What happened
On January 29, 2001, a Beech Baron BE-55, registration HB-GHG, was conducting a flight training mission from Geneva to Annemasse and Bern. The crew, consisting of an instructor and a student pilot, began the flight with a completed pre-flight inspection, which included a visual check of the landing gear struts.
During the takeoff rotation, the student pilot, who was at the controls, felt the right wing dip toward the ground but managed to maintain control using the ailerons. Following takeoff, the crew noticed that upon retracting the landing gear, the green indicator light for the right gear remained unlit, while the red transit light stayed illuminated. After attempting a gear cycle and requesting a low-altitude flyover for visual verification by ground personnel, the crew attempted a manual extension of the system, but the mechanism reached its limit.
Following established checklists for landing gear malfunctions, the crew prepared for an approach at minimum speed. Approximately 150 meters after touchdown, the right landing gear strut collapsed. The aircraft slid along the runway on its right wing and propeller before veering onto the grass. The crew evacuated the aircraft without injury.
The investigation
SUST examined the wreckage and found that the right wheel had retracted into the internal door of the gear strut. The investigation revealed that the forward pivot axis for the retraction strut was missing, along with the screw that normally secures this axis.
Maintenance records showed that the landing gear had undergone a general overhaul in November 1999. The most recent 100-hour inspection, performed on September 7, 2000, included the checking and lubrication of the strut axes. Investigators confirmed that the retaining screw was present and tightened during that 100-hour inspection. Since that time, the aircraft had completed 140 flight hours and 285 landings.
Findings
The investigation determined that the loss of the retaining screw for the retraction strut axis caused the collapse of the right landing gear. While the exact circumstances of the screw's disappearance could not be determined, investigators considered several technical possibilities, including over-torquing the screw and self-locking nut, the use of an incorrect or too-short screw, the use of a worn self-locking nut, defective materials, or insufficient thread engagement due to an improper washer.