Serious incident to the ATR 42-500 registered F-GPYF operated by HOP! on 25/03/2018 at beginning of descent to Aurillac airport (Cantal)

Cat. 1 investigation report: ICAO-format report, published after a major or complex investigation.

The crew took off from Paris-Orly at about 18:30 and, during the climb, completed a first level flight at FL110.
The findings made during the investigation of the door hinge assemblies of the aircraft’s left main landing gear determined that the nut on the door’s rear hinge assembly had gradually become loose in service until it fell off, causing the landing gear door to become slightly misaligned with the fuselage. This misalignment resulted in excess drag on the aircraft. This abnormal position of the door also placed additional stress on the other hinge assembly points on the landing gear door, resulting in their successive failure in flight.
At the beginning of the descent to FL180, the door knocked against the fuselage, causing a thud that was heard by people on board, and then separated from the rest of the plane, causing the other damage found on the aircraft:
- damage to wing root fairings;
- scratches to cabin window and surrounding skin;
- tear on lower surface skin of left flap;
- scratches on lower surface skin of left wing;
- small dents on vertical stabilizer.

Not understanding what had just happened, but seeing that the flight parameters were normal, the crew decided to continue the descent and landed at Aurillac as normal. It was only on the apron that the damage to the aircraft was observed.

The investigation was unable to identify the exact cause of the loss of the nut on the rear hinge assembly of the main landing gear door, leading to the loss of the door. However, the investigation did highlight the possibility that the nut in question and the torque applied did not comply with the configuration specified by ATR during the initial design of the fastener.

The investigation also showed that lack of information in the manufacturer’s generic maintenance documentation and difficulties in identifying the relevant information can lead, through a combination of organisational and human factors, to the installation of screw/nut combinations that do not reflect the state of the art. The consequences of these deviations from best practice on the performance of the screwed joint could not be precisely determined during the investigation, but may lead to the malfunction of the fastener.

Consequently, the BEA has issued ATR with two safety recommendations. These relate to a review of the manufacturer’s generic maintenance documentation on fasteners and particularly, on the notions of the tightening torque to be applied, interchangeability of parts and reuse of self-locking nuts.