Background
On 25 March 2018, an ATR 42-500 registered F-GPYF, operated by HOP!, was conducting a commercial flight from Paris-Orly to Aurillac, France. The aircraft took off at approximately 18:30 UTC and climbed to FL110 before continuing to its destination.
Incident Details
During the initial phase of descent to Aurillac, at FL180, the crew heard a thud and the aircraft experienced a loss of the left main landing gear (L MLG) door. The door struck the fuselage, causing damage to wing root fairings, scratches to a cabin window and surrounding skin, a tear on the lower surface skin of the left flap, scratches on the lower surface skin of the left wing, and small dents on the vertical stabilizer. The crew, not understanding the event but observing normal flight parameters, continued the descent and landed at Aurillac without further incident. The damage was discovered on the apron.
Investigation Findings
The French civil aviation safety investigation authority (BEA) launched an investigation. Examination of the door hinge assemblies revealed that the nut on the rear hinge assembly of the L MLG door had gradually loosened in service until it fell off. This caused the door to become slightly misaligned, increasing drag and placing additional stress on the other hinges, which failed successively in flight.
The investigation was unable to determine the exact cause of the nut loss. However, it highlighted the possibility that the nut and the applied tightening torque did not comply with the configuration specified by ATR during the initial design of the fastener. Additionally, the investigation found that a lack of information in the manufacturer's generic maintenance documentation and difficulties in identifying relevant information could lead, through a combination of organizational and human factors, to the installation of screw/nut combinations that deviate from best practices. The consequences of these deviations on the performance of the screwed joint could not be precisely determined but may lead to fastener malfunction.
Safety Actions
As a result of the investigation, the BEA issued two safety recommendations to ATR. These recommendations concern a review of the manufacturer's generic maintenance documentation on fasteners, particularly regarding tightening torque, interchangeability of parts, and reuse of self-locking nuts.