CHI93IA140

The domestic revenue flight was in cruise flight en route to Oshkosh, Wisconsin, when the airplane experienced a loss of power in the right engine. Before the right engine could be relit and power restored, there was a loss of power in the left engine. Both engines were restored to their original power settings. As the pilot was commencing the descent for the final approach, the left engine lost power. The engine relit and recovered. On final approach the right engine lost power. The pilot shutdown the right engine and made a single-engine landing. The investigation revealed a lack of bonding between the nacelle module 3 (engine inlet) and the airframe on both engines. This condition allows a static charge to build up in the module, and when it discharges, it results in interruption of the engine de- ice timer operation. This condition leads to ice ingestion by the engine, and a subsequent loss of power.

On April 11, 1993, at 0820, a British Aerospace ATP, N855AW, operated by Air Wisconsin, Inc., as United Express Flight 5010, experienced several losses of power on both engines as the flight was en route to Wittman Field, Oshkosh, Wisconsin. There were no injuries to the crew of four. There were no passengers aboard the airplane. The flight, operating under 14 CFR Part 121, originated at the Outagamie County Airport, Appleton, Wisconsin, at 0810. Instrument meteorological conditions prevailed at the time, and an IFR flight plan was filed.

The flight crew reported the temperature was 32 degrees Fahrenheit with moderate snow at the time of the power losses. The flight was in cruise at 3000 feet msl when the right engine lost power. At the time the right engine began to restore power, but before recovering to the original power setting, the left engine lost power. The left engine then recovered to its original power setting. As the pilot commenced final descent for landing at Oshkosh, the left engine lost power and then recovered. On final for landing the right engine lost power and started to recover, but the pilot manually shut it down and made a single-engine landing without further incident.

The airplane and engines were examined at Oshkosh. The engines were ground run without incident, and the airplane was ferried to Appleton for further examination.

During the examination of the airplane at the Air Wisconsin maintenance facility by Air Wisconsin, British Aerospace, and FAA personnel, a lack of bonding between the nacelle module 3 (intake section) and the airframe was found on both engines. No other defects were found. British Aerospace, the Federal Aviation Administration, and the Civil Aviation Authority (Great Britain) determined the lack of bonding between engine nacelle module 3, and the airframe allows the build-up of a static charge which may result in the interruption of the Engine De-ice Timer operation. This could lead to ice ingestion by the engine, and a subsequent loss of engine power.

To correct the situation British Aerospace issued Service Bulletin ATP-24-55 on April 22, 1993. The Service Bulletin detailed the bonding test procedures for the airplane, and the corrective action necessary to ensure adequate bonding of components. On April 24, 1993 they issued Revision 1 to the Service Bulletin which added a bonding strap for the Throttle Stepper Motor which is located in nacelle module 3.

British Aerospace Service Bulletin ATP-24-55 Revision 1, was complied with, and the airplane was released for service.