Incident Overview
An aircraft on its first flight of the day, carrying eight parachutists and a pilot, took off with approximately 70 US gallons of fuel. The flight was intended to drop parachutists, but upon reaching about 11,000 feet, the decision was made to cancel the detail due to cloud cover obscuring the dropping zone.
In-Flight Failure
During the subsequent descent, while rolling out of a left turn at roughly 4,500 feet, a loud bang was heard. The left aileron was observed to have become partially detached. This was followed by a violent lateral oscillation of the control column lasting several seconds, after which the right aileron also became partially detached.
Evacuation and Descent
At this point, the pilot ordered the parachutists to abandon the aircraft, which they successfully did. The aircraft then adopted an angle of bank to the right of approximately 45°, despite the pilot applying full left rudder. The descent continued until the pilot realized the aircraft was approaching the village of Yarwell; he applied engine power to ensure clearance over the houses. The bank angle increased to 60°, and the aircraft struck the ground right wing low, coming to rest upright facing the opposite direction.
Injuries and Damage
The pilot sustained minor injuries, and there was no fire.
Probable Cause
Subsequent examination showed that, on each aileron, the center of the three attachments to the wing had suffered an in-flight failure, leading to the subsequent detachment of one half of each aileron control surface. Detailed inspection revealed the presence of fatigue in each of the rear angle brackets, located within the wing, to which are attached the aileron supports. Preliminary indications are that the point of primary failure occurred at the left aileron rear angle bracket; the remaining brackets had failed subsequently as a result of overload.