What happened
On April 9, 1964, a Sokol M1D aircraft, registration HB-TAM, was conducting a local flight circuit involving several stops, including Lommis airfield. During the takeoff roll from the grass runway at Lommis, the pilot heard a loud bang immediately upon rotation. Following the takeoff, the pilot proceeded to retract the landing gear as parted with the ground. However, the pilot quickly discovered that the ailerons were significantly restricted, with the control stick unable to move fully to the left and only limited movement available to the right.
Deciding to abort the flight, the pilot initiated a wide right-hand turn to return to the airfield. Upon extending the landing gear, the aileron control was restored to normal functionality. Despite the normal approach, the landing gear collapsed upon touchdown, resulting in damage to the aircraft's propeller, engine cowling, and fuselage underside. No injuries were sustained by the pilot or the passenger.
The investigation
The investigation examined the mechanical state of the Sokol M1D and the landing gear mechanism. The aircraft's maintenance records showed the last annual inspection had been completed in July 1963, and no technical defects were found in the aircraft's overall condition.
Investigators inspected the landing gear's retraction mechanism, which is operated by a hand crank. The examination revealed that the sacrificial rivets (designed as shear points) in the landing gear struts had failed on both sides. This failure caused the drive shafts to shift out of alignment with the worm gear axis. Furthermore, the left aileron control rod was found to be broken.
Findings
- The investigation concluded that the aircraft likely took off with the landing gear unlocked.
- The initial loud bang heard during takeoff was caused by the failure of the sacrificial rivets due to mechanical stress on the bumpy grass runway while the gear was not properly locked.
- When the pilot attempted to retract the gear, the misaligned drive shafts caused the mechanism to interfere with the aileron control linkage, resulting in the observed control blockage.
- The final collapse of the gear during landing occurred because the gear was not fully locked in the extended position.
- The pilot could not definitively confirm if the pre-flight check of the gear lock had been performed.
Safety action
No specific safety recommendations were recorded in the final report, as the investigation attributed the event to pilot oversight regarding the gear lock verification.