History of Flight
After an uneventful flight from Rotterdam, the aircraft landed at Norwich Airport and taxied to Stand 5. With brakes applied and before engine shutdown, the pilot began a standard procedure to check the hydraulic system. This involved shutting down the right engine first, leaving the left engine running at idle to supply hydraulic pressure via its engine-driven pump. The pilot then selected the landing gear selector lever to GEAR-DOWN from the GEAR-NEUTRAL position (the lever's normal position after gear-down selection in flight). Within seconds, the nose landing gear (NLG) retracted. As the nose dropped, the left propeller struck the ground, abruptly stopping the engine. The pilot shut down the aircraft and both occupants exited via the rear stair-door, jumping to the ground without injury. Later, the pilot re-entered and found the gear selector at GEAR-NEUTRAL. Inspection revealed both main landing gear doors open and the NLG collapsed.
Hydraulic and Landing Gear System
The Piper Navajo's hydraulic system uses two engine-driven pumps supplying pressure to landing gear and brakes. The landing gear system includes a power pack connected to the selector lever. When selecting gear down, the lever is pulled aft and moved to GEAR-DOWN, held by a release valve. A door solenoid opens the gear doors; pressure then opens a priority valve, directing fluid to lower the gear. Once locked, limit switches close the doors; a time delay valve then returns the lever to GEAR-NEUTRAL. On the ground, a squat switch on the left main gear's upper torque link (compressed when aircraft is on ground) activates an anti-retraction solenoid, preventing the lever from moving beyond neutral. A metal guard also blocks movement past neutral unless the handle is pulled aft against a spring.
Engineering Investigation
Investigation revealed that the squat switch failed to actuate when the gear was compressed; the switch was damaged but the cause could not be determined. Additionally, the spring in the gear selector lever was weak, reducing the handle's ability to return to its stowed position. UK Piper servicing agency indicated this spring weakness is not uncommon. With the defective squat switch, the anti-retraction solenoid remained disengaged. Consequently, when the lever was moved to DOWN during the post-flight check, it began returning to NEUTRAL as expected. However, due to the weak spring and disengaged solenoid, the force applied may have moved the lever past the guard and solenoid balk toward the UP position, initiating retraction. This would unlock the aft-retracting NLG down lock and open the doors. The aircraft's weight kept the sideways-retracting main gear from moving. How the lever returned to neutral after the event was not determined.