What happened
On August 13, 1975, at approximately 14:50 CEST, a Mucha 100 A glider, registration HB-688, was involved in a ground accident during the takeoff roll at the Gruyères aerodrome. The aircraft, operated by Aéro-Gruyère Société d’aviation sportive, was being prepared for a training flight.
Following the assembly of the wings to the fuselage, the glider was aligned on runway 36. As the tow plane reached the glider's takeoff speed, the pilot applied slight back pressure on the control column to compensate for a small undulation in the runway surface. At this moment, the right wing lifted unexpectedly. The pilot was unable to correct the developing right turn and subsequently released the tow cable. The glider performed a ground loop and eventually overturned, resulting in the destruction of the airframe, including a broken fuselage, a torn right wing root, and a damaged cockpit canopy. The pilot escaped the wreckage uninjured.
The investigation
The investigation focused on the assembly process of the aircraft's wings. The wing-to-fuselage connection relies on a system of tenons and metal conical pins located within the main spar. This mechanism is secured using a special wrench that expands the pins via a worm gear.
Investigators found that while the upper pin on the right wing was correctly seated, the lower pin had failed to fully penetrate the wing tenon. Consequently, the right wing was only secured at its upper surface. The investigation established that the person responsible for the assembly performed a visual check on the left wing but failed to do so on the right wing due to a distraction. The pilot also failed to independently verify the connection of the lower pins before commencing the flight.
Findings
- The primary cause of the accident was an omission in the pre-flight verification procedures.
- The right wing was improperly attached to the fuselage because the lower conical pin had not fully engaged the tenon.
- The assembly technician failed to visually inspect the lower pin position on the right wing due to a momentary distraction.
- The pilot relied on the technician's work and did not personally verify the integrity of the wing attachment.
- The aircraft's weight and center of gravity were within limits, and the aircraft was airworthy prior to the assembly error.