What happened
On January 20, 2015, a Fokker 100 was parked at position 30 of Nuremberg International Airport undergoing de-icing operations. The aircraft, operated by a Swiss airline, was being treated with Type I de-icing fluid. While the de-icing personnel were working on the right side of the aircraft near the horizontal stabilizer, the Auxiliary Power Unit (APU) began to experience a rapid increase in RPM.
The de-icing operator noted a significant increase in the noise frequency and exhaust output. Shortly after, two loud explosions occurred, causing the de-ing vehicle to shake. The force of the event caused a maintenance hatch on the fuselage to burst open, releasing a two-meter-long flame. Inside the cabin, the shockwaves were felt by the crew, and an APU error message appeared on the cockpit displays before the unit automatically shut down. A fragment from the APU compressor wheel pierced the rear pressure bulkhead, creating a 100mm hole and causing smoke to enter the cabin. Passengers were moved to the forward section of the aircraft and evacuated via buses. There were no fatalities and no injuries.
The investigation
The BFU examined the wreckage, finding that the APU housing had ruptured, the turbine wheel was heavily damaged, and the compressor wheel had shattered. Investigators noted that the area around the APU air intake lacked clear markings. The investigation also reviewed the training of the de-icing personnel, who were found to be properly qualified according to AEA standards, and analyzed the technical history of the aircraft type, noting that similar APU failures had occurred previously.
Findings
- The primary cause of the APU failure was the ingestion and combustion of de-icing fluid through the air intake.
- The rapid increase in RPM caused the turbine to disintegrate, with fragments penetrating the pressure bulkhead.
- Visibility was reduced during the process due to heavy mist formed by the temperature difference between the de-icing fluid and the aircraft surfaces.
- The lack of clear markings around the APU air intake made it difficult for the operator to identify the hazard zone.
Safety action
- The aircraft manufacturer should clearly mark the area around the APU air intake on the Fokker 100.
- The BFU recommended that EASA intensify its oversight of de-icing activities and consider bringing ground de-icing services under formal regulatory supervision, similar to aircraft maintenance.