher the data.

The briefing by Hall seemed only to emphasize the need to retrieve the other black box, the cockpit voice recorder, which experts said could capture the sound of an explosion or mechanical failure, or a discussion among people in the cockpit about the reason for the descent. While it began as a controlled descent, radar data indicate it later became faster than what the airplane is designed for. Investigators said on Wednesday, however, that they think it did not become supersonic.

At the crash scene, the Navy has pulled back one ship and plans to withdraw three more, including the Grapple, which is the support ship for one of the robotic submarines, because seas are expected to be too rough today for them to work. Instead, search officials plan to resupply them today and send them back out in time to work on Friday, when smoother seas are predicted. A commercial vessel, the Carolyn Chouest, remains on site with another robot submarine, and will work ``as long as weather conditions permit,'' Hall said.

CAPTION(S):

photo

Photo: The flight data recorder from EgyptAir Flight 990 is displayed in Washington on Wednesday.

Doug Mills/Associated Press