A commuter plane rolled rapidly to the right, corrected momentarily and then rolled over on its back before plunging 9,000 feet to the ground, killing all 68 people on board, federal investigators said Wednesday.
National Transportation Safety Board Chairman Jim Hall said Monday's disastrous plunge was preceded by some unusual--and thus far unexplained--maneuvers.
The new developments intensified concerns that the plane involved in the crash, a European-built ATR72, is especially susceptible to icing and that ice building up on the wings may have crippled flight control surfaces.
Hall told a news conference that American Eagle Flight 4184 from Indianapolis was holding at 10,000 feet and a speed of approximately 201 m.p.h. over Roselawn because of stormy weather at Chicago's O'Hare International Airport when air traffic controllers asked the plane's crew to descend to 8,000 feet.
Hall said voice and flight-data recordings recovered from the wreckage showed that as the airliner descended, its speed increased to approximately 213 m.p.h. and an alarm went off in the cockpit, warning that damage to the plane's flaps could occur at that speed.
The recordings showed that the flaps--flat metal slabs that can be extended back from the wings to increase their lift--were deployed at a 15-degree angle.
Such deployments usually are made only at low speed, when the plane is taking off or landing, and veteran pilots said deployment at such a high speed is uncommon.
Hall said that as the flaps began to retract, either automatically or through controls moved by the pilot, "the onset of an abnormal descent occurred."
The ATR72 prop-jet rolled to the right, recovered briefly, then rolled right again, over onto its back, "and a recovery was not accomplished," Hall said.
Greg Feith, the investigator in charge, said both rolls resulted from movement of the plane's ailerons--hinged plates beside the flaps that move up and down to make a plane bank and turn.
Whether the pilot used the control wheels to move the ailerons deliberately, or whether they moved on their own through some control malfunction is not yet clear, Feith said.
"We don't know exactly what this (cockpit) crew was doing," he said.
Another ATR72 encountered similar control problems in December, 1988, over central Wisconsin. The officials said that plane's ailerons malfunctioned when ice built up on the wing, but the pilot recovered and the plane did not crash.
Hall said the pilot of still another ATR72 approaching Chicago on Monday reported that his plane picked up 1/8 of an inch of ice while holding near Flight 4184.
That plane suffered an engine flameout, but the engine was restarted and the plane landed without incident.
The additional information shed new light on the mystery of what caused Monday's crash. Until Wednesday, most of the attention focused on the possibility that icing overburdened the airliner and that the plane may have broken up in the air before slamming nose down into a soybean field.
But evidence was still fragmentary, and the NTSB said it probably will be months before the cause of the crash is determined.
The airliner--a state-of-the-art aircraft fabricated largely of modern composite materials--shattered like glass on impact, scattering fragments of debris over a square mile of muddy farmland.
Each fragment is part of the puzzle that the NTSB must reassemble before its investigators can say definitively why Flight 4184 plunged to the ground.
More than 30 investigators and support personnel fanned out across the field Wednesday morning, marking the crash site into carefully measured grids.
Their survey work completed, they began retrieving body parts. The human remains were placed in plastic body bags and transported to a local armory, which is serving as a temporary morgue.
Officials said some of the remains are so fragmented that positive identification may never be possible.
Flight 4184 took off from Indianapolis International Airport at 3:10 p.m. CST Monday and headed for Chicago.
At the controls were pilot Orlando Agular, 29, and co-pilot Jeffrey Gagliano, 30. Both men were experienced aviators with more than 1,500 hours apiece in ATR72s. Agular's family resides in Palm Desert, Calif.
Stormy weather--which had delayed their takeoff by about an hour--continued to harass the plane as it headed northwest, and weather delays at O'Hare forced the plane into the holding pattern over Roselawn at 3:25 p.m.
The holding pattern is an oval loop about 10 miles long that planes circle at different altitudes while awaiting clearance to land.
Flight 4184 was circling at 10,000 feet while rain was continuing to fall and temperatures had dropped to below freezing.
These conditions are conducive to a buildup of ice that can overburden a plane with added weight and distort the shape of the wings and tail surfaces, reducing their ability to provide lift. The plane's anti-ice devices were on, officials said.
"You don't want to be in an ATR under those conditions," one veteran pilot said. "They don't handle ice well at all."
At 3:56 p.m., a controller told Flight 4184 to descend to 8,000 feet, remaining in the pattern.
One of the men in the cockpit acknowledged the call, ending his transmission with a courteous, "Thank you." Those are the last words the controllers heard.
Seconds later, the plane hit the ground.
The wide dispersal of the wreckage is seen as evidence that the plane may have broken up on the way down.