The airline transport pilot reported that, during a formation flight, the Yak 52’s engine started to run rough. Unable to maintain level flight, he detached from the formation to troubleshoot the problem.
He noted there were about 29 gallons of useable fuel onboard and all other indications were normal. The engine continued to lose power, and about 500′ above the ground, he elected to land in a vineyard in Porterville, California.
During landing, the airplane touched down, came to an abrupt stop, nosed over, and came to rest inverted.
Post-accident engine examination revealed small pieces of fabric in the fuel screen. In addition, some metal shavings and a small piece of metal were found in the fuel intake and carburetor inlet screen.
The origins of the metal piece and shavings could not be determined.
It is unlikely that the observed debris was enough to clog the fuel system, however it is possible that there were other, non-observable pieces of debris in the fuel system that could have resulted in decreased fuel flow and the partial loss of engine power.
Probable cause: A partial loss of engine power due to contaminants in the fuel system that decreased the fuel flow, the source of which could not be determined.
NTSB Identification: WPR17LA094
This April 2017 accident report is provided by the National Transportation Safety Board. Published as an educational tool, it is intended to help pilots learn from the misfortunes of others.