Incident
A business jet was holding short for takeoff during a positioning flight with a two-person aircrew when the crew noticed a Master Caution and a Pack & Aux Overheat warning light illuminated on the cockpit annunciator panel. This was followed by a Rear Baggage Smoke Detector warning light. The crew shut down the APU and the captain declared an emergency, after which the crew evacuated the airplane. Fire crews arrived shortly and determined that the fire had extinguished itself.
Investigation
Examination of the aft pressure bulkhead by the manufacturer revealed metal discoloration and soot residue. 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, consistent with damage caused by an electrical arc. No evidence of chafing on the hydraulic line was found. Copper, cadmium, and nickel material were acid rinsed from the crater; none are listed as alloying elements in stainless steel. Copper wire is commonly used as an electrical circuit conductor.
Findings
A pinhole leak in a hydraulic line at 3,000 psi could be expected to atomize the hydraulic fluid, which was Aeroshell Fluid 41 with a reported flash point between 221°F and 230°F. The electrical looms and cables in the aft equipment bay were disposed of prior to the Safety Board initiating the investigation; therefore, determination or identification of the electrical arc source was not possible.