Amateur-built RV 6A N4269Y accident near Petaluma, California

Probable cause

A partial loss of engine power during initial climb after takeoff due to a loose left magneto, and the pilot's decision to turn back to the airport and his exceedance of the airplane's critical angle of attack, which resulted in an aerodynamic stall/spin and impact with terrain.

NTSB analysis

The commercial pilot was departing in his experimental, amateur-built airplane. During the initial climb, when the airplane was between 500 to 800 ft above ground level, the engine backfired several times before losing power. The airplane entered a 180° turn back toward the airport; however, during the second half of the turn, the bank angle increased, and the airspeed slowed. The airplane entered an aerodynamic stall and spin and subsequently impacted terrain.

Postaccident examination of the engine revealed that the left magneto was loose on its mounting pad and free to rotate by hand. This would have resulted in erratic engine-to-magneto timing for that magneto and would likely account for the engine backfiring and the partial loss of engine power. There were no additional mechanical anomalies that would have precluded normal operation. No maintenance records were recovered, and whether any magneto-related maintenance was performed before the accident could not be determined.

The area beyond the departure end of the runway comprised a golf course and open fields that would have been favorable for a straight-ahead, forced landing. The pilot's decision to turn back toward the runway at low altitude and his subsequent exceedance of the airplane's critical angle of attack during the turn resulted in an aerodynamic stall/spin and a loss of control.

Text from the NTSB. Open the NTSB docket for WPR18FA150 for the full record.