On June 19, 2007, a Boeing 747-251B N632NW operated by Northwest Airlines Inc. was involved in an aviation accident near Winnipeg, Manitoba, 150 nm N, CA. No fatalities were reported. Investigators recorded the probable cause as: Adjacent wire bundles in a high-vibration area in the number four engine were secured in a non-standard way, leading to wire chafing and a high-voltage power surge that caused the on-board fire. This summary draws on records from the Transportation Safety Board of Canada (TSB).
A Northwest Airlines Boeing 747-200 freighter (N632NW) diverted to Winnipeg after a cockpit fire caused by chafed wires in the number four engine area. The crew managed the fire, and the aircraft landed safely with no injuries.
Incident Overview and Emergency Response On January 10, 2008, the Transportation Safety Board of Canada released its final report on an incident involving Northwest Airlines Flight 909, a Boeing 747-200 freighter with registration N632NW. The aircraft was en route from Wilmington, Ohio, to Anchorage, Alaska, with a crew of three on board. During cruise flight at flight level 360, the crew detected smoke and fire emanating from a cockpit circuit breaker panel located behind the second officer's position. The crew completed the related emergency cockpit checklist procedures and declared an emergency with air traffic control. The fire self-extinguished during the action of the checklist procedures. The aircraft diverted to Winnipeg, Manitoba, landing safely at 0649 central daylight time with airport rescue and fire-fighting vehicles standing by. There were no injuries. The weather, both at cruise altitude and at landing, did not play a factor in this occurrence. A special aerodrome weather report was issued for Winnipeg at 0622 central daylight time, indicating wind 320° true at 12 knots, visibility 15 statute miles with a few clouds at 1100 feet, broken clouds at 2200 and 9500 feet, and a temperature of 12°C. The crew accomplished the “Engine Fire Detection Light and/or Fault Light Illuminated” company operations manual (COM) Section 2.26.32 checklist in the emergency/abnormal procedures manual. The fire-detection loop was selected from the loop “A” to the loop “B” position and the fault light extinguished. Soon after the completion of the checklist, an electrical smell was detected. The second officer confirmed that the smell was coming from behind the P6 panel located at the rear of the cockpit. The second officer looked through the designed viewing ports and saw flames and smoke within the panel. The crew members donned their oxygen masks and smoke goggles and control of the aircraft was given to the first officer. The first officer declared an emergency and requested a direct clearance to Winnipeg. The captain and second officer began the “Electrical Fire or Smoke” COM Section 2.26.6 checklist, during which time the Engine Fire Detect 3A-4A circuit breaker on the P6 panel popped. The fire began to increase in intensity and stopped only after the crew tripped the number one and number two generator breakers in accordance with the checklist instructions. The fire damage was contained within the interior of the P6 panel and the visibility inside the cockpit was normal at all times. When the fire was out, the crew members removed their oxygen masks and smoke goggles. The captain took control of the aircraft and approximately 10,000 pounds of fuel was dumped to bring the aircraft down to its maximum landing weight. The fuel dump was terminated above 10,000 feet above sea level and a visual approach was made to Runway 36 at the James Armstrong Richardson International Airport in Winnipeg, where a safe landing was accomplished. ## Technical Findings and Maintenance Issues The wire chafing occurred as a result of relative movement between individual wires secured together in adjacent wire bundles in a non-standard way. The bundles are situated in an engine area identified as a high-vibration area. Excess wire length in the number 4 engine fire-detection loop was folded back and secured to the adjacent PMG wiring harness. The Boeing Standard Wiring Practices manual specifies in part that all wires must be parallel before a wire harness tie is assembled on the wire harness. The excess wire length in the number 4 engine fire-detection loop was not secured in this manner. The wires chafed to the conductor core, which allowed the generator PMG 60 VAC to transfer to the 28 VDC “A” loop fire-detection circuit and cause a current surge back through to the fire-detection control electronics box in the cockpit. The current surge was dissipated by resistors on the A4 circuit board warning card, causing a rise in temperature in the resistors to the point of ignition. There is no circuit breaker protection in the circuit between the circuit card and the engine fire loop; thus, the system would overheat until the short occurred. It could not be established when the subject wires were secured improperly, but it is likely that it occurred at some point at or after engine overhaul. Subsequent shop visits, and under-cowl maintenance checks did not identify the wiring discrepancy or wire chafing. Such a discrepancy would be hard to identify without disassembly of the wire harness. Ongoing maintenance checks only require a visual inspection/check of the wire harness as installed. The following TSB Engineering laboratory reports were completed: LP 057/07 - DFDR/CVR Analysis, Boeing B747-200, N632NW; LP 059/07 - Fire Detection Control Box Examination, Boeing B747-200, N632NW. These reports are available from the Transportation Safety Board of Canada upon request. Adjacent wire bundles in a high-vibration area in the number four engine were secured in a non-standard way. The improperly secured wire bundle was not detected by Northwest Airlines Inc. maintenance or quality control personnel during initial installation, modification of the bundle, or the subsequent under-cowl inspections. The wires chafed to the conductor core, allowing a high-voltage power surge back through to the fire-detection control electronics box in the cockpit. The power surge subsequently shorted the resistors on an internal circuit card, causing the on-board fire. ## Safety Action Taken Northwest Airlines Inc. initiated a Boeing 747 fleet campaign to look for wire routing and chafing issues similar to those found on the occurrence aircraft. No similar discrepancies were found. Northwest Airlines Inc. also enhanced its maintenance program by adding a new step to the existing 600-hour “under-cowl check.” The step was designed specifically to identify and correct poorly routed wiring in the area of the chafe as noted on the occurrence aircraft. This report concludes the Transportation Safety Board's investigation into this occurrence. Consequently, the Board authorized the release of this report on 10 January 2008.
Probable cause
Adjacent wire bundles in a high-vibration area in the number four engine were secured in a non-standard way, leading to wire chafing and a high-voltage power surge that caused the on-board fire.
