What happened
On 23 August 2016, an Alpi Aviation Pioneer 400, registration G-CPPG, was returning to Thruxton Aerodrome for a private flight. During the approach, the pilot attempted to extend the retractable landing gear, but the system failed to lock into position. The landing gear unsafe light remained illuminated, and the circuit breaker for the gear system tripped repeatedly upon attempts to reset and recycle the mechanism.
In an effort to secure the gear, the pilot used the manual hand crank, but the handle would only rotate half a turn. While the pilot could observe through a transparent footwell panel that the nosewheel had exited its bay, the gear was not confirmed as locked. The pilot subsequently performed a landing on a grass strip parallel to the main runway. During the landing sequence, the right-seat observer shut off the fuel and magnetos. The aircraft settled onto its belly and pitched nose-down as it came to a stop. There were no injuries to the two crew members, though the aircraft sustained damage to its underside.
The investigation
Investigators examined the aircraft's retractable tricycle landing gear system, which utilizes an electric motor to drive three screwjacks. The examination revealed that all three screwjacks had fractured due to the collapse. Crucially, the investigation identified that the nose gear screwjack extension rod had become bent.
It was determined that this bent rod likely prevented the overcentre mechanisms from engaging, which is necessary to lock the gear legs in the down position. The bend in the rod may have occurred during a previous takeoff when the aircraft encountered a bump on the runway. This deformation also likely caused the slight sticking of the throttle lever previously noted by the crew, as the rod passes beneath the throttle quadrant.