Missoula, MT, USA
N148Z
SHORT BROS SD3 60 SHERPA
The airline transport pilots reported that, during a ferry flight of an airplane that had just had been repainted, while on approach at its destination, the nose gear extended but failed to lock and subsequently collapsed on landing. Postaccident examination of the nose gear confirmed the nose gear's failure to lock, which led to its collapse. Observation of the nose gear down-lock actuator piston revealed that it had been painted, which prevented the piston from engaging and locking the nose gear during extension. Before the flight, maintenance personnel and the flight crew did not observe that the nose gear down-lock actuator piston area was painted. The airplane manufacturer issued a Service Information Letter (SIL) in 2007, that described a similar incident where the nose gear failed to lock in the down position on the first flight after an airplane had been repainted. The subsequent investigation revealed paint spray on the exposed piston of the nose landing gear down-lock actuator. The SIL stated "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 preflight checks to be below the specified pressure. The accumulator was serviced to an adequate pressure prior to takeoff. Subsequently, the pressure bled down during the incident flight, and when the emergency landing gear extension was used by the flight crew, the accumulator did not have adequate pressure. According to the airplane manufacturer, the emergency landing gear extension is reliant on the main system pressure being depleted so that the emergency pressure can change a control valve position. Since the airplane's main hydraulic system was operative during the flight, the inability to use the landing gear emergency extension system, because of the lack of adequate pressure, would have been inconsequential, since the main system pressure was not depleted.
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.
The failure of the nose landing gear down-lock piston to engage due to paint accumulation, which resulted in a nose landing gear collapse during landing. Also causal was the failure of the facility that performed the painting to comply with manufacturer guidance regarding painting of the airplane.
Source: NTSB Aviation Accident Database
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