What happened
On December 6, 1977, a Beechcraft BE-95 Travelair, registration HB-GBM, was conducting IFR training flights as part of a flight school program. The flight, operated by Bodmer Air Taxi AG, began at Zurich Airport with a flight instructor and two students. After performing several approaches in Zurich and subsequent maneuvers in Bern, the crew departed Bern for further training.
Shortly after takeoff from Bern, during the climb-out checks, the instructor noticed that the red "gear-up" warning light failed to illuminate, despite the landing gear lever being in the down position and electrical fuses being intact. The crew attempted to follow emergency procedures for landing gear extension while holding near Schüpberg. Despite following the manual and attempting to extend the gear through positive G-loading, the gear remained partially retracted, with the main gear stuck approximately 26 cm from the wing underside.
Due to the inability to extend the gear, the crew decided to return to Zurich. To mitigate the impact of a potential gear-up landing, the crew requested that Zurich Tower prepare a 1000-meter-long foam cushion on Runway 14. During the approach, the crew even consulted a mechanic via radio to attempt further manipulations. At approximately 12:20, smoke was reported in the cabin, likely due to an electrical short in a motor.
At 13:04, the aircraft landed on Runway 14. The engines were shut down just before touchdown, but the aircraft struck the runway 105 meters before the foam cushion, eventually coming to a halt after skidding 270 meters. The landing resulted in one fatality and two injuries.
The investigation
The investigation focused on the mechanical state of the landing gear system and the failure of the emergency extension method. Investigators examined the landing gear switch, the nose gear indicator, and the physical condition of the retraction mechanism.
Technical analysis revealed that the nose gear was retracted and the drive shaft for the nose gear had a broken side. The manual emergency hand crank could rotate without resistance, but it could not engage the drive shaft. Crucially, the drive shaft and worm gear had shifted toward the drive motor, preventing the manual crank from functioning.
Findings
- The investigation determined that a failure occurred in the relay responsible for regulating the speed of the landing gear drive motor.
- Because the relay failed to slow the motor down at the end of the retraction cycle, the motor continued at full speed (7500/min) until the gear hit its limit.
- This sudden stop caused the destruction of a roller bearing that was already compromised due to inadequate lubrication.
- The destruction of the bearing allowed the drive shaft to shift axially, which blocked the manual emergency extension mechanism.
- Additionally, the coil for the "gear-down" direction in the landing gear drive motor had burned out.