What happened
On October 13, 2016, a Short Bros SD3-60 Sherpa, registration N148Z, was performing a ferry flight from Kingman, Arizona, to Missoula, Montana. The aircraft, operated by the USDA Forest Service, was returning to Missoula International Airport (MSO) after undergoing a repainting process.
During the approach, the flight crew received an unsafe nose gear indication. Despite multiple attempts to extend and lock the gear, the crew was unable to secure a down-and-locked indication and proceeded to land. During the landing roll, the nose gear collapsed as the aircraft's nose was lowered, causing the underside of the fuselage to contact the runway. The two pilots were not injured.
The investigation
An examination of the aircraft revealed minor damage to the bottom of the fuselage. Investigators found that the nose gear down-lock actuator piston (plunger) had been coated with paint. This paint accumulation prevented the locking piston from engaging properly, which prevented the gear from locking in the extended position.
Maintenance and flight crew pre-flight inspections had not identified the paint on the chrome surface of the piston. The investigation also noted that the landing gear emergency accumulator pressure had bled down during the flight. While the accumulator was serviced to adequate pressure before takeoff, it lacked sufficient pressure when the crew attempted to use the emergency extension system. However, because the main hydraulic system remained operative, the lack of pressure in the emergency accumulator was not a factor in the gear's failure to lock.