Icing possible cause of Michigan plane crash

Latest developments:

• Plane's de-icing system was repaired twice
• Icing was problem at Detroit airport
• Worn part, defective valve replaced
• FAA was aware of icing problems
• Bitter cold hampers investigation
• Propeller blades were replaced in August

RAISINVILLE TOWNSHIP, Michigan (CNN) -- Flights in and out of Detroit's airport were icing up before a commuter plane, trying to land in a snowstorm, went into a roll and nose- dived into a field, killing all 29 people aboard.

Also, records of the plane involved in the crash indicate that on at least two occasions, the plane's de-icing system failed and had to be repaired.

While the weather is an obvious possibility as to what caused the crash of Comair Flight 3272, National Transportation Safety Board member John Hammerschmidt said Friday it was too soon to speculate about the cause.

The plane, an Embraer 120 twin-engine turboprop operated by Comair, went down at dusk Thursday, 18 miles short of Detroit Metropolitan Airport without any mention of trouble from the pilot.

On the flight recorders:		(217K/20 sec. AIFF or WAV sound)
On the condition of the recorders:		(151K/14 sec. AIFF or WAV sound)

By late Friday, the NTSB had found the two black box flight recorders that might yield clues to the cause of the accident. Crews were still working to recover bodies from the burned and splintered wreckage.

Icing was problem at Detroit airport

According to Detroit Metropolitan Airport spokesman Mike Conway, planes at the Detroit airport had to be de-iced frequently in Thursday's snow and cold.

"We had precipitation all day, and the airlines were de-icing aircraft all day," he said.

Ice disrupts airflow over the wings and can cause planes to stall or roll. One witness to the Comair crash, Ted Rath, said he saw the plane roll three times before it crashed into a snowy field.

At a press conference Friday night, Hammerschmidt said conditions at the time of the crash were light snow, mist, light winds and moderate turbulence. The forecast also called for "light to moderate icing," and he noted that the pilot of a small plane had reported severe icing near Finlay, Ohio.

But Hammerschmidt noted that the Brazilian-made Embraer 120 was equipped with systems to de-ice the wings, propellers and engines during flight.

Worn part, defective valve replaced

Those de-icing systems include inflatable, leading-edge icing boots on the wings, heated, leading edges on the propellers and heated inlet probes in the engines, he said.

However, Federal Aviation Administration records indicate that the Embraer 120 involved in the crash experienced two failures of de-icing systems in the past.

The system that prevents icing on the wing's airfoil failed during a flight on March 19, 1994. A part, described as worn, was replaced and the problem was considered fixed, according to records obtained by CNN.

On February 11, 1993, the system that prevents icing on the engine intake inlets failed during a flight. A valve deemed to be defective was then replaced.

Last year, the Federal Aviation Administration issued an "Airworthiness Directive" concerning problems found with icing of wings and propellers on the Embraer 120.

FAA was aware of icing problems

The directive ordered aircraft owners to post a notice in their flight manuals which warned: "Flight in freezing rain, freezing drizzle, or mixed icing conditions may result in ice build-up on protected surfaces exceeding the capability of the ice protection system, or may result in ice forming aft of the protected surfaces. This ice may not be shed using the ice protection systems, and may seriously degrade the performance and controllability of the airplane."

The directive warned pilots to keep a close eye for clues to icing on the wings and propeller blades and ordered them not to use the autopilot system under those circumstances.

In 1994, an American Eagle ATR-72 that had been in a holding pattern in icy weather for nearly 40 minutes suddenly rolled and crashed near Roselawn, Indiana, killing 68 people. Investigators blamed ice on the wings.

The Comair flight, however, had not been in a holding pattern, and the NTSB's Hammerschmidt noted the plane was equipped with three de-icing systems.

Bitter cold hampers investigation

At the crash scene, sheriff's deputies carried victims' remains to a hangar nearby. The wreckage and the bodies were scattered over a 100-by 200-yard area in weeds and snow, and in a 4-foot-deep crater gouged out by the plane.

The investigation was slowed by snow and below-zero wind- chill readings that prevented investigators from working outside for more than 15 or 20 minutes at a time, Hammerschmidt said.

According to FAA records, maintenance on the plane's propeller and de-icing equipment was performed in 1993 and 1994. The plane's last major maintenance check was on November 20, the airline said.

In fatal crashes of Embraer 120s in 1991 and 1995, propellers were cited as the cause. In the latter, near Carrollton, Georgia., a section of a prop blade on the left engine broke off and fell to the ground miles from the crash.

Propeller blades were replaced in August

The NTSB later concluded that an undetected crack in the blade caused the failure which led to the crash.

A spokesman with Hamilton Standard, which manufactures the propellers used on Embraer 120s, said blades on all Comair planes were repaired by last August.

Among those killed in the Comair crash were a woman who was on the way to the funeral of her brother, who was himself killed in a cargo jet crash in Virginia; a Procter & Gamble executive recruiting at the University of Michigan; and a university student heading home from an interview with Procter & Gamble.

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