What happened
On 30 October 2004, a Yak-50, registration G-OJDR, was performing a local flight for landing practice at Wellesbourne Mountford Airfield. Following takeoff, the pilot attempted to retract the landing gear, but an audible air leak occurred, and the cockpit indication showed that only the left main gear had successfully locked. The right main gear remained in the unlocked position.
The pilot climbed to 2,000 feet to allow the engine-driven compressor to recharge the pneumatic system. Despite multiple attempts to extend the gear using both the normal and emergency pneumatic systems, the right gear failed to lock. The pilot also attempted various flight manoeuvres, including pitching and yawing, to encourage the gear to lock, but with no success. After conducting two low-level fly-bys to assess the gear's state, and with fuel levels becoming critical, the pilot elected to land on the grass adjacent to the paved runway.
During the landing roll, both main gears collapsed almost simultaneously. The aircraft's underside struck the ground, causing the propeller tips to break and tearing the oil cooler and its housing from the belly. The pilot was able to exit the aircraft unassisted, and no injuries were reported.
The investigation
The AAIB investigation focused on the pneumatic system and the landing gear mechanism. When the aircraft was recovered, the gear was manually extended and found to lock correctly on the ground. However, testing revealed that the pneumatic pressure was depleting at a rate of approximately 10 kg/cm² per second during extension attempts in flight.
Detailed examination of the right main gear actuator revealed that the O-ring seals on the piston had rolled. Specifically, the seal used for extension had a rolled lip, and the seal used for retraction was severely rolled. This condition created a significant leak that prevented the actuators from overcoming the aerodynamic drag forces encountered during flight.
Findings
- The primary cause of the accident was that both main gears were not locked down at the time of landing.
- The failure to lock the gear was due to insufficient pneumatic pressure to fully extend the actuators in flight.
- The loss of pressure was caused by a leak within the right main gear actuator resulting from rolled O-ring seals on the piston.
- The design of the downlock microswitch allowed for a 'green' indication even if the gear was not fully engaged.
- It is possible that improper lubrication during the installation of the seals following a recent overhaul contributed to the seals rolling.