What happened
The flight was conducting an arrival when air traffic control vectored the aircraft for an instrument landing system approach to runway 3R. At that time, automated terminal information service reports indicated moderate mixed icing within clouds. The captain reported that the aircraft encountered cloud tops at 5,500 feet and remained in the clouds for approximately five minutes, accumulating about 1.5 inches of rime ice. Because the accumulation was rapid, the crew decided to wait until they were in clear air before cycling the de-ice boots to prevent bridging.
After intercepting the glide slope on a coupled approach, the aircraft was slowed to extend the gear and flaps. A call for "flaps 15" was made, and approximately three seconds later, the autopilot disconnected as the aircraft descended through about 1,900 feet above ground level at approximately 146 knots. At roughly the same time, the de-ice boots were cycled as the aircraft broke out of the weather. The aircraft then went out of control, entering bank oscillations of up to 44 degrees and abruptly losing about 500 feet before the crew recovered. Despite the loss of control, the crew landed successfully.
The investigation
The investigation revealed that the recently trained crew had not received sufficient training in the use of de-ice equipment from company instructors. These instructors had been trained at the factory. Following this incident, the manufacturer revised the flight manual to provide more specific information regarding de-icing procedures.