Disintegration of the auxiliary power unit (APU) duct liner led to smoke and the evacuation of a FlyBe Bombardier Q400 at Belfast City Airport last fall, and the ground crew working the same aircraft two days earlier found a just-ejected piece of the liner, but did not link the part to the aircraft.
The October 2016 incident took place as the crew was preparing to push back with 77 passengers and two cabin crew onboard. The pilots were notified that smoke was "emanating from the rear of the aircraft around the APU,” an UK Air Accidents Investigation Branch (AAIB) report found. The crew confirmed the report and ordered a precautionary evacuation. The APU, which had been running, shut itself down soon after the command to evacuate.
"At no time were any fire or smoke warnings received on the flight deck, nor were any fumes or smoke observed inside the aircraft, although a burning odor became apparent as the incident progressed,” AAIB said.
Investigators quickly determined that the tail cone was subjected to excessive heat. They found blistered paint and electrical cables with signs of heat damage.
“It was concluded that the disintegration of the APU exhaust duct liner allowed hot exhaust gases into the exhaust duct volume outside the liner and into the tail cone itself,” AAIB said. The damaged wire is believed to have triggered an automatic shut-down of the APU.
Investigators discovered that a piece of the duct liner was found two days earlier as the same aircraft was being prepared for takeoff at Birmingham Airport. Soon after the aircraft was cleared to push back, a tug driver found a piece of “hot” debris just behind the aircraft and notified the flight crew. The captain "examined the object and conducted a walk-round inspection of the aircraft,” AAIB said. "There was no obvious aircraft damage and the object bore no markings, such as a part or serial number.”
The APU had been running for about 5 min. in preparation for engine start and push-back.
"In the absence of any evidence to indicate that the object was associated with the aircraft, the pilots assumed that it had come from a passing truck on the roadway behind the aircraft,” AAIB said. The FlyBe crew decided to continue the flight.
The object was photographed and placed in a foreign object damage bin. Investigators probing the Belfast incident could not locate it, however. "The nature of the failure was not determined, although chances of obtaining more information may have been improved had the ejected piece of the duct liner been correctly identified and retained,” AAIB said.
Bombardier told AAIB it had reports of 17 Q400 APU duct-liner cracks since 2003. "Most of the failures were cracks or separation along the weld lines” on aircraft with between 2,500 and 25,000 hours. The FlyBe aircraft had 16,700 hours when the Belfast incident occurred.
While in-service degradation of the the duct liner is expected, the part does not have an in-service limit or inspection interval, the AAIB report said. "The duct’s location in the airframe means it is difficult to inspect effectively unless the APU is removed,” AAIB explained. "Such removals, and consequent ‘opportunity’ inspections, seem to occur at relatively frequent, although necessarily irregular, intervals."
The Q400 APU is approved for use on the ground only. If the system is not shut down manually prior to takeoff, an automatic shut-down takes place when the aircraft’s wheels leave the ground.
None of the previous incidents resulted in a fire, AAIB said. “In addition, it is considered that there is little likelihood of a duct liner failure resulting in an airborne fire as the APU is not capable of being used in the air."