The autopilot system aboard EgyptAir Flight 990 became disengaged and the crew put the plane into a relatively gentle “controlled descent,” investigators said Wednesday, citing the first evidence from the plane’s flight data recorder.
The findings contradicted initial reports based on radar that the Boeing 767 broke into pieces as it dived out of the sky at nearly supersonic speeds.
Air safety officials also said information from the just-retrieved flight data recorder ruled out, at least for now, the possibility that one of the 767’s two engine thrust reversers deployed in flight and caused the jet to plunge. Thrust reversers help slow an aircraft and are designed to be activated only after landing.
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A number of possible scenarios could account for the findings announced Wednesday: Depressurization of the cabin or some other emergency might have prompted the veteran captain to seek a lower altitude, or the flight crew might not have realized the autopilot shut off and the plane rolled over and out of control.
The new but still preliminary information led the National Transportation Safety Board to scuttle its confidently stated assertions last week that the troubled aircraft plunged at speeds of nearly Mach 1–the speed of sound–into the North Atlantic off the Massachusetts coast.
Safety board Chairman Jim Hall declined to reveal Flight 990’s newly tabulated rate of descent– key information that the aircraft’s flight data recorder normally would collect. Initially, investigators said that the Boeing 767 abruptly dived with engines revving, losing 24,000 feet a minute, which experts said would have caused the plane to break apart.
Locating and retrieving Flight 990’s other “black box,” the cockpit voice recorder, took on added urgency as the mystery of the crash deepened. The Navy’s USS Grapple has located an electronic “pinger” beacon from the voice recorder.
The voice recorder, which is fed by three microphones inside the cockpit, would pick up conversations between the pilots as well as any other voices in the cockpit, radio transmissions, engine noise, control switches such as the autopilot being flipped, and other sounds that are a critical part of the effort to reconstruct the accident.
Investigators said nothing has been ruled out–mechanical or maintenance failures, pilot error, terrorism, an unusual weather phenomenon, even a suicide mission–as the probe concluded its second week.
A survey by the Tribune of aviation accident records, meanwhile, indicated that cases of 767s’ autopilots becoming disconnected without a pilot’s command are rare. Safety board documents included only one case, in 1996 when a Martinair Holland 767 sustained minor damage during an emergency landing in Boston after the autopilot turned itself off amid a series of electrical problems.
In a case involving a different type of aircraft, an Evergreen International Airlines Boeing 747 temporarily lost control in 1991 when the autopilot computer deviated from the programmed flight path. There were no injuries in either incident.
The possibilities are endless– and sometimes bizarre. The 1994 crash of an Airbus A310 leased by the Russian airline Aeroflot was attributed to a pilot letting his 15-year-old son sit in the captain’s seat. The youth disconnected the autopilot, the plane’s recorders later revealed. Seventy-five people on the Hong Kong-bound flight died.
Hall said the first hint of trouble on Flight 990 occurred when the jetliner’s autopilot, which controls the altitude, heading and throttle speed, disconnected for unknown reasons shortly after the plane reached its cruising altitude of 33,000 feet.
Eight seconds later, the New York-to-Cairo flight “begins what appears to be a controlled descent” to about 19,000 feet, Hall said. The flight recorder stopped working five to 10 seconds later when the aircraft apparently lost electrical power, Hall said, adding that the recorder’s magnetic tape was still being studied in the safety board’s laboratory in Washington.
The NTSB, citing Air Force radar data, originally said the aircraft plunged several miles to 16,700 feet, then rose to 24,000 feet before plunging again.
The plane’s last transponder transmission was sent from about 10,000 feet. Some experts initially questioned the safety board’s interpretation of the radar data, suggesting instead that the plane continued its downward direction and that the reading at 24,000 feet was actually the severed tail section of the plane falling at a slower rate.
Accident investigators and veteran pilots said the disconnection of the autopilot coupled with the controlled descent of the 767 appeared to suggest the EgyptAir captain and first officer responded correctly to an unfolding emergency.
“A relatively rapid but controlled descent could mean a cabin depressurization problem, in which case you want to get down to 10,000 feet as quickly as you safely can,” said Grant Brophy, who has 20 years’ experience as an air accident investigator.
No distress call was received from the crew, but Brophy said that was not necessarily the pilots’ first priority.
“Remember, they are struggling to put their oxygen masks over their radio headsets and English is their second language. You become so task-oriented in an emergency that even someone with good English proficiency loses the ability to convert from Arabic in the cockpit to English when speaking to the air-traffic controllers and then back again,” Brophy said.
A veteran pilot commented, “They appeared to have had time to radio. I’d like the FAA to know that I’m coming down and to get other airplanes out of the way.”
Pilots discounted the possibility that the crew accidentally disengaged the autopilot, because a warning horn and lights would alert the crew.
One pilot presented a scenario in which the aircraft begins an imperceptible turn downward, perhaps only one-half degree, drawing the plane off-course.
Add to the mix possible crew fatigue–the EgyptAir plane was operating well past midnight and had been late arriving to New York from Los Angeles–and disaster could occur, the pilot said.
All 217 passengers and crew aboard the plane were killed. Most of the wreckage, and the remains of nearly all the victims, are still 270 feet at the bottom of the ocean, 65 miles southeast of Nantucket Island.
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Safety board officials declined to say Wednesday, in light of the flight recorder data contradicting the idea of a supersonic crash, whether sonar and deep-sea robots mapping the 6-square-mile debris field had detected large pieces of the fuselage or intact bodies.