Incident Summary
On 11 April 1999, at approximately 1545 UTC, a Piper PA-34-200T (registration G-BHFH) was at Bembridge Airport, Isle of Wight, for a private flight. The pilot reported that after selecting landing gear down on arrival, no green landing gear indications were observed, though a mirror on the No 1 engine nacelle showed the nosewheel correctly positioned. A fly-past confirmed all three gear appeared extended, and an uneventful landing followed, after which the three green lights illuminated.
During engine start-up for the return flight, the right engine started successfully, but four attempts to start the left engine failed. A strong smell of burning was noted, followed by smoke entering the cabin. External observers saw smoke and flames in the nose area. The pilot shut down the aircraft and ordered an evacuation; another pilot assisted with a fire extinguisher. No injuries occurred.
Examination
Initial examination via the baggage compartment door revealed extensive heat and smoke damage inside the forward baggage hold. The Piper PA-34 series has a fixed GRP nose fairing housing the forward baggage hold, nosewheel bay, battery, and landing gear power pack. The panel separating the nosewheel bay from the baggage hold was largely burnt and melted, allowing visibility of the ground from within the hold. The forward section of this panel lay atop the battery.
The battery was severely heat-damaged: the upper part was melted, exposing plates in several cells, and electrolyte separation was lost. However, the lower case was undistorted, and the recovered electrolyte, though containing black fragments, was clear—differing from typical internal battery failure patterns.
Battery Analysis
Detailed examination of the battery by the AAIB and a battery manufacturer revealed that the negative end was relatively undamaged, while the lid over cells 1–5 at the positive end was melted and burnt. The vent stoppers and rope handle were missing, presumed melted. Top-lead straps appeared undamaged. Part of the positive terminal (wing nut and stud) remained attached to the aircraft connector but separated from the battery. A voltage check after electrolyte removal showed about 9 volts still present, indicating no internal failure.
Investigators concluded that considerable heat had been present at the positive terminal area. A high resistance there, combined with extensive engine cranking, could generate sufficient heat to ignite adjacent combustible materials or residual hydrogen gas from battery charging.
Background
The CAA had issued an Airworthiness Notice (No 12 Appendix 44, 12 November 1990) and later a leaflet (11 to 12 Appendix 24-5 of CAAIP) addressing past problems with battery terminals and connections. The leaflet references FAA General Aviation Airworthiness Alert AC 43-16, which advises proper terminal fit, cleanliness, and torque. At least one previous incident resulted in similar damage. The aircraft had undergone a landing gear investigation shortly before the flight, but the battery was not disconnected, and no cable disconnection or rectification occurred.
Findings
The exact reasons for the landing gear indication problem and the battery terminal overheating could not be definitively determined. A reasonable explanation for the overheating is a poor contact at the positive terminal due to a loose terminal, corrosion, or insufficient wing nut tightening. Such a poor contact, present before the fire, would worsen with each engine start, progressively increasing resistance and heat. Softening of the lead encapsulating the terminal bolt could further degrade contact, eventually leading to arcing and ignition if current was not interrupted.