What happened
During a positioning flight, the two-person crew of a business jet was holding short for takeoff when they observed several warnings on the cockpit annunciator panel. These included a Master Caution, a Pack & Aux Overpend warning light, and a Rear Baggage Smoke Detector warning light.
In response to the indications, the crew shut down the Auxiliary Power Unit (APU). The captain declared an emergency, and the crew evacuated the aircraft. Upon arrival, fire crews determined that the fire had extinguished itself.
The investigation
An examination of the aft pressure bulkhead by the manufacturer revealed soot residue and metal discoloration. It was determined that the heat caused a loss of material temper, resulting in structural damage to the bulkhead.
Following the incident, the operator inspected the aft equipment bay and discovered a pinhole leak in a stainless steel hydraulic line. The line featured a small, crater-like deformation with evidence of recast metal and a 0.005-inch hole at the base of the crater. This damage was consistent with an electrical arc. There was no evidence of chafing on the line.
Investigators found traces of copper, cadmium, and nickel that had been acid rinsed from the crater; none of these are alloying elements in stainless steel, though copper wire is a common electrical conductor. The leak in the hydraulic line, which operated at 3,000 psi, could have atomized the Aeroshell Fluid 41 hydraulic fluid. Because the electrical cables in the aft equipment bay were disposed of before the investigation began, the specific source of the electrical arc could not be identified.