Incident Overview
On 28 July 2013, a Piper PA-34-200T Seneca II, registration G-CLUE, suffered a left main landing gear collapse during the landing roll at Durham Tees Valley Airport. The aircraft was on a private flight with two pilots on board; no injuries were reported. Damage was limited to the left engine, propeller, main landing gear, flap, and aileron.
Flight and Landing
The pilot initially intended to land at a private airstrip approximately 20 nautical miles from Durham Tees Valley Airport. During the approach, only the green down-and-locked indicator lights for the right main and nose landing gears illuminated, indicating that the left main gear had not extended. The pilot recycled the gear selection and climbed to 2,000 feet to assess the situation. After checking fuses and swapping indicator bulbs without success, he decided to return to Durham.
Upon arrival, the pilot requested a fly-by of the control tower to visually inspect the landing gear. He selected gear down again, but this time no green down-and-locked lights appeared, and the tower reported that all gears appeared retracted. The pilot then performed the emergency extension checklist, deploying the landing gear via free-fall. The tower confirmed that all three gears appeared down, but the pilot still had no green lights. He proceeded to land gently on all three gears at stall speed. After touchdown, as he applied gentle braking, the left side of the aircraft sank, the left propeller contacted the runway, and the aircraft came to a halt. Both pilots evacuated via the normal door, which had been left ajar as a precaution. Emergency services attended, but no fire occurred.
Aircraft System Description
The Piper PA-34 uses a single reversible electric hydraulic pump to raise and lower the landing gear. When hydraulic pressure is lost, the gears are designed to free-fall under their own weight. The main gear downlock mechanism consists of a conventional over-centre sidestay held in position by hooks engaging a pin. In a normal powered extension, the actuator engages the hooks; during free-fall, a spring is used. A microswitch actuated by the hooks illuminates the green down-and-locked light.
Post-Accident Examination
During recovery, the downlock hooks on the left main gear would not engage, requiring an improvised strut for towing. In a hangar, the gear was retracted using a hand pump and then extended via free-fall. The nose and right main gears locked normally, but the left gear remained partially retracted and had to be pulled down manually. After slightly slackening the centre pivot bolt and applying lubrication to the downlock mechanism, further free-fall extensions still did not allow the hooks to engage; levering the hooks with a screwdriver was necessary to overcome stiffness caused by corrosion and lack of lubrication. Inspection of the electro-hydraulic pump revealed that the brushes had completely burned out and the commutator was excessively worn.
Maintenance Background
The aircraft was operated under a CAA Light Aircraft Maintenance Programme (LAMP) requiring detailed visual inspection of landing gear components and functional checks every 150 flying hours. G-CLUE had undergone maintenance and an annual inspection starting in August 2012, completed in November 2012, with an Airworthiness Review Certificate issued on 1 March 2013. However, during the first takeoff after maintenance, the landing gear would not retract. The aircraft was flown to Blackpool with the gear extended, where a touch-and-go resolved the issue. Additional maintenance at Blackpool included adjusting a microswitch for the 'Undercarriage light on' indication and a retraction test. The aircraft departed Blackpool on 21 March 2012 (as recorded) and accumulated approximately 8 hours 30 minutes of flight time before the accident.
Discussion
According to the investigation, the inability of the left main landing gear to lock down resulted from two separate factors: the failure of the electro-hydraulic pump, which prevented normal gear extension, and the inability of the left main gear to fully lock down under gravity due to an overtight pivot bolt in combination with corrosion and lack of lubrication. The engineer who examined the aircraft noted that intermittent landing gear operation is often a symptom of excessive brush and commutator wear in the pump.