Cirrus Design Corp SR22 N903SR accident near Pocatello, Idaho

Probable cause

The distributor gear teeth failure of the right magneto that resulted in severe engine vibration and partial loss of engine power, which progressively led to the failure of the left magneto distributor drive gear teeth. Contributing to the airplane’s continued operation with the partial loss of engine power was the pilot’s failure to execute all steps in the engine partial power loss procedure.

NTSB analysis

The pilot reported that, at 17,000 ft mean sea level, the engine suddenly started vibrating severely and partially lost power. He declared an emergency and an air traffic controller provided vectors for an instrument approach into a nearby airport. The pilot stated that the vibrations increased in severity and available engine power was decreasing. The pilot adjusted the mixture and throttle to no effect; he did not cycle the magnetos because he didn't want to risk losing engine power completely. After descending through the 2,000-ft broken cloud layer on the instrument approach, he determined that the airplane was not going to make it to the runway. At 1,000 ft above ground level, he deployed the Cirrus Airframe Parachute System, which brought the airplane down into an open field. He and his passenger rapidly exited the airplane before it was dragged away by the parachute in a 30-knot wind.

Engine data indicated that, 2 hours 56 minutes into the flight, the engine rpm started to fluctuate. Two minutes later, the cylinder head temperature (CHT) of the No. 6 cylinder increased and peaked at 331 degrees F; 7 minutes later, it had decreased to 248 degrees F. At this point, the CHT for the No. 3 cylinder increased to 315 degrees F. About 3 hours 13 minutes into the flight, the engine exhaust gas temperatures (EGT) of cylinder Nos. 2, 4, 5, and 6 dropped off while the EGTs for cylinder Nos. 1 and 3 increased.

Examination of the left and right magnetos revealed that the right magneto distributor drive gear had 10 teeth fractured off in the same gear sector, and the left magneto had 3 teeth broken in the same sector; all of the fracture surfaces on both gears exhibited crack arrest marks and river patterns consistent with progressive fracture.

Based on the right magneto distributor gear damage, it is likely that the failure of the distributor drive gear teeth allowed the magneto distributor to stop rotating in proper firing order and allowed unsequenced repeated firing of the No. 6 cylinder and later the No. 3 cylinder, as reflected by the increase in CHT, which resulted in severe engine vibration and a partial loss of power. The unsequenced firing of the Nos. 6 and 3 cylinders also precipitated erratic power pulses through the engine that affected the left magneto distributor drive gear, which in turn initiated the left magneto distributor gear teeth failure.

The pilot operating handbook lists the steps the pilot should take in the event of an engine partial power loss. Step seven of the engine partial power loss emergency procedures calls for the pilot to cycle through the left and right magnetos using the ignition switch. It is likely that, if the pilot had selected the left magneto after the initial indications of partial power loss and vibration, power could have been restored by isolating the right magneto and operating the engine entirely on the left magneto.

Text from the NTSB. Open the NTSB docket for WPR14FA091 for the full record.