Aircraft: Piper Arrow III. Injuries: 3 Minor. Location: Kalispell, Mont. Aircraft damage: Substantial.
What reportedly happened: The airplane had just taken off and was at an altitude between 300 and 500 feet AGL over a residential area when the engine started to sputter and lose power.
The pilot selected a long street on which to make a forced landing. He lowered the flaps and slowed the airplane to a minimum controllable airspeed.
During the emergency landing the airplane collided with a number of vehicles and trees. The Piper’s left wing was torn off, then the fuselage embedded in front of a house.
The post-accident 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 wearing would permit the distributor gear to intermittently disengage from the drive gear.
Once the distributor gear 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.
Investigators determined that 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 five years after the date of manufacture or last overhaul, or four years after the date placed in service, whichever occurs first, without regard to accumulated operating hours since new or last overhaul.
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 Identification: WPR12LA092
This February 2012 accident report is provided by the National Transportation Safety Board. Published as an educational tool, it is intended to help pilots learn from the misfortunes of others.