Plane Crash Prompts Warning to Crews : Aviation: Company prohibits flight procedures that immediately preceded Indiana tragedy that killed 68. Bulletin is called ‘precautionary.’

Three days after the crash of an American Eagle ATR72 commuter plane that killed all 68 on board, the airline’s parent company issued a bulletin Thursday prohibiting some of the procedures used by the cockpit crew.

Marty Heires, a spokesman for the AMR Co., said the bulletin was “precautionary” and was not intended to imply that either the crew or the propjet plane were at fault.

The first prohibited procedure involved flying in a holding pattern with the plane’s flaps extended.

Flaps--flat metal slabs that can be extended back from the wings to increase their lift--usually are extended only on takeoffs and landings.

Flight 4184 was flying at a speed of about 210 m.p.h. with its flaps extended at a 15-degree angle as the plane circled over Roselawn, descending slowly from 10,000 feet.

An alarm sounded, warning the pilot that damage to the flaps could occur at that speed, and he retracted them.

The second procedure prohibited in the bulletin is using the plane’s autopilot during conditions in which ice can build up on the wings and fuselage, overburdening the aircraft.

Even though the cockpit crew of Flight 4184 was aware of the icing hazard, the plane was being flown on autopilot in the holding pattern.

Heires said the flap advisory was not new and was simply upgraded from the “not recommended” list to the “prohibited” list. He said the autopilot advisory is new.

Even though the airline did not relay National Weather Service information about icing conditions to the cockpit crew--the pilots of Flight 4184 were aware of them.

Recording devices recovered from the plane show that the propjet’s de-icing equipment had been turned on more than 15 minutes before the crash.

Why the pilot had the flaps deployed in the holding pattern is not clear.

However, sources close to the investigation believe that when he retracted them in response to the alarm, that is when Flight 4184’s troubles intensified. The sources believe the wings were already losing their lift because of accumulating ice, and that the extended flaps had been adding lift that helped stabilize the plane.

They say that retracting the flaps may have reduced the already diminished lift even more. Seconds later, in an automatic correction, the plane’s autopilot deflected the ailerons--hinged plates next to the flaps that move up and down to bank and turn the plane.

The sources believe this aileron movement may have tilted the plane enough to deprive the wings of that last critical amount of lift needed to support the aircraft.

The ATR72 went into a stall, rolling over on its back and plunging 9,000 feet to the ground.

Whether the cockpit crew could have done anything to prevent the stall or recover from it is still a matter of speculation.

Investigators have noted similarities between this crash and an incident in Wisconsin in December, 1988, involving an ATR42, a smaller version of the ATR72.

The pilot of the ATR42 was flying through subfreezing temperatures and moderate to heavy rain when the plane rolled and went into a stall. The plane was on autopilot and sources said this may have prevented the pilot from sensing the impending stall.

However the pilot was able to recover from the stall and land.

As a result of the incident in Wisconsin, the Federal Aviation Administration issued the following warning to ATR42 pilots:

“Prolonged operation in freezing rain should be avoided. Ice accretion due to freezing rain may result in asymmetric wing lift.”

In addition, the FAA called for wing modifications that were intended to reduce such problems.

Questions have arisen whether these modifications, incorporated into the ATR72s, have solved the problem.

National Transportation Safety Board Chairman Jim Hall said Wednesday night that an American Eagle ATR72 flying near South Bend, Ind., Monday night experienced “buffeting” and rolled into a turn after ice built up.

The pilots avoided a stall and landed safely.