What happened
On August 8, 2023, a Boeing A75N1 (registration N56907) was involved in an accident near Red Lodge, Montana. The aircraft was being operated as a Part 91 personal flight by a pilot and a passenger.
Shortly after departing runway 16 at Red Lodge Airport (RED), the aircraft crossed a steep hill approximately 200 feet high. During this phase of flight, the airplane began to descend. The pilot reported that while he did not hear an audible decrease in engine RPM, he did not check the engine tachometer. The pilot verified that the throttle and mixture were full forward, the ignition switch was set to both magnetos, and the fuel selector was on.
The aircraft continued its descent and impacted trees. A witness near the scene reported seeing the airplane stop climbing, descend, hit trees, and then nose over. The aircraft came to rest upright in the trees in a nose-low attitude. The impact caused substantial damage to the upper and lower wings and the left horizontal stabilizer. The pilot and passenger were not injured.
The investigation
Mechanical examination of the engine revealed that no spark was present at any of the spark plug leads. The engine utilized a Bendix-Scintilla magneto with dual mechanical points. Upon rotating the magneto drive, only one set of points opened.
Investigators found several issues within the magneto assembly:
- The screws holding the advance points in place were loose, preventing the advance-side points from moving away from the cam to open.
- A P-lead spring found in the advance-side coil area was the incorrect part and appeared to have been modified to fit.
- The advance-side condenser was non-functional.
- The advance-side timing was so far out of specification that it could not be properly measured.
- The retard-side timing was found to be 7.5° from the advance-side timing, whereas the correct specification was 4.5°.
- On the left-hand ignition harness, three spark plug wires failed continuity tests when connected to the distributor.
Maintenance records indicated that the most recent 100-hour inspection occurred on November 2, 2022, and included a check of the magneto timing, with no anomalies recorded at that time.