Amateur-built RV-9A N629BJ accident near Santa Maria, California

Probable cause

A partial loss of engine power during cruise flight for reasons that could not be determined because postaccident examination and testing did not reveal any anomalies that would have precluded normal operation.

NTSB analysis

After the pilot leveled the experimental amateur-built airplane for cruise flight, the engine began to vibrate and lose power. He immediately applied full rich fuel mixture, then conducted a series of troubleshooting procedures, including recycling power to the electronic ignition system and switching between the left and right electronic magneto. The vibration continued with no increase in engine performance, and, unable to maintain altitude, he performed a forced landing into a plowed field, where the airplane sustained substantial damage.

Subsequent examination of the engine revealed no anomalies that would have precluded normal operation. The ignition modules were examined and tested, and, while one unit exhibited wear consistent with imminent failure, both units functioned correctly. If one of the ignition units had failed in flight and lost its timing reference, it could have resulted in the observed engine vibration. However, if this was the case, the troubleshooting steps performed by the pilot would have remedied the problem. During examination, compression was not attained for one of the engine cylinders. This was most likely due to postaccident cylinder contamination and did not cause the loss of engine power.

Text from the NTSB. Open the NTSB docket for WPR12LA141 for the full record.