What happened
On 22 December 2002, a Piper PA-31 Navajo Chieftain, registration PH-MBL, arrived at Norwich Airport from Rotterdam for a private flight. After taxiing to Stand 5, the pilot began standard post-flight procedures to test the aircraft's hydraulic system. This procedure involved shutting down the right engine while keeping the left engine running at idle to provide hydraulic pressure via the engine-driven pump.
During the test, the pilot moved the landing gear selector lever from the gear-neutral position to the gear-down position. While the pilot expected the lever to return to neutral after the cycle, the nose landing gear instead retracted, causing the nose of the aircraft to drop to the ground. This impact caused the left engine propeller to strike the runway, resulting in a sudden engine stoppage and shock loading. The main landing gear doors also opened during the event. The pilot and the single passenger exited the aircraft via the rear stair-door by jumping to the ground; no injuries were sustained by either occupant.
The investigation
Investigators examined the hydraulic and landing gear systems to determine why the gear retracted while the aircraft was on the ground. The investigation focused on the functionality of the squat switch and the landing gear selector lever mechanism.
Technical analysis revealed that the squat switch, located on the left main landing gear upper torque link, had failed to activate despite the aircraft being in a compressed state on the ground. Additionally, the spring within the gear selector lever was found to be weak, which hindered the handle's ability to return to its proper stowed position.
Findings
- The defective landing gear anti-retraction (squat) switch failed to signal the anti-retraction solenoid, meaning the system did not provide a mechanical balk to prevent the lever from moving into the up position.
- A weak spring in the gear selector lever reduced the resistance needed to keep the handle from moving past the neutral position.
- The combination of these two failures allowed the selector lever to move past the gear-neutral position and into the retraction sequence during the hydraulic check.
- The nose gear collapse and subsequent propeller strike caused damage to the nose gear doors, the left propeller, and the engine.