What happened
A commercial pilot was operating a float-equipped aircraft on a V/FR cross-country flight as part of his guiding service. Approximately 20 minutes after departing one lake for another, the engine power dropped to idle RPM. The pilot attempted to push the throttle forward and maneuvered the aircraft toward a nearby lake; however, the airplane descended into trees before reaching the water. The impact caused extensive damage to the wings and fuselage. The pilot reported no known mechanical issues with the aircraft prior to the flight.
The investigation
An examination of the aircraft revealed that the support shaft for the throttle and mixture bell cranks was missing. This loss of the shaft disconnected the throttle control, which allowed the engine to transition to idle RPM. Investigators also inspected the air induction box assembly (PN 1650012-1). While the air box itself was the correct part, it had undergone a weld repair around the shaft support bushings on both sides. During this repair, the original solid shaft bushings were replaced with earlier generation roller bearings. This improper installation prevented the support shaft from being properly pinned in place and removed the redundant safety features of the original design. The incorrect configuration allowed the shaft to migrate out of the air box.
Findings
- The missing support shaft disconnected the throttle control, causing the engine to go to idle.
- The air induction box had been repaired using incorrectly substituted parts, specifically replacing solid bushings with roller bearings.
- Maintenance log books contained no entries regarding the repair performed on the air box assembly.