What happened
On 11 April 1999, a private flight involving a Piper PA-3/4-200T, registration G-BHFH, arrived at Bembridge Airport, Isle of Wight. Upon arrival, the pilot noted a lack of green landing gear indications, although visual inspections via an engine nacelle mirror and a fly-past of the control tower suggested the gear was correctly extended. Following a successful landing, the crew prepared for a return flight.
During the engine start-up sequence for the second leg, the right engine started without issue, but the left engine failed to start after four attempts. Shortly thereafter, a strong burning smell and smoke entered the cabin. External observers also witnessed flames and smoke emanating from the aircraft's nose area. The pilot immediately shut down the engines and ordered an evacuation. The fire was addressed by the pilot of another aircraft using an extinguisher, followed by the use of the aircraft's own onboard extinguisher.
There were no fatalities and no injuries to the two occupants or the two dogs on board.
The investigation
Investigators examined the nosecone and forward baggage compartment, finding extensive heat and smoke damage. The inspection revealed that the battery compartment cover had melted, making the ground visible from inside the baggage hold. The aircraft battery itself had sustained severe thermal damage; the upper portion of the casing had melted, and the internal cell structure was compromised, allowing electrolyte to flow between cells.
Technical analysis by the AAIB and battery specialists determined that the lower battery casing remained undistorted and the electrolyte was clear, which ruled out a primary internal battery failure. Instead, the damage was concentrated at the positive terminal. The investigation also noted that the aircraft had recently undergone a landing gear system check where the battery remained connected during testing.
Findings
- The fire originated in the region of the positive battery terminal.
- The investigation identified that high electrical resistance at the positive terminal likely caused significant heating during the repeated engine cranking attempts.
- This heating could have ignited adjacent combustible materials or residual hydrogen gas produced during battery charging.
- Potential causes for the high resistance included a loose terminal, corrosion, or improper tightening of the wing nut, all of which would degrade electrical contact over time.
- The investigation could not definitively determine the exact cause of the initial landing gear indication issue or the precise origin of the terminal's overheating, but noted that the thermal process likely involved a progressive degradation of the connection through repeated use.