Background
The aircraft, a Fletcher registered ZK-BHT, was fitted with a 6.6 hp Kohler 4-stroke fan-cooled auxiliary power unit (APU) to drive the spray system. This arrangement had been approved for spraying non-inflammable liquids by the Airworthiness Division of the CAA. However, after January 15, 1957, the operator began using the aircraft to spray diesel fuel for burning-off purposes without informing or seeking approval from the Airworthiness Division for the use of an inflammable liquid.
Accident Sequence
At 1345 on the day of the accident, diesel oil was loaded for dispersal over a scrub-covered ravine on a farm in the Omahina Valley near Waverley. After the fifth sortie, the pilot reported to his loader driver that during the flight he had briefly operated the hopper jettison control to place a heavier deposit of diesel on thick scrub. The cockpit immediately filled with dense, black smoke, leading him to believe the aircraft had caught fire. Inspection revealed a film of diesel oil inside the fuselage aft of the hopper, extending to the tail cone, as well as on the underside of the fuselage. A leaking connection in the supply pipe to one of the spray booms was tightened, and the oil was mopped up as best possible. The operation continued with two uneventful sorties.
On the eighth sortie, as the Fletcher climbed out of the ravine to land on a strip 300 to 400 feet above, two witnesses observed a plume of flame from the underside of the fuselage, about midway between nose and tail. The aircraft was seconds from landing. It leveled out, turned 90 degrees to the right, and disappeared behind a ridge. After turning away from the approach, the aircraft plowed through trees on a steep slope, dived almost vertically, and then subsided to a more level position. The right wing detached, and a small fire broke out in the engine bay. This ignited fuel from a broken fuel line from the right wing tank, augmented by diesel from the hopper when the jettison system was sheared off. Additionally, petrol from the Kohler APU's tank added to the blaze when the vent pipe from that small tank was broken off.
Probable Cause
The accident inspector concluded that a fire had broken out in the air, causing an emergency that affected the pilot's ability to fully control the aircraft. This fire was most probably caused by the ignition of oil fumes and residue in the interior of the fuselage, with the source of ignition being the exhaust pipe of the APU, which had been seen to become red hot at times.