Incident Overview
On September 24, 2015, about 1515 Pacific daylight time, a Cessna 177 Cardinal, registration N3416T, experienced an on-ground fire while the pilot attempted to start the engine at Jacqueline Cochran Regional Airport in Thermal, California. The owner, who was also the passenger, was operating the airplane under 14 CFR Part 91. The private pilot and passenger were not injured; the airplane sustained substantial damage.
The cross-country personal flight was to originate from Thermal with a planned destination of Boulder Municipal Airport, Colorado. Visual meteorological conditions prevailed and no flight plan was filed.
Pilot Account
The pilot reported that the airplane was based in Boulder, Colorado, and earlier that morning he and his passenger completed a flight from there to Palm Springs, California. They departed Palm Springs and landed in Thermal with the intent of purchasing fuel. After shutting down the engine, they decided not to refuel at that location and moved to the other side of the field, where they topped off the wing tanks with 25 gallons.
Following fueling, the pilot attempted to start the engine, but it would not turn over. With the mixture in the full rich position, he pumped the primer once and continued cranking. The pilot and passenger observed a fire start in the muffler area. The pilot cranked the starter in an attempt to suck the fire through the muffler. The passenger exited the airplane and discharged the entire contents of a fire extinguisher on board. The pilot exited from the rear of the airplane. At no time did the engine actually start, and the propeller was not turning. The pilot noted that the engine was running very hot that day as seen from the oil temperature gauge, which he thought was due to outside air temperatures reaching over 100 degrees Fahrenheit and the duration of the flight.
Post-Accident Examination
A certified airframe and powerplant mechanic completed a teardown examination under the supervision of Federal Aviation Administration inspectors. Fuel system continuity was established from the firewall to each cylinder. The boost pump was activated with no anomalies. Removal of the spark plugs revealed all were uniformly dark in color, indicating an oil-fouled operating condition, but a compression check showed normal operations. Examination of the combustion chambers through the upper spark plug holes using a lighted borescope showed no mechanical damage and no evidence of foreign object ingestion or detonation. No valve-to-piston face contact was observed. The gas path and combustion signatures at the spark plugs, combustion chambers, and exhaust system components displayed coloration consistent with a rich operation. No oil residue was present in the exhaust system gas path.
The fire appeared to have started on the lower right side of the engine, where there was white coloration and excessive heat damage to the firewall and belly skin in that area. The mechanic opined that the source of fuel that ignited the fire was likely due to the pilot overpriming the induction system, which would have filled the carburetor air box with fuel located in that position.
Pilot's Operating Handbook Guidance
According to the airframe Pilot's Operating Handbook, "No priming is required when the weather is warm," and that "weak intermittent firing followed by puffs of black smoke from the exhaust stack indicates over-priming or flooding." It further states that "excess fuel can be cleared from the combustion chambers by the following procedure: Set the mixture control full lean and the throttle full open; then crank the engine through several revolutions with the starter," and to "repeat the starting procedure without any additional priming."