Primary finding
Probable cause
The total loss of engine power due to the separation of the idler gear shaft bolts as a result of improper installation, which allowed for the idler gear to become disengaged.
Investigator assessment
Analysis narrative
The pilot reported that during the flight, he switched fuel tanks and shortly after, the engine began to sputter. The pilot turned on the electric fuel boost pump and switched the fuel selector valve to the original position, and the engine began to operate normally. However, a short time later, the engine quit, and despite his attempts, he was unable to restore engine power. The pilot initiated a forced landing to a nearby road during which the left wing struck the ground, the airplane veered to the left, exited the surface of the road, and impacted rising terrain. Postaccident examination of the engine revealed that the crankshaft idler gear shaft was separated from its mount and the idler gear was disengaged from the crankshaft and camshaft. Fretting was observed within the area of the idler gear shaft mount. The idler gear shaft bolts were separated. No evidence of a locking plate or safety wire was observed. It’s likely that idler gear bolts had become loose due to improper installation, which allowed the idler gear shaft to shift and the idler gear to become disengaged from the crankshaft and camshaft, which would result in a loss of engine power.
Source record
Factual narrative
On July 26, 2020, about 1110 mountain daylight time, a Vans RV7, N514BS, was substantially damaged when it was involved in an accident near Henefer, Utah. The pilot sustained minor injuries. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The pilot reported that while flying at an altitude of about 10,000 ft mean sea level, he switched fuel tanks and shortly after, the engine began to sputter. The pilot turned on the electric fuel boost pump and switched the fuel selector valve to the original position, and the engine began to operate normally. However, a short time later, the engine began to sputter a second time. Despite his attempts, he was unable to restore engine power and initiated a forced landing to a nearby road. During landing, the left wing struck the ground, and the airplane veered to the left off the surface of the road and impacted rising terrain. Both wings and fuselage were structurally damaged. Examination of the Lycoming IO-540-C4B5 engine by a Federal Aviation Administration inspector revealed that the crankshaft idler gear shaft was separated from its mount and the idler gear was disengaged from the crankshaft and camshaft. The idler gear shaft bolts were separated with no locking tabs or safety wire present. Photos supplied by the inspector showed that both the left idler gear shaft bolt holes were elongated, and the entire mounting area was severely worn and exhibited areas of fretting. The left idler gear interconnects the crankshaft, camshaft, left magneto, and fuel pump drive gears. (See figure 1) Figure 1: View of the idler gear and idler gear shaft assembly.