What Happened
On the evening of 16 November 2021, a flight instructor and a student pilot were preparing for a night navigation training flight at Cape Town International Airport (FACT). The aircraft, a Piper PA-28-161 Warrior II with registration ZS-KBK, was operated by Cape Town Flying Club under Part 141 training rules. The flight instructor had earlier completed a 1.9-hour training flight with another student on the same aircraft, after which no defects were recorded in the flight folio. After a 35-minute safety briefing, the duo conducted a pre-flight inspection and found no abnormalities. The aircraft had 36 US gallons of Avgas LL100 fuel.
Engine Start and Fire
The student pilot attempted to start the engine following the Pilot's Operating Handbook (POH) procedure. The first start attempt failed, and after a cool-down period, a second attempt also failed. Shortly thereafter, the flight instructor observed flames emanating from the lower engine cowling. The fire intensified. The instructor stopped the cranking, switched off the electrics, leaned the mixture, closed the throttle, and turned off the ignition. He retrieved the hand-held Halon fire extinguisher but was unable to remove the safety pin as it was stuck. The pair exited the aircraft and shouted for help. A nearby pilot from another aircraft brought his fire extinguisher, successfully removed its safety pin, and extinguished the fire. The aerodrome Aircraft Rescue and Fire-fighting (ARFF) team arrived shortly after and secured the scene. The aircraft sustained substantial damage; neither occupant was injured.
Post-Accident Examination
The aircraft maintenance organisation examined the aircraft and tested the fuel system, finding no leaks or incorrect operations. Fire damage was concentrated on hoses and electrical wiring along the right side of the engine, as well as the engine mount and the base of the bottom and top cowlings. Examination of maintenance records and the flight folio revealed no abnormalities, open defects, or deferred maintenance items. The aircraft was certificated and maintained in accordance with regulations.
The report noted that the likely source of the fire was excessive fuel pumped into the carburettor venturi by the accelerator pump when the throttle lever was advanced. With the engine stopped, excess fuel could drip down and pool in the carburettor heat box. The heat from the hot exhaust system could then vaporise the pooled fuel and act as an ignition source.
Probable Cause
The investigation determined that it was likely that during engine start, the pilot most probably over-primed the engine, causing excess fuel to drip down and collect in the heat box. As the engine was still hot, this resulted in fuel vapour catching fire. A contributing factor was incorrect engine start technique.