Boeing A75N1(PT17) N56907 accident near Red Lodge, Montana

Probable cause

A partial loss of engine power during the initial climb out due to improper maintenance of the magneto.

NTSB analysis

The pilot was departing the airport with a passenger for a local flight when the accident occurred. After takeoff, and as the airplane was crossing a steep drop off in terrain, it began to descend. The pilot verified that the throttle and mixture were full forward, and ensured the ignition switch and fuel selector were both on. He did not detect a reduction in engine speed but did not check the engine tachometer. The airplane continued to descend until it impacted trees, nosed over, and came to rest in the trees, which substantially damaged the wings and the left horizontal stabilizer.

Postaccident examination revealed that one set of magneto points did not open and close when the magneto drive was rotated. Further examination revealed that there was an incorrect P-lead spring installed in the advance-side coil area. The screws that hold the advance-side coil in place were loose, but the yellow tamper indicator was still intact. When placed on a test bench the retard-side points did not begin firing until 400 to 500 rpm, when they should have begun firing at 280 rpm. The advance-side points did not fire at all, and the advance-side condenser did not function. The advance-side timing was so far off that it could not be properly measured, and the retard-side timing was about 3° off proper timing. Examination of the ignition harness revealed three of the nine leads from the left-hand harness did not pass a continuity test when the leads were attached to the distributor but passed when disconnected from the distributor.

The last 100-hour inspection occurred on November 2, 2022, at an airframe time of 559.47 hours, and included checking the timing of the magneto. No anomalies were noted in the maintenance records.

Given the improper maintenance to the magneto, it is likely that the engine ignition system did not provide adequate spark at the correct timing, which resulted in a partial loss of power and subsequent descent into terrain.

Text from the NTSB. Open the NTSB docket for WPR23LA307 for the full record.