In the final minutes of American Eagle Flight 4184, the twin-engine turboprop plane rolled sharply to the right, straightened itself, rolled right again and then turned over on its back as it plummeted toward an Indiana farmfield in a driving rainstorm.
That short, agonizing portrait emerged from an analysis of the flight data recorder and the co*ckpit voice recorder, the head of the federal team investigating the crash said Wednesday.
Although investigators are unclear on what caused the plane to roll and fall out of the sky, weather-related problems, such as icing, have not been ruled out, officials said. All 68 people on board were killed in the Monday afternoon crash.
Shortly after the first right roll, a loud warning signal was picked up on the co*ckpit’s voice recorder, said National Transportation Safety Board Chairman Jim Hall at an evening news conference in Merrillville, Ind. The signal has been identified as a warning that the plane was traveling too fast for the safe deployment of the plane’s flaps, which cause a plane to descend by slowing it down.
The data recorder, which plots the settings and performance of engine and wing functions, showed that the plane’s autopilot was engaged when the first roll began, said Greg Feith, the board’s chief investigator for the Flight 4184 crash.
The autopilot then clicked off, either on its own or through the action of the pilot. The flaps were pulled back, and the ailerons on the plane’s wings moved sharply, the data recorder shows, indicating that the pilot was fighting to right the plane as it picked up speed and lost altitude.
For a moment, the plane nearly leveled off. Then another roll to the right began, but this time, the plane rolled all the way over on its back.
“The pilot’s recovery effort was not successful,” Hall said.
Radar at the air-traffic control center in Aurora recorded the plane falling rapidly until contact was lost at 4,000 feet, Hall said.
The role that the rainy, windy weather may have played in the crash remains unclear, Feith said.
The flight data recorder showed that the plane’s de-icing equipment was on, but another pilot, flying the same model of airplane at about the same time, reported that his plane had picked up about an eighth of an inch of ice.
That pilot was in a holding pattern about 10 miles to the north of Flight 4184, which circled for 32 minutes waiting for clearance to land at O’Hare International Airport.
The grim task of recovering and identifying the remains of the crash victims began Wednesday, Hall reported. A temporary morgue has been set up in a National Guard armory in Remington, Ind.
The painstaking reconstruction of the aircraft will begin Thursday as investigators continue to sift the evidence in the search for a reason why the ill-fated aircraft spun out of control.
As the investigation into the crash of American Eagle Flight 4184 finished its second full day, the weather conditions at the time of the crash late Monday afternoon continued to stand out as one of the most promising avenues in the search for explanations for the disaster.
Several conditions have to be met for ice to form on an airplane’s wings, but evidence that those conditions were present in the Chicago metropolitan region led the National Weather Service to issue an aviation advisory for light-to-moderate icing while Flight 4184 was in the air.
Ice is dangerous for airplanes because it can build up quickly on the wings and rob a plane of the lift it needs to stay in the air. It can decrease thrust when it builds up on propellers or jet-engine air-intakes. It can hamper communications with the ground when it coats a plane’s antennas.
And when ice builds on a plane’s tail, it can lead to disastrous control problems.
Ice played a role in a 1987 crash of a smaller version of the Super ATR-72 that plunged at least 8,000 feet into a farmfield. An ATR-42 smashed into a mountain in the Italian Alps after flying through an ice zone. The accident killed 37 people. Investigators determined that the plane was not flying fast enough to prevent stalling in those conditions.
Turboprops, like the Super ATR-72, have shown a greater tendency to have icing problems on their tails than jet aircraft because the horizontal tail pieces tend to be proportionally smaller than those on jets, making any ice that builds up on their tails a potentially greater problem.
Also, because they are used on short-haul routes, like Flight 4184’s hop from Indianapolis to Chicago, they spend more time in the altitude range where icing is most prevalent, 3,000 to 15,000 feet.
“They hang out in ice a lot more,” said Tom Ratvasky, an aerospace engineer at the National Aeronautics and Space Adminstration’s Lewis Research Center in Cleveland.
Tail icing is especially dangerous because the tail exerts a downward pressure on an airplane, which keeps it flying level. If ice builds up on the tail, it ceases to function aerodynamically.
The so-called “tail stall” that results “will push the plane down violently and put it into a dive,” said Bob Sweginnis, associate professor of aeronautical science at Embry-Riddle Aeronautical University’s Prescott, Ariz., campus. “There have been cases where it’s taken both pilots pulling on the controls to recover from the nose-down attitude the plane goes into.”
In the last 30 years, 16 plane crashes around the world have been blamed on tail icing; 159 people died in those accidents, most of which involved turboprops.
Ice can form on a plane even when the ground temperature is above freezing, as it was Monday. Temperatures drop about 3.5 degrees for every 1,000 feet in altitude. At 10,000 feet, the altitude at which Flight 4184 circled for 32 minutes, the temperature may have been as low as 9 degrees.
How much ice accumulates depends on a variety of factors, including the plane’s speed and the shapes of its components, said James Johnson, a Chicago pilot and meteorological consultant. A fat wing, for example, pushes air in front of it, impeding ice formation, Johnson said.
Thinner leading edges-tail surfaces, for example-are more prone to ice accumulation.
“You have to assume that if you get any at all on the wing, it will be at least that bad on the tail,” said professor Thomas Carney of Purdue University’s department of aviation technology.
Passenger planes are equipped with systems designed to prevent ice formation or to get rid of it once it begins to accumulate. Some aircraft, for example, have heating elements in the wings, some laser-drilled holes in the wings that ooze a glycol solution and others rubber “boots” on the leading edges of the wings and tail that can be inflated and then retracted to break off ice as it forms.
That was the kind of de-icing device on the American Eagle ATR-72, and investigators will try to determine if the pilots had turned on the device and whether it worked properly.
