History of the Flight
The Yak-50, registered RA44461, was returning to Shoreham Airport after participating in the Eastbourne Air display, accompanied by another Yak. When the pilot selected the landing gear down, the starboard (right) main leg failed to extend. Both aircraft climbed to hold at 2,000 feet in an attempt to resolve the problem. The landing gear was cycled at various airspeeds and 'g' load conditions, and the emergency system was used, but the right main gear remained retracted. After visually confirming the right main gear was fully retracted, the pilot raised the remaining gear and burned off fuel to minimum levels in anticipation of a gear-up landing on grass Runway 21. A normal approach was made; on short finals, the magneto switches were selected off, the fuel shut-off valve was pulled, and the battery turned off. The aircraft touched down wings level and decelerated rapidly. When it came to a stop, the pilot exited normally. Airport fire services attended, but there was no fire.
Aircraft Examination
The aircraft was raised on airbags and inspected by a licensed engineer. The right gear up-lock was found engaged, and a turnbuckle linkage rod in the up-lock release mechanism had fractured. The up-lock was released manually, and the gear lowered normally. During gear extension, the initial actuator movement moves a bell-crank connected to the up-lock mechanism input lever by a short turnbuckle linkage. This linkage consists of a rigid rod with a turnbuckle-type eye-end at each end. The eye-end fitting at the actuator end of the rod had broken under the head of the fork. The eye-end itself was very stiff, which would cause bending each time the landing gear was operated. The other end of the linkage was undamaged and moved freely. The maintenance organisation considered that over tightening, subsequent repainting, or lock wire misrouting might all have been factors in restricting free movement of the eye-end. Detailed examination at the AAIB showed that the eye-end fitting had fractured due to a reverse bending fatigue mechanism oriented in the plane of the bending induced by the stiffness in the linkage.
Follow-up Action
As a consequence, the maintenance organisation amended the Maintenance Schedule to include a close visual inspection of the turnbuckle linkage and a check for free movement. One other aircraft was found to have a stiff turnbuckle linkage, although no cracks were found. The maintenance organisation now renews any turnbuckle linkage eye-ends found to be stiff.