What happened
On May 29, 2008, a Cessna 560XL was performing a flight from Oberpfaffenhofen to Vienna. Approximately two to three minutes after takeoff, while climbing through an altitude of roughly 3,600 ft, the crew experienced sudden and severe vibrations. The crew immediately decided to abort the flight and returned to Oberpfaffenofon, where the aircraft landed safely with no injuries.
Prior to the flight, the aircraft had undergone maintenance. As part of a specialized surface treatment process, the engine cowlings had been removed. During the transport of the upper cowling halves from a separate painting hangar back to the maintenance hangar, the four lower external quick-release fasteners on each cowling were secured using only the bottom-most fasteners, which were accessible from the ground. Following this procedure, the aircraft was released for flight.
The investigation
The BFU investigation focused on the structural integrity of the engine cowlings and the fastening mechanisms. Post-flight inspections revealed that the upper halves of the cowlings on both engines had detached. Specifically, a triangular section of the left upper engine cowling, measuring approximately 80 cm by 105 cm, was missing.
Further examination of the fastening hardware showed that 16 of the lower quick-release fasteners (four per cowling half) had been forcibly pulled out of the engine mounting frames. The remaining fasteners, which secured the cowlings to the fuselage side, remained intact and undamaged.