What happened
On April 26, 2009, at approximately 0415 EDT, a Douglas DC3, registration N136FS, was taxiing on the "juliet" taxiway toward runway 10 at Luis Munoz Marin International Airport (TJSJ) in San Juan, Puerto Rico. The aircraft, operated by Four Star Aviation Inc., was conducting a cargo flight to Charlotte Amalie, United States Virgin Islands, under 14 CFR Part 135 regulations.
While taxiing, the instrument panel and cockpit floor suddenly erupted in flames, and smoke filled the cockpit. The crew, which included an airline transport pilot, a commercial pilot, and two crewmembers, attempted to shut down the engines. Due to the intensity of the fire and smoke, the pilots and two cargo handlers evacuated the aircraft. There were no injuries to the four occupants. Weather conditions at the time were visual meteorological conditions (VMC) with a 100/04kt wind and 10 miles of visibility.
The investigation
FAA inspectors examined the wreckage and found intense fire damage extending from the bulkhead behind the pilots' seats to the front of the airplane. The damage was severe enough to melt instruments, switches, tubing, hoses, and the aircraft skin, with more significant damage noted on the right side of the fuselage. The left propeller was found partially feathered.
Investigators focused on the main junction box, where they found that most wires were relatively undamaged except for two specific wires that were missing insulation. These wires, which were connected to the battery relay, ran through exposed terminal studs and toward the forward cockpit area where the fire originated. Heat damage was observed on other components in contact with these wires. While the fire prevented investigators from identifying the specific system these wires served, it was noted that the propeller feathering system is one system connected to the battery relay.
Additionally, inspectors examined the fuel pressure indicating system, which uses rigid aluminum lines routed on the right lower side of the fuselage. Maintenance records showed the aircraft had undergone a Phase D inspection on July 14, 2008, and a Phase B inspection on March 22, 2009. At the time of the accident, the aircraft had flown 313.1 hours since its last Phase D inspection.