History of the Flight
The aircraft, a Beech 76 Duchess (registration G-CBBF), was returning to Bournemouth Airport from Haverfordwest. The weather was good, with a cloud base at 3,500 feet and wind from 030° at 13 kt. The pilot contacted Bournemouth Radar and received vectors for an ILS approach to Runway 08. The pilot reported lowering the landing gear while level at 2,000 feet and confirmed three green lights.
On final approach, two stages of flap were selected, and a slight crab was adopted to offset the crosswind from the left, which was reduced as the aircraft descended. Touchdown occurred near the runway centerline on both main wheels and initially appeared normal. However, the right wing began to drop, and the right propeller contacted the runway surface. The pilot applied full left aileron, and initially there was sufficient airspeed to lift the right wing, but as speed decayed, the wing dropped again. The aircraft curved to the right, coming to rest on grass after turning more than 90 degrees. The pilot shut off electrical switches and fuel, and both occupants exited safely.
Aircraft Examination
Initial inspection revealed that the right main landing gear leg had folded upward, while the other two legs remained down and locked. The aircraft was recovered to a maintenance hangar, placed on jacks, and the landing gear system was tested. The gear position is controlled by a two-position electrical switch; hydraulic power for the actuators is provided by an electrically driven pump. During extension, the pump supplies pressure to the extend side until all three downlock microswitches close. If any microswitch opens while the gear is selected down, the pump reactivates to maintain pressure.
A significant internal leak was found in the nose landing gear actuator, allowing hydraulic fluid to bypass between the extend and retract sides. This would substantially reduce the extend pressure available at all actuators if a downlock switch opened due to movement of a leg. Dimensional inspection of all three landing gears, as per the manufacturer's maintenance manual, showed the right landing gear's overcentre travel slightly outside the minimum dimension. This check is not part of the normal inspection schedule.
It is most likely that during the landing roll, the crosswind generated a side load on the right main landing gear leg, causing it to move from its overcentre position. Because of the internal leak in the nose gear actuator, the hydraulic pump could not generate sufficient extend pressure to prevent the right leg from folding upward, resulting in the right wing dropping.