On October 13, 2016, about 1645 mountain daylight time, a Short Bros SD3-60 Sherpa airplane, N148Z, sustained minor damage following a nose gear collapse during landing, at the Missoula International Airport (MSO) Missoula, Montana. The two airline transport pilots, were not injured. The airplane was registered to and operated by 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, about 1253.
The pilots reported that prior to landing, they had an unsafe nose gear indication. After multiple unsuccessful attempts to get the nose gear to extend and indicate that it was down and locked, they decided to come in for a landing. During the landing roll, as the airplane's nose was lowered, the nose gear collapsed, and the forward section of the bottom of the fuselage, made contact with the runway surface. Once the airplane came to a stop, both pilots egressed.
Examination of the airplane by the operator revealed minor damage on the underside of the fuselage.
The airplane was returning to MSO, on its first flight, after being re-painted by a vendor in Kingman, Arizona. According to the operator, examination of the nose gear revealed that it failed to lock in the extended position and collapsed during landing. The flight crew attempted to extend and lock the nose gear during the flight but were unsuccessful.
Post incident examination of the nose gear down-lock actuator piston (plunger), revealed that its chrome surface had been painted, which resulted in the locking piston not engaging, and prevented the locking of the nose gear during landing gear extension. The painting of the nose gear down-lock actuator piston was not observed by maintenance personnel or the flight crew on their pre-flight exterior checks.
The airplane manufacturer Shorts issued a SD3-60 Sherpa Service Information Letter (SIL) 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. The SIL described a similar incident where the nose gear failed to lock in the down position, on the first flight after the airplane had been re-painted. The subsequent investigation revealed paint spray on the exposed chrome piston of the nose landing gear down-lock actuator. The SIL stated "to ensure that all masking material has been removed from the exposed piston after cleaning/painting, and that the piston is free from any paint or cleaning materials."
Additionally, the landing gear emergency accumulator was observed during the pre-flight checks to indicate below the specified pressure. The accumulator was serviced by maintenance to an adequate pressure, prior to 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, when activated, a fully serviced emergency accumulator forces the rapid application of hydraulic pressure into the system, however, according to the airplane's manufacturer, this is reliant 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 be greater than the residual pressure of the failed main hydraulic system.