What happened
On 25 March 2018, an ATR 42-500, registration F-GPYF, operated by HOP!, was performing a scheduled commercial flight from Paris-Orly to Aurillac. During the initial climb, the aircraft reached FL110, where the crew received a "CRUISE SPEED LOW" alert. This alert persisted during the cruise phase at FL190, with the indicated airspeed recorded at 212 knots rather than the expected 224 knots.
As the aircraft began its descent through FL180, passengers and crew heard a loud thud and felt a jolt. The aircraft' and autopilot trajectory remained stable, and the crew continued the descent to Aurillac without further incident. However, upon landing, the crew discovered that the left main landing gear door was missing and that the aircraft had sustained significant damage, including damage to the wing root fairings, scratches to the cabin windows and wing skin, a tear in the left flap, and small dents on the vertical stabilizer.
The investigation
The BEA investigation focused on the hinge assemblies of the left main landing gear door. Investigators examined the components of the front and rear hinge assemblies, recovered debris, and reviewed maintenance records. The technical inspection of the remaining hardware revealed that at the rear hinge assembly, the screw had exited its housing, and the associated nut and washer were missing. At the front hinge assembly, while the screw remained in place, a section of the male hinge had broken.
Findings
Investigation of the hinge assemblies determined that a nut on the rear hinge assembly had gradually loosened during service until it fell off. This loss caused the landing gear door to become misaligned with the fuselage, creating excess drag. This misalignment placed additional stress on the remaining hinge points, eventually leading to their failure in flight.
While the exact reason for the nut's initial loosening could not be identified, the investigation highlighted that the nut and the applied torque may not have complied with the configuration specified by the manufacturer. Furthermore, the investigation noted that ambiguities in the manufacturer's maintenance documentation regarding fastener combinations and torque could lead to the installation of improper hardware.
Safety action
The BEA issued two safety recommendations to ATR regarding the review of generic maintenance documentation, specifically concerning tightening torque applications, the interchangeability of parts, and the policy for reusing self-locking nuts.