What happened
On February 12, 1979, Allegheny Airlines Flight 561, operating an Boeing 737 with registration N29824, was scheduled for a flight from Benedum Airport in Clarksburg, West Virginia, to Washington, D.C. The aircraft had previously attempted a flight to Morgantown but returned to Benedum due to low visibility and a malfunctioning instrument landing system. Following this return, the aircraft was refueled and treated with an unheated deicing fluid between 12:20 and 12:35.
As the flight prepared for its rescheduled departure, snow continued to fall. Although ground personnel believed the aircraft was clean following deicing, witnesses observed approximately 1/4 inch of wet snow on the horizontal surfaces of the plane as it taxied toward runway 21. During the takeoff roll, some observers noted that while some snow blew off, a portion remained adhered to the wings and tail.
Shortly after liftoff, the aircraft experienced severe lateral instability. Passengers reported the plane rolling rhythmically from right to left. Witnesses on the ground observed the right wing dipping significantly before the aircraft struck the ground in an inverted position near the end of the runway. The accident resulted in 2 fatalities and 23 injuries.
Findings
Investigations concluded that the primary factor was the decision to depart with snow accumulated on the wings and empennage. This accumulation caused the outboard sections of the wings to stall prematurely, leading to a loss of lift and diminished aileron effectiveness. The resulting lateral oscillations caused the aircraft to lose altitude and impact the ground.