Piper PA31 N4112K accident near Fairbanks, Alaska

Probable cause

The improper installation of a hydraulic line at the right main landing gear (MLG) unlock actuator, which resulted in a total loss of system hydraulic pressure and failure of the landing gear to extend. Contributing to the accident was the failure of the right MLG shuttle valve due to corrosion, which prevented the right MLG from extending during the emergency blowdown activation.

NTSB analysis

The pilot was approaching the destination airport for landing when the landing gear did not extend. The pilot utilized the manual pump system, the emergency pneumatic blowdown system, and conducted maneuvers at altitude to extend and lock the gear down; the nose and left main landing gear (MLG) extended and locked, but the right MLG did not. The pilot subsequently landed with the right MLG retracted, and the right wing impacted the runway during the landing roll, resulting in substantial damage.

An examination of the hydraulic system revealed no fluid in the hydraulic reservoir and a leak at the right MLG unlock actuator due to a loose fitting that exhibited stripped threads at the connection. The right MLG hydraulic shuttle valve was stuck and unable to move. After disassembly, water and corrosion was evident inside the cylinder.

The hydraulic lines were all replaced during a phase inspection about one week before the accident.

It is likely that the hydraulic line fitting to the right MLG unlock actuator was insufficiently tightened or was cross-threaded during installation, which resulted in wear of the threads after numerous landings. The subsequent loss of system pressure prevented the landing gear from extending normally and with the hand pump.

How water intruded into the shuttle valve could not be determined, but the resulting corrosion likely prevented the valve from moving during the emergency blowdown system activation, which prevented pneumatic pressure from activating the right MLG actuator. The condition was not evident until the emergency system was utilized.

Text from the NTSB. Open the NTSB docket for ANC20LA013 for the full record.