San Francisco, CA, USA
N520QS
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.
HISTORY OF FLIGHT On September 11, 2004, at 0938 Pacific daylight time, a British Aerospace BAE 125-1000A, N520QS, experienced an aft equipment bay fire while holding short of the runway for takeoff at San Francisco International Airport, San Francisco, California. Net Jets, Inc., was operating the airplane under the provisions of 14 CFR Part 91. The pilot and copilot were not injured, and the airplane was substantially damaged. Visual meteorological conditions prevailed, and an instrument flight plan had been filed for the positioning flight destined for North Las Vegas, Nevada. The Maintenance Director for Net Jets reported to the National Transportation Safety Board investigator that the aircrew was to perform a positioning flight to North Las Vegas. While the airplane was holding short for takeoff, the crew noticed that the Master Caution and a Pack & Aux Overheat warning light on the cockpit annunciator panel was illuminated, followed by a Rear Baggage Smoke Detector warning light. The crew shut down the auxiliary power unit (APU) and the first officer proceeded to examine the rear baggage compartment. When the first officer opened the baggage compartment door, smoke filled the cabin. 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 revealed metal discoloration and soot residue. The manufacturer determined that there was structural damage to the aft pressure bulkhead. The initial damage assessment performed by the operator that was relayed to the Safety Board investigator indicated that no structural damage had occurred and that there was smoke but no fire in the aft equipment bay. Once the operator determined that structural damage to the aft pressure bulkhead had occurred and the Safety Board investigator was informed, much of the equipment and material in the aft bay had been removed, examined by the airplane manufacturer, or disposed of. AIRCRAFT INFORMATION The business jet style airplane is powered by two turbofan engines, has a nine passenger capacity, and is operated by a crew of two. Examination of the maintenance logbook revealed that a 300, 600, 1,200, and 2,400-hour Approved Aircraft Inspection Program (AAIP) inspection was completed and recorded on August 26, 2004. At the time of the inspection the airplane had 6,364.7 hours of total flight time. The number one engine had 6,171.9 hours on it, and the number two engine had 8,785.7 hours on it. The operator reported that the airplane had flown an additional 49.2 hours since the inspection on August 26. TESTS AND RESEARCH Aircraft Examination The airplane was examined by engineers from the Raytheon Aircraft Company on September 23rd and 24th. The damage to the aft equipment bay was surveyed. Inspection of the baggage bay revealed that the left-hand side (port side) of the rear pressure bulkhead was discolored and showed signs of thermal damage. Various damaged equipment and trim had been removed, and stored in the cabin and baggage bay. This equipment included the 'GG' electrical panel, the hydraulic accumulators, the hydraulic accumulator support, and baggage bay liner panels. The APU had been removed and was stored in a hangar. During the course of the investigation additional piping, wiring, and baggage bay liner panels were moved or removed to gain access for testing. The rear pressure bulkhead assembly had evidence of damage with scorched, burnt, and blistered paint on both the forward and aft faces of the left-hand web sections (port side). The airframe components suspected to have suffered heat damage were tested for change in material temper using a Sigmatest D conductivity meter. The conductivity reading obtained from the rear pressure bulkhead assembly web sections 25-9FC867-3 and 25-9FC867-5 were consistent with the material temper being altered by the incident. The detailed engineering report can be viewed in the official docket section of this investigation. Failed Hydraulic Line Assembly Nondestructive Evaluation After the incident the operator inspected the aft equipment bay and identified a hydraulic line with a pinhole leak. The line was removed and sent to the Raytheon Aircraft Metallurgical Engineering Laboratory for nondestructive evaluation. The examining engineer reported to the Safety Board investigator that he did not observe any evidence of chafing on the pipe and that he saw a single point contact that may have bounced around on the pipe a bit before or after making the deep penetration. The hydraulic line material is 3/8-inch stainless steel tube, T72/73 22 SWG, with a part number of 25-9SF31-293S. A small crater measuring approximately 0.100-inch by 0.060-inch was identified in the bend of the line. A small pinhole, approximately 0.005-inch in diameter, was present at the bottom of the crater. The walls of the crater revealed a reddish copper color. Some hydraulic fluid was observed in the bottom of the crater. The pinhole, viewed from the inside of the tube using a borescope, exhibited thin wall rupture features. No discoloration was found on the tube interior. Both the clean and reddish copper colored surface of the crater displayed a pebble like texture, similar to recast observed on electro discharge machined surfaces. The surface of the pinhole, where the wall was radically thinned at the bottom of the crater, exhibited overload fracture features. The pinhole location was flushed with acetone and the fluid collected. Some oil residue was noted in the fluid. No solid particulate was detected in the collected fluid. A reddish copper appearance was still present on the crater surface. The pinhole location was next rinsed with dilute nitric acid, similar to that used to passivate T27/73 stainless steel. The collected rinse solution was subjected to inductively coupled plasma (ICP) spectrographic chemical analysis. Copper was identified as the dominant element present, along with lesser quantities of cadmium and nickel. It should be noted that copper and cadmium are not listed as alloying elements in stainless steel T72 and T73 alloys, which are ferrous based, containing 17- 19 percent chromium and 8-12 percent nickel. Copper wire is commonly used in aircraft electrical systems. The full metallurgical engineering laboratory report is available in the official docket of this investigation. Applicable Airworthiness Directives (AD) and Service Bulletins (SB) There are two Airworthiness Directives (94-15-15 and 96-07-11) that addressed possible damage to hydraulic lines or electrical looms/cables in the aft equipment bay from chafing that could result in a pinhole in a line. Examination of the airplane records revealed that these AD's, which involved an inspection of the equipment bay, were complied with on August 11, 1994, and May 1, 1996, respectively. Additionally, the specified inspection action was previously announced to operators in the following two service bulletins: SB A29-92 (May 19, 1994) and SB 29-95 (Mar 24, 1995). Aeroshell Fluid 41 Material Data The aircraft operates a 3,000 psi hydraulic system using Aeroshell 41 (equivalent to MIL-H-5606) as the working medium per the airplane's maintenance manual specifications. The director of maintenance stated that MIL-H-5606 or an equivalent fluid was used on the airplane. The Aeroshell Fluid 41 Material Data Safety Sheet (MSDS), located on the Shell Corporations website, states that the flash point is between 105 Celcius (C) to 110C (221 Fahrenheit (F) to 230F) and no data was available for the auto-ignition temperature. The flashpoint is the lowest temperature at which a heated liquid's vapors when mixed with air can be ignited (flashed) by a flame or spark, or other ignition source. The auto-ignition temperature is the lowest temperature at which a heated liquid's vapors will self-ignite and burn without exposure to any ignition source.
An electrical arc from an undetermined source initiated a hydraulic line rupture, which resulted in an equipment bay fire.
Source: NTSB Aviation Accident Database
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