Incident Overview
On October 13, 2016, at about 1645 mountain daylight time, a Short Bros SD3-60 Sherpa airplane, registration N148Z, experienced a nose gear collapse while landing at Missoula International Airport (MSO), Missoula, Montana. The two airline transport pilots were not injured. The airplane was registered to and operated by the United States Department of Agriculture (USDA) as a public aircraft in support of the Forest Service. Visual meteorological conditions prevailed, and a visual flight rules (VFR) flight plan was filed for the ferry flight, which originated from Kingman Airport, Kingman, Arizona, at approximately 1253.
Sequence of Events
Prior to landing, the pilots received an unsafe nose gear indication. After multiple unsuccessful attempts to extend the nose gear and confirm it was down and locked, they decided to proceed with the landing. During the landing roll, as the airplane's nose was lowered, the nose gear collapsed, causing the forward underside of the fuselage to contact the runway surface. The airplane came to a stop, and both pilots egressed.
Examination Findings
Examination by the operator revealed minor damage on the underside of the fuselage. The nose gear was found to have failed to lock in the extended position and collapsed during landing. The flight crew had attempted to extend and lock the gear during the flight without success.
Post-incident examination of the nose gear down-lock actuator piston (plunger) showed that its chrome surface had been painted. This paint prevented the locking piston from engaging, thus preventing the nose gear from locking when extended. The painting of the piston was not detected by maintenance personnel or the flight crew during pre-flight exterior checks.
Related Service Information
The manufacturer, Shorts, had issued Service Information Letter (SIL) SD3-60 Sherpa L-23 in April 2007, titled "Servicing/Painting – Removal of masking material/paint from nose landing gear down-lock actuator piston after cleaning/painting operations." This SIL described a similar incident where the nose gear failed to lock after re-painting. The investigation in that case also found paint spray on the exposed piston. The SIL emphasized ensuring that all masking material is removed and the piston is free of paint or cleaning materials.
Additional System Issue
During pre-flight checks, the landing gear emergency accumulator was observed to be below the specified pressure. Maintenance serviced it to an adequate pressure before takeoff. However, the pressure bled down during the incident flight, and when the emergency landing gear extension was used, the accumulator did not have adequate pressure. According to the operator's maintenance inspector, a fully serviced emergency accumulator would force rapid hydraulic pressure into the system, but the airplane's manufacturer stated that this depends on the main system hydraulic pressure being depleted so that the emergency pressure can change a control valve position. In this situation, the emergency pressure would have been greater than the residual pressure of the failed main hydraulic system.