Air Crash Investigation Calls for Change in Anti-Icing Procedures

Reinforcing suspicions that an accumulation of ice and inadequate pilot response may have caused January’s crash of an Embraer 120 commuter plane near Detroit, federal officials on Wednesday called for changes in procedures followed by pilots flying the planes in icing conditions.

Comair’s Flight 3272--flying as part of the Delta Connection--rolled unsteadily and then crashed nose down into a cornfield in Raisinville Township as it approached Detroit’s Metropolitan Airport during a snowstorm. All 29 aboard the twin-engine turboprop were killed on impact.

The National Transportation Safety Board stressed Wednesday that the official cause of the Jan. 9 crash has yet to be determined.

However, the board said that known icing conditions in the area at the time of the crash, coupled with evidence of the airplane’s “degraded aerodynamic performance” and a lack of evidence that its wing de-icing boots had been activated, “strongly suggest that ice had accumulated on the airframe but may not have been seen or recognized as a hazard by the flight crew of Comair Flight 3272.”

In reaching its conclusions, the board cited six other incidents during which the flight crews of Embraer 120s had difficulties controlling the planes in icing conditions.

Accumulating ice can overburden an aircraft and disrupt the smooth flow of air over the wings and flight control surfaces. At the worst, icing can precipitate a stall, a condition in which the wings no longer provide adequate lift and the plane noses over into an uncontrolled dive.

Sounds taped by Flight 3272’s cockpit voice recorder and flight information compiled by the plane’s flight data recorder indicate that may be what happened.

The NTSB said the plane had leveled off its descent to the airport at 4,000 feet when air traffic controllers asked the cockpit crew to make a gradual left turn. Responding to the request, the plane’s autopilot, under guidance from the crew, rolled the plane to the left at about a 20-degree angle.

But when the autopilot tried to limit the left turn roll to 20 degrees by turning the control wheel to the right, the left turn roll angle increased instead. The cockpit crew added power in an effort to straighten out the plane, but to no avail. The roll angle increased to a steep 45 degrees.

As the autopilot disconnected automatically, an alarm went off, warning the pilots that the plane was in danger of stalling. The control wheel deflected from the right to the left, and the left turn roll steepened to 140 degrees.

The pilots could be heard swearing and shouting, “Oh!” “Oh!”

The plane began to dive at an angle that increased to 80 degrees just before impact.

In reviewing this and other Embraer 120 icing incidents, the NTSB noted that Comair pilots were told not to use the wing de-icing boots until they noticed that half an inch of ice had built up. This was because it was thought that premature activation of the inflatable boots, which expand like a balloon to break and shed ice on the leading edge of the wings, could result in new ice forming in the shape of the inflated boot, “making further attempts to de-ice impossible.”

The NTSB said Wednesday that this phenomenon is too rare to justify delays in activating the boots.

The board recommended that manuals and training procedures be changed to improve the response of Embraer 120 pilots to potential icing conditions. The board also recommended that automated ice-detection systems be installed on the planes.