What happened
On 25 June 1999, a Yak-18T, registration RA81584, was conducting a private flight from Henstridge to White Waltham Airfield. Upon arrival at the airfield, the pilot attempted to extend the landing gear. While the nose gear and left main gear indicators showed a green light signifying they were locked, the indicator for the right main gear failed to illuminate.
To verify the status of the gear, the pilot performed a serviceability check on the indicator light and confirmed it was functioning. A visual inspection of the mechanical indicator above the wing showed the gear was in the fully extended position. Furthermore, the pneumatic system pressure gauge indicated normal operating levels. The pilot attempted to recycle the landing gear three times, performing various maneuvers to encourage the gear to lock. Despite the mechanical indicator showing the gear was extended, the cockpit green light remained unlit. A low pass over the airfield's fire and rescue service was conducted, which confirmed that the landing gear was indeed fully extended.
During the approach to the grass runway 07, the pilot deployed the flaps, which operated normally via the pneumatic system. The aircraft touched down smoothly at approximately 115 kph. However, as the aircraft decelerated to roughly 50 kph, the right main landing gear collapsed. The aircraft subsequently came to a stop after striking the ground with the right rear corner of the flap.
The investigation
Investigators examined the right main landing gear operating mechanism to determine why the gear failed to lock despite being visually extended. The inspection focused on the pneumatic components and the mechanical linkage of the gear assembly.
Findings
- The investigation established that the over centre elbow joint hinge had become too stiff for the pneumatic ram to overcome, which prevented the gear from physically locking into the down position.