History of the Flight
The pilot and two passengers departed Bristol Airport at 0930 hrs on 11 July 2018 for a flight to Cotswold (Kemble) Airport, intending to return later that day. The pilot configured the aircraft for landing on Runway 08 with the intention to perform a touch-and-go. The wind was 6 kt from 060°. The pilot confirmed three GREEN landing gear indications and touched down at approximately 90 kt IAS at 1002 hrs. Before reapplying power for takeoff, the pilot raised the flaps and noticed the left wing drop. He corrected using ailerons and aborted the takeoff. The pilot maintained the aircraft on the runway until the left wing and propeller contacted the runway, and the aircraft veered left, coming to rest on the grass between the tarmac and grass runways with the left main landing gear collapsed. The pilot and passengers exited unaided without injury.
Accident Site
Examination of the runway showed impact marks from the left propeller after the aircraft passed the intersection with Taxiway Bravo. Fifteen strike marks were noted, with spacing consistent with an aircraft speed of 70–80 kt. Further along, there were additional strike marks and scraping from the left wing, aileron, and aft fuselage. The aircraft halted facing northwest on the grass between the tarmac and grass runways.
Aircraft Information
The Piper PA-34 Seneca II has a fully retractable, hydraulic tricycle landing gear. It is raised and lowered by an electrically-driven, reversible hydraulic pump. Each leg is held down by a spring-loaded downlock hook. When downlocks engage, the pump switches off and three GREEN indications illuminate in the cockpit. A system releases hydraulic pressure to allow gravity extension in case of hydraulic power loss.
Aircraft Recovery and Examination
As the aircraft was lifted by a recovery vehicle, the right main landing gear collapsed, though the nose gear remained down. Examination at the site found that neither main landing gear downlock was engaged. The recovery team attempted to lock the gear down but were prevented by hydraulic pressure in the system. The cockpit emergency landing gear release lever was operated, releasing the pressure, and the downlocks engaged. The aircraft was towed to a storage facility. Subsequent examination of both main landing gears and linkages found no defects that could have caused the gears to unlock. Microswitches and GREEN indicators were checked and were defect-free. The left propeller blade tips were eroded from runway contact, along with the left aileron and its hinges and a section of the aft fuselage.
Recorded Information
Closed-circuit camera footage from a hangar at the Woodside Apron captured the landing from Taxiway Delta until the aircraft veered off the runway. The footage was consistent with the left wing dropping and propeller contact, followed by correction and further contact along the runway.
Analysis
The pilot reported seeing three GREEN landing gear down indicators on approach. It is possible that during the landing sequence, a short application of hydraulic pressure to the retraction system occurred, sufficient to start the retraction sequence and unlock the main landing gears but stopping before the gears began to raise. This pressure would have been released during recovery when the emergency release was operated. The cause of such pressure could not be determined.
Two similar incidents of PA-34 main landing gear collapse have been reported in AAIB Bulletins (G-FILE in 1988 and G-BEHU in 2002). In both, extensive examination found no cause for the unlocking.
Conclusion
At an unknown point after the pilot confirmed three GREENS, the main landing gear became unlocked, leading to the left landing gear collapsing during landing. During recovery, pressure in the hydraulic system prevented downlock engagement; this was released by the emergency extension system. Subsequent examination found no defects, and the cause of the pressure could not be determined. Despite a similar incident to the same aircraft in 1988, no further action is to be taken as the aircraft is beyond economic repair.