What happened
During a commercial passenger flight on 26 May 2017, a DHC-8-402, registration G-PRPH, was descending through 12,000 feet when the flight crew received a triple chime and a red warning indicating low oil pressure on the number two engine. The pilot observed the pressure dropping and, despite attempting to reduce power, decided to shut down the engine to prevent further issues. The crew followed standard emergency procedures, including the NITS briefing for cabin crew and notifying air traffic control. The aircraft subsequently completed a successful single-engine landing at Manchester Airport without further incident.
The investigation
Post-flight inspections of the number two engine revealed that a cap on the propeller overspeed governor test solenoid had detached. This separation allowed the engine's lubrication oil to leak under pressure, depleting most of the system's contents. Investigators examined the component and found that the four bolts securing the cap to the governor unit had failed. Laboratory analysis of the bolts indicated that the failure was primarily due to fatigue, with one bolt showing evidence of a complex fracture and another showing a transition from fatigue to overload. While the investigation noted that the wire locking orientation was slightly unusual, it was found to be within acceptable parameters.
Findings
- The loss of oil pressure was caused by the detachment of the solenoid valve cap on the propeller overspeed governor.
- The attachment bolts failed predominantly due to fatigue.
- Evidence of fretting on the mating surfaces suggested that reduced bolt tension had existed for an extended period.
- The investigation could not definitively determine the root cause of the fatigue, though the possibility of insufficient torque during assembly could not be ruled out.
- The engine's vibratory environment likely contributed to the cyclic loading that led to the fatigue failure.