LAX04LA334 · British Aerospace BAE 125-1000A

The business jet experienced a fire in the aft equipment bay while holding short for takeoff. The two-person aircrew were performing a positioning flight. While the airplane was holding short for takeoff, the crew noticed a Master Caution and a Pack & Aux Overheat warning light illuminated on the cockpit annunciator panel, followed by a Rear Baggage Smoke Detector warning light. The crew shut down the APU. The captain declared an emergency and the crew evacuated the airplane. Fire crews arrived shortly thereafter, and determined that the fire had extinguished itself. Examination of the aft pressure bulkhead by the manufacturer revealed metal discoloration and soot residue, and it was determined that there was structural damage to the aft pressure bulkhead due to a heat induced loss of material temper. After the incident the operator inspected the aft equipment bay and identified a hydraulic line with a pinhole leak. The stainless steel hydraulic line had a small crater like deformation with evidence of recast metal and a 0.005-inch hole at the base of the crater, which is consistent with damage caused by an electrical arc. There was no evidence of chafing on the hydraulic line. Copper, cadmium, and nickel material were acid rinsed from the crater; none of which are listed as alloying element in stainless steel. Copper wire is commonly used as an electrical circuit conductor. A pinhole leak could be expected to atomize the hydraulic fluid that was at 3,000 psi. The hydraulic system working fluid used was Aeroshell Fluid 41, which has a reported flash point between 221F and 230F. The electrical looms/cables that were in the aft equipment bay were disposed of prior to the Safety Board initiating the accident investigation; therefore, determination or identification of the electrical arc source was not possible.

Verbatim NTSB analysis and probable cause from the NTSB dataset