Piper PA-28R-201T N38906 accident near Kalispell, Montana

Probable cause

The partial loss of engine power due to magneto malfunction. Contributing to the accident was the lack of adherence to the manufacturer’s recommended magneto overhaul schedule.

NTSB analysis

After takeoff, between 300 and 500 feet above ground level over a residential area, the airplane's engine started to sputter and lose power. The pilot selected the longest street on which to make a forced landing, lowered the flaps, and slowed the airplane to a minimum controllable airspeed. The airplane collided with a number of vehicles and trees, and, in the process, the left wing separated from the fuselage. The airplane rotated inverted and embedded itself into the front of a residential house.

Postaccident examination and testing of the left magneto revealed that the magneto’s distributor block bushing was worn to an extent that it provided significant radial play between the bushing and distributor block. The bushing, which holds the distributor gear axle in place, would permit the distributor gear to intermittently disengage from the drive gear. Once the distributor gear had disengaged from the drive gear, the internal timing of the magneto would be off, which could disrupt the normal ignition sequence and operation of the engine. If the pilot had switched to the right magneto, engine power would have likely been restored. The most recent magneto overhaul was performed in 1989. The engine manufacturer recommends that magnetos be overhauled or replaced 5 years after the date of manufacture or last overhaul, or 4 years after the date placed in service, whichever occurs first, without regard to accumulated operating hours since new or last overhaul.

Text from the NTSB. Open the NTSB docket for WPR12LA092 for the full record.