Fairbanks, AK, USA
N4112K
Piper PA31
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.
On January 12, 2020, about 1223 Alaska standard time, a Piper PA-31-350 airplane, N4112K, sustained substantial damage when it was involved in an accident at Fairbanks International Airport (FAI), Fairbanks, Alaska. The airline transport pilot and seven passengers were uninjured. The airplane was operated as a Title 14 Code of Federal Regulations Part 135 scheduled commuter flight. The pilot reported that as the airplane approached the destination airport, the landing gear did not extend, the landing gear indicator lights did not illuminate, and the selector did not return to the neutral position. She repeated the landing gear extension procedures again; however, the landing gear still did not extend. She then checked that all the circuit breakers were in and used the checklist to perform the emergency landing gear extension procedure. The pilot pumped the hand-operated emergency gear extender about 70 times, but there was low feedback pressure in the system. When the landing gear still did not extend, she diverted back to FAI, where emergency services and a maintenance crew were available. As the airplane approached FAI, the pilot activated the emergency blowdown pneumatic system for both the main and nose landing gear. The green down-and-locked indicator lights for the nose and left main landing gear illuminated, but the right main landing gear (MLG) light did not illuminate. The pilot performed a fly-by of the control tower and the tower controller reported that the right MLG door was open, but the gear remained retracted. The pilot then flew to a nearby training area, climbed to 5,000 ft, and attempted to swing the right MLG out with higher-G maneuvers, but was unsuccessful. The pilot performed a straight-in visual approach to FAI runway 2L. Once the landing was assured, she shut down and secured both engines. During the landing roll, the right wing impacted the runway, resulting in substantial damage to the right wing and aileron. All occupants evacuated normally. The airplane was recovered to the company hangar, where a Federal Aviation Administration inspector examined the hydraulic system. The hydraulic reservoir was almost depleted and minimal fluid existed in the system. After servicing and testing the hydraulic system, a leak was observed at the fitting between the right main landing gear door close line and the right MLG unlock actuator. The hydraulic line was removed, and the threads on the actuator assembly connection were stripped. The fitting was capped and the system functioned normally. The company also tested the pneumatic blowdown system and discovered that the right MLG shuttle valve was stuck in place. The shuttle valve was removed and disassembled, and water and corrosion were present inside. According to the maintenance records, all of the landing gear hydraulic lines were replaced during a recent phase inspection about one week before the accident.
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.
Source: NTSB Aviation Accident Database
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