Introduction
On 30 October 2004, a Yak-50 aircraft, registered G-OJDR, experienced a landing gear malfunction during a private flight at Wellesbourne Mountford Airfield, Warwickshire. The pilot was the sole occupant and reported no injuries. The aircraft sustained damage to its underside, propeller, and oil cooler.
History of the Flight
The pilot intended a short local flight for landing practice. After a normal takeoff, selecting the landing gear lever to UP resulted in a venting noise and only the left main gear locking up. The right main gear remained unlocked. The pilot departed the circuit and attempted to rectify the issue using both normal and emergency pneumatic systems, as well as aircraft manoeuvres. After two low fly-bys to confirm the gear state, and with limited fuel remaining, the pilot decided to land on the grass beside the paved runway. During the landing roll, both main gears collapsed. The aircraft slid on its belly for approximately 100 metres. The pilot vacated the aircraft unassisted; there was no fire.
Aircraft and Landing Gear System
The Yak-50 is a low-wing, single-seat aircraft with a tailwheel configuration. The main landing gear retract aft into the wing. Extension is pneumatically powered. The system has a main and emergency air reservoir. The gear selector has three positions: UP, NEUTRAL, and DOWN. The downlock mechanism uses spring-loaded hooks that engage a fixed stop, triggering a microswitch for the cockpit indication.
Examination and Findings
After recovery, the main landing gear could be extended and locked by hand. At a maintenance facility, a compressed air tank was used to extend the gear successfully on the ground, though without the aerodynamic drag encountered in flight. Detailed examination of the landing gear system revealed that the O-ring seals on the piston of the right main gear actuator were rolled. The extension seal had a rolled lip; the retraction seal was severely rolled. This condition allowed air leakage during both extension and retraction. No anomalies were found in the downlock mechanism.
The maintenance organisation noted that at the maximum gear extension speed, at least 30 kg/cm2 pressure is required to lock the gear down. The pilot observed pressure depleting at about 10 kg/cm2 per second when selecting gear down. Since extension takes at least four seconds, sufficient pressure was never available to lock both legs in flight. The leak also reduced the effectiveness of the emergency reservoir. The venting noise heard by the pilot was caused by air leaking past the piston and venting at the valve behind the gear selector.
Conclusions
Both main gears collapsed on landing because they were not locked down. The gear did not lock down due to insufficient pneumatic pressure, which resulted from the rolled O-ring seals in the right main gear actuator.