N56907Boeing A75N1(PT17) · 1941

On August 8, 2023, about 0900 mountain daylight time, a Boeing A75N1 airplane, N56907, was substantially damaged when it was involved in an accident near Red Lodge, Montana. The pilot and passenger were not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight.

The pilot reported that he and a passenger departed runway 16 at the Red Lodge Airport (RED), Red Lodge, Montana. As the airplane crossed over a cliff line, described as a “steep hill about 200 ft,” it began to descend. The pilot did not detect an audible decrease in engine rpm and did not look at the engine tachometer. He checked that the throttle and mixture were full forward, the ignition switch was on both, and the fuel was on. The airplane continued to descend and subsequently impacted trees. A witness, located about 1 mile away, reported the airplane stopped climbing and descended, hit trees, then nosed over.

The airplane came to rest upright in trees, positioned in a nose-low attitude, which substantially damaged the upper and lower wings and the left horizontal stabilizer. Recovery efforts resulted in additional damage to the airplane.

The pilot reported that at the time of departure the temperature was about 60°F and the wind was from the south-southeast at about 4 knots.

Postaccident examination of the engine revealed no spark to any of the spark plug leads. The engine was equipped with a Bendix-Scintilla magneto with dual mechanical points. When the magneto drive was rotated, only one set of points opened. The magneto was shipped to a facility that catered to radial engine magnetos for further examination.

The components of the magneto were identified as belonging to the advance-side or retard-side of the magneto. Examination of the magneto revealed that the retard points had a coat of grease or oil burned onto them, but the points themselves were not burned. The screws that held the advance points in place were loose, which caused the advance-side points to back away from the cam and not open. The yellow tamper indicator was still intact but fell off when the screw was tightened before the bench test run. Figure 1o is a view of the magneto and the dual points configuration.

Figure 1. Bendix-Scintilla Magneto with dual points.

Upon removal of the advance-side coil cover, a P-lead spring fell out. Examination of this spring indicated that it was not the correct part and appeared to be modified to fit the magneto.

A test run of the magneto revealed that the retard-side points began firing at 400 to 500 rpm. According to the maintenance manual, the points should begin firing consistently at 280 rpm. The advance-side points did not fire at all but should have fired at 180 rpm.

Both coils were removed and checked to be operating within tolerance. The advance-side condenser was checked and was not functional. The retard-side condenser was checked and found to be functional.

The magneto drive was placed on a timing wheel to check for internal timing. Examination revealed the advance-side timing was so far off that it could not be properly measured. The retard-side timing was dependent on correct advance-side timing, so as a control, the advance-side was timed properly, and the retard-side timing was then checked. The retard-side timing was found to be 7.5° from the advance-side timing. The correct timing was 4.5° from the advance-side timing for this model of magneto.

Examination of the distributor and spark plug wires revealed no anomalies from the right-hand side. The left-hand side exhibited three spark plug wires with no continuity from the distributor to the end of the plug wire. The spark plug wires were then removed from the distributor and continuity was subsequently verified.

A review of the maintenance logbook revealed that 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.