N29304Cessna 177 · 1968

The pilot submitted a written statement and reported that following a successful runup, he departed from the airport. During the climb, the engine began to experience a partial loss of power and the pilot believed that it was due to vapor lock. The pilot verified the cockpit settings and leaned the mixture. The engine returned to normal power. Approximately 30 seconds later it happened again so the pilot leaned the mixture again. Power returned, but then it happened a third time. The pilot contacted the tower because the airplane was not climbing. He requested a return to the airport and turned back. Just after making the turn, the engine lost total power. The pilot force-landed the airplane in a field and the airplane impacted a fence. During the accident sequence, the firewall was buckled and the right wing leading edge was damaged. Post accident examination of the airplane and test run of the engine revealed no mechanical anomalies. Just prior to departure, the pilot had added 10 gallons of fuel, for a total of 22 gallons onboard the airplane, and no contamination was observed. The temperature at the time of the accident was 102 degrees Fahrenheit.

According to FAA publication Advisory Circular (AC) AC 65-12A, fuel normally remains in a liquid state until it is discharged into the air stream and changes into a vapor. Under certain conditions the fuel may vaporize in the lines, pumps, or other fuel components. The vapor pockets formed by the premature vaporization can restrict the fuel flow through units that were designed to handle liquids rather than gases. The resulting partial or complete interruption of the fuel flow is called vapor lock. The AC further states that transfer of heat from the engine tends to cause vaporization of fuel in the lines and the pump. This tendency is increased if the fuel in the tank is warm, commonly as a result of high atmospheric temperatures. Vapor lock can become serious enough to block the fuel flow completely and stop the engine. Even small amounts of vapor in the inlet line can restrict the fuel flow to the engine driven pump and ultimately reduce its output pressure.