Amateur-built Lancair IV P N731SJ accident near Stafford, Virginia

Probable cause

The pilot's decision to install voltage regulators in the engine compartment where they were susceptible to water intrusion, which resulted in a malfunction of the standby electrical system's voltage regulator and a subsequent in-flight fire.

NTSB analysis

The airplane was equipped with a dual battery system. After takeoff the pilot turned on the standby battery switch to charge the standby battery. He smelled smoke which became “very heavy and thick.” After landing, he opened the door and the smoke evacuated. He observed "billowing clouds of black and gray acrid smoke," toward the rear of the airplane. After opening the baggage compartment door, and removing the compartment floor, the pilot discharged a fire extinguisher into the area of the standby battery. He observed a flame, and discharged the fire extinguisher again. He then observed a red glow in the battery box. During construction of the airplane, the pilot had coated the inside of the battery box with room temperature vulcanizing (RTV) silicone. Examination revealed, remnants of a charred wood block (a combustible organic material), and the RTV silicone which had charred, melted, and adhered to the top of the standby battery and its partially melted vent caps. Holes with charring along the upper edge of one side of the battery case were evident where the battery outgassed since its vents caps were blocked. Lead acid batteries must be protected from overcharging and sources of ignition, as hydrogen gas can explode when ignited. Sulfuric acid in contact with combustible and/or organic materials can cause fire or explosion. The airplane’s voltage regulators were mounted in the engine compartment. Their cases were metal, they were rectangular in shape, and a plastic terminal strip protruded from a slot in the case on one side. The regulators were not potted, the internal circuit board was not conformally coated, and no weather seal existed between the plastic terminal strip and the case. Water staining and evidence of arcing were present on the inside of the standby regulator. The manufacturer recommended the regulators be installed in a location that would protect them from heat, vibration, and water on the pilot side of the firewall, or inside the cabin. The pilot was aware of the recommendation, but thought that the heat in the airplane would be worse in the cockpit than in the engine compartment. He also believed the regulators were "sealed in Bakelite," and it would provide "ease of access" to mount them on the firewall.

Text from the NTSB. Open the NTSB docket for NYC08FA292 for the full record.