German investigators have concluded that the uncontained failure of a parked Fokker 100's auxiliary power unit was triggered by ignition of de-icing fluid.
The Helvetic Airways aircraft had been preparing for departure from Nuremberg after passengers and crew had boarded.
It was undergoing de-icing treatment on its vertical fin when the APU speed started to increase sharply, before two separate loud explosions were heard and the APU shut down.
German investigation authority BFU says that APU fragments were subsequently found beneath the empennage. A maintenance access door was ruptured, the APU housing was torn open, and the turbine and compressor were badly damaged.
The BFU adds that, inside the aircraft, a fractured section of the compressor wheel had punctured the area below an aft flight attendant's seat and left a 10cm hole in the pressure bulkhead.
None of the aircraft's occupants was injured.
Investigators state that de-icing fluid had been ingested through the APU air intake, ignited, and caused an increase in rotation speed.
Although the APU automatically shut down, and the fuel supply cut off, the ignition of the de-icing fluid meant that the fuel shut-off "had no effect", says the BFU. The rotation speed continued to increase until the APU disintegrated.
The BFU says that, although the de-icing worker had been trained as a pilot, and was aware that fluid should not enter the APU intake, the absence of markings made identification of this area "difficult".
"It would improve safety if such areas were clearly marked and it would also make the work of the de-icing personnel easier," the authority adds in its inquiry into the 20 January 2015 event.
Investigators point out that the de-icing fluid is warm when sprayed, and this creates a misting effect in the cold air temperatures characteristic of de-icing scenarios. The mist can limit the vision of de-icing workers, and this contributed to the APU fluid ingestion.
In its recommendations the BFU is advising that the European Aviation Safety Agency should consider placing de-icing operations under a regulatory authority, like aircraft maintenance, and that the European Commission should establish a legal framework to put de-icing and ground services under EASA supervision.