After boarding was completed, a Fokker 100 needed to be de-iced before commencing its passenger flight to Zurich (Switzerland) from Nürnberg Airport (Germany). With the required checklist completed the de-icing crews got to work. The aircraft was to be de-iced, using Type-I de-icing fluid.
The failed APU (© & Source BFU)
As the de-icing crew was applying the de-[icing fluid to the leading edge of the righthand stabiliser, the rotation speed of the APU increased, while the noise emitted by the APU increased significantly. At the same time, the APU exhaust started to emit more and more smoke. The crew immediately stopped the de-icing. A loud bang occurred, followed by more banging sounds, with the maintenance door at the bottom of the fuselage blowing open, emitting a 2-metre-long flame. The pressure waves emitted with the bangs from the APU rocked the de-icing truck. The cockpit crew also heard the bangs and felt the aircraft rock. As an APU error message illuminated the crew shut the APU down. The APU had failed catastrophically, emitting parts, with one piece even penetrating the aft pressure bulkhead.
The maintenance door damaged and blown open by the APU failure (© & Source BFU)
The incident was reported to The German Federal Bureau of Aircraft Accident Investigation (German: Bundesstelle für Flugunfalluntersuchung, BFU) which investigated the incident.
In their report, they stated that, as de-icing fluid was ingested by the APU via the intake the APU speed increased, up to the point where an APU protective shutdown occurred. However, the APU speed increase was caused by the ignition of the de-icing fluid, this process was not stopped by the closure of the APU fuel supply. The APU rotation speed continued to increase to a point where the APU structurally failed. Parts of the APU compressor penetrated the aft pressure bulkhead entering the cabin below the cabin crew seats in the rear of the cabin, The hot pieces of APU cause the development of smoke in the aft cabin.
The area where APU debris penetrated the cabin, through the aft pressure bulkhead, under the crew seat.(© & Source BFU, amended by V2 Aviation - Training & Maintenance)
After previous (similar) occurrences the NTSB had issued recommendations, regarding the markings of the APU air inlet. These recommendations were not followed in Europe as the aircraft manufacturer had described the effects of de-icing fluid entering the APU air inlet at the base of the vertical stabiliser. The BFU issued three safety recommendations aimed at preventing re-occurrence, they were aimed at the manufacturer, the EASA and the European Commission. They stated in their report that the APU inlet area should be clearly identified and clearly marked to alert the de-icing crew of the hazards.
The BFU report (in German), which served as the source for this blog can be accessed by clicking on the .pdf file below;
** Editorial note **
V2 Aviation - Training & Maintenance has not been able to obtain an investigation report in English on this accident. This blog is therefore based on the original report (In German) and several internet sources. Should there be inconsistencies in the blog don't hesitate to get in touch with us. There are two possibilities to do that, via the comments function at the bottom of this page or via the contact page of the website.