Grumman Acft Eng Cor-Schweizer G 164C N6674K accident near Maxwell, California

Probable cause

A total loss of engine power due to an uncommanded closing of the fuel shutoff valve, which terminated fuel to the engine. The reason for the fuel shutoff valve’s closure could not be determined based on the available information.

NTSB analysis

The pilot departed the private agricultural airstrip to the south, orbited over the area just west of the airstrip, then proceeded east. Just east of the airstrip, the pilot turned left heading north then made a 180° left turn back toward the south and onto a right downwind in preparation to land to the north. The pilot stated that about midway down and parallel to the runway, the engine surged about two to three times then completely lost power. The pilot elected to perform a forced landing in a plowed field, but during a short landing roll, the airplane nosed over on its back, which resulted in substantial damage to both upper wing panels, both horizontal stabilizers, the rudder, and the elevator.

Postaccident examination of the engine revealed damage consistent with an engine that was not producing power at the time of impact.

Functional tests of the fuel control unit identified fuel schedules that were out of specification limits, and operation of the engine at low power conditions with the out-of-specification fuel control settings most likely resulted in unstable engine operation below 100% engine speed. The fuel shutoff valve (FSOV), which terminates fuel flow to the engine, was found in the electrically commanded closed position after removal from the airplane. Testing of the valve indicated that it required a higher voltage to open than specified, and had a higher leakage through the output port than was specified, but it passed all other tests and neither condition would cause an uncommanded closing of the valve.

The results of the engine teardown examination coupled with the functional testing of various engine components failed to reveal what precipitated the reported engine surge condition before the subsequent loss of engine power.

Text from the NTSB. Open the NTSB docket for WPR18LA128 for the full record.