What happened
On 28 July 2013, a Piper PA-34-200T Seneca II, registration G-CLUE, was conducting a private flight when the pilot identified a malfunction in the landing gear indication. While attempting to land at a private airstrip approximately 20 nm from Durham Tees Valley Airport, the pilot noted that the green down-and-locked indicator for the left main landing gear failed to illuminate, though the nose and right gear appeared secure.
After attempting to rectify the issue by recycling the gear selection and checking fuses, the pilot decided to return to Durham Tees Valley Airport. Upon arrival, the pilot requested a fly-by of the control tower to visually confirm the gear status. The tower reported that all landing gear appeared to be retracted. Following further attempts to extend the gear via the cockpit controls, the pilot opted to use the emergency extension system.
During the approach, the pilot deployed the emergency extension, which allowed the gear to drop via gravity. Although the tower confirmed all three gears appeared down, the cockpit indicators remained unconfirmed. The pilot performed a gentle touchdown at stall speed. However, during the landing roll, as braking was applied, the left main landing gear collapsed. The aircraft's left propeller made contact with the runway, causing damage to the engine, propeller, landing gear, and airframe components. There were no fatalities and no injuries to the two crew members.
The investigation
Investigators examined the aircraft's hydraulic and mechanical systems to determine why the gear failed to lock. The inspection of the hydraulic pump revealed that the electric motor brushes had burned out and the commutator was excessively worn.
Regarding the mechanical failure of the downlock, the investigation found that the left main landing gear remained partially retracted. The centre pivot bolt was found to be overtightened, and the mechanism suffered from corrosion and a lack of lubrication. This stiffness prevented the spring-loaded downlock hooks from engaging properly during the gravity-fed emergency extension.
Findings
- The primary cause of the gear extension failure was the failure of the electro-hydraulic pump due to worn brushes and a damaged commutator.
- The subsequent collapse of the gear was caused by the inability of the left main landing gear to fully lock into position.
- This mechanical failure was attributed to a combination of an overtightened pivot bolt, corrosion, and insufficient lubrication within the downlock mechanism.