The cockpit voice tape from the Air Florida jetliner that crashed into the Potomac River three weeks ago strongly suggests that the pilots took off even though they knew that ice or snow was on the plane's wings, sources close to the investigation said yesterday.
Preliminary transcripts show that the pilot and copilot commented to each other repeatedly on the heavy snow falling on Jan. 13 as they taxied to the runway. Their words suggest that at one point they peered out cockpit windows specifically to check for ice or snow on their wings, noted that there was some, but took off anyway, one source said.
About 30 seconds after takeoff, the plane crashed. Copilot Roger Alan Pettit's final words were, "We're going down, Larry," according to one source, and pilot Larry Wheaton responded: "I know it." Then came the sound of impact as the jet struck the 14th Street bridge at about 150 mph. Pettit, Wheaton and 72 other people aboard were killed, along with four people on the bridge.
Last night, officials at the National Transportation Safety Board, which is heading the investigation, declined to comment on the contents of the cockpit tape, saying it is still being studied. Final transcripts of the tape will probably be released later this month.
Investigators have cautioned that cockpit voice tapes, which record cockpit sounds on a continuous 30-minute loop, are subject to varying interpretation because pilots' words are often terse, indistinct and spoken in conjunction with nods, gestures and other nonverbal communication.
One source kept open the possibility that the pilots' words referred to ice or snow on other planes, not their own. But others close to the investigation said they were convinced the pilots had been speaking of their own plane.
Federal Aviation Administration regulations specify that "no pilot may take off an airplane that has frost, snow or ice adhering to" engines, windshields, wings or control surfaces.
Ice on wings can be a major problem for airplanes because it alters the contour of the wings and reduces their lifting power. Investigators seized on icing as a possible factor almost immediately after the crash. About 43 minutes elapsed between the time the plane was sprayed with a de-icing solution and its takeoff. One Braniff pilot who saw Flight 90 as it was taking off told them that it had snow or ice on the wings and fuselage.
The sources were unable to provide direct quotes for most of the tape's transcript. But they said the crew ran through a normal preflight checklist and repeatedly commented on the weather as they awaited takeoff permission. One pilot made a remark to the effect that the snow would close down schools, a source said.
As the plane sped down the runway, the pilots' words indicate that they were concerned it was not accelerating fast enough.
Examination of the jet's wreckage, now laid out on the floor of a National Airport hangar, has shown that the pilots did not activate devices to de-ice engines. Investigators have theorized that probes measuring engine thrust were frozen at takeoff and gave the crew artificially high readings on their engine power.
A spokesman for the safety board said that Air Florida's flight manual requires that engine de-icers be turned on in conditions of wet snow but not in dry snow. The contents of the last weather report that the crew got, including whether the snow was described as "wet" or "dry," remain unclear.
One source said that preliminary electronic analysis of the jet whine, as recorded on the cockpit tape, indicates that the engines were giving out only about 80 percent of normal power. Another monitoring device recovered from the plane, the flight data recorder, has shown that it took 47 seconds to accelerate to takeoff speed instead of the normal 30 or so.
Seconds after liftoff, the 737's "stick shaker," a device that gives off a loud rattle to warn that the plane is about to stall--a fall due to insufficient lift--can be heard on the tape, sources said.
One source said that the rattle began just after the plane reached 166 mph, 24 mph faster than the 737's normal stall speed. That could support theories that ice or snow had reduced the wings' ability to generate lift, making the plane stall at a higher speed than normal.
The pilots' comments indicate that they may have been fighting to pull the plane's nose down after takeoff, sources said. Witnesses have said that the plane climbed away from the airport with its nose pitched upward excessively. Such a position in the air can damage a plane's ability to fly.
The plane's manufacturer, Boeing, has issued two bulletins to airlines operating the 737, warning that in icy conditions, the plane had shown a tendency to pitch up suddenly shortly after takeoff. Investigators have speculated that the Air Florida plane did so, further complicating lift problems it may have been experiencing.
The sound of the shaking stick continued until impact, sources said.
Investigators have said that the plane's pilot apparently had little experience in cold weather aviation. With plane after plane taking off safely in similar conditions ahead of their 737, the crew may not have realized the danger they were in.