Aeronca 7AC N2659E accident near Novato, California

Probable cause

The pilot's use of a non-vented fuel cap, which resulted in a total loss of engine power due to fuel starvation.

NTSB analysis

The pilot reported that while in cruise flight, the engine suddenly started running rough and the engine rpms rapidly decreased from 2,100 to 1,900. The pilot immediately applied full carburetor heat, but the engine continued running rough so he headed toward the nearest airport. While on the base leg of the traffic pattern, the engine lost all power. The airplane subsequently touched down short of the runway and collided with a fence. Postaccident examination of the wreckage found that the left auxiliary fuel tank was empty and the right auxiliary tank was about one-third full. The auxiliary tank fuel selector under the instrument panel was found in the off position. The main fuel tank, which was configured to feed the engine, was full and the fuel indicator also indicated full. Further examination of the engine revealed no evidence of mechanical malfunctions or failures that would have precluded normal operation.

The pilot reported that the main fuel tank fuel cap was misplaced when the airplane was refueled. The pilot then borrowed a fuel cap from a local mechanic and installed it on the main tank filler port. An examination of the airplane revealed that the borrowed fuel cap was a non-vented type; because there was no other vent for the main tank, the only vent would be through a vented cap. As fuel was used from the main tank, the lack of a vented cap caused a vacuum within the main tank that eventually reduced the fuel flow, starving the engine. Had an auxiliary tank been selected, adequate venting would have occurred because both auxiliary tanks had vented fuel caps.

Text from the NTSB. Open the NTSB docket for WPR11LA131 for the full record.