History of the Flight
On 31 January 2016, the pilot of Piper PA-28-161 Cherokee Warrior III, registration G-CEXO, had completed two previous flights that day. The first start required 50 seconds of electric priming due to a cold engine, following the recommendation of the aircraft's engineer to address starting difficulties. The second start, with a warm engine, used 25 seconds of priming. During the second flight, the pilot noticed a faint smell of fuel during high-power, low-airspeed phases. After landing, he discussed the smell with two instructors who had also noticed it, and the aircraft was inspected by the engineer, who found no leaks.
For the third start, the engine was still warm, and the pilot again primed for 25 seconds. The engine was reluctant to start, so the pilot leaned the mixture while continuing to crank. When the engine sounded close to starting, he returned the mixture to full rich, and the engine started and ran up to 1,000–1,200 rpm. Witnesses then alerted the pilot to flames outside the aircraft around the lower cowling. He shut down the engine, evacuated with his passenger, and transmitted a MAYDAY before turning off the master switch. A bystander extinguished the fire using a hand-held BCF extinguisher directed into the fuel water drain access hole.
Discussion
The fire was most probably caused by overpriming the engine, leading to a fire in the airbox/carburettor and cowling. The club's standard starting technique typically used only about six seconds of priming, but recent starting problems had led some pilots to prime for up to 50 seconds. The pilot had reservations about the extended priming but was reassured it was acceptable if required to start the engine. The Pilot's Operating Handbook gave no limits on priming duration but recommended no prime initially and to prime as required if the first attempt failed.
The damage was limited to minor paint under and aft of the engine cowling and a scorched ADF aerial.