What happened
While operating a Northwest Airlines Inc. flight from Wilmington, Ohio, to Anchorage, Alaska, the crew of a Boeing 747-20 and registration N632NW encountered an emergency during cruise flight at 36,000 feet. The incident began when the second officer observed temperature fluctuations in the number four engine fire-detection loop "A" indicator, which progressed to a steady master fire-detection light in the cockpit.
Following established procedures, the crew switched the detection loop from "A" to "B," which initially extinguished the fault light. However, an electrical odor soon emerged, and the crew observed flames and smoke within the P6 panel located behind the second officer's seat. The crew donned oxygen masks and smoke goggles, and the first officer declared an emergency, requesting a direct diversion to Winnipeg, Manitoba.
During the execution of emergency checklists, a circuit breaker popped, and the fire intensified. The crew managed to suppress the fire by tripping the number one and number two generator breakers. To reach a safe landing weight, the captain dumped approximately 10,000 pounds of fuel. The aircraft landed safely at James Armstrong Richardson International Airport with no injuries reported.
The investigation
Investigators examined the wiring within the number four engine area, a high-vibration zone. They discovered that excess wire length in the fire-detection loop had been folded back and secured to an adjacent wiring harness in a manner that violated Boeing Standard Wiring Practices, which requires wires to be parallel before being tied.
This non-standard installation caused the wires to chafe against the conductor core. This degradation allowed a 60 VAC voltage from the generator PMG to surge into the 28 VDC fire-detection circuit. The resulting current surge traveled back to the cockpit electronics box, where it heated the resistors on a circuit board to the point of ignition. The investigation found that because there is no circuit breaker protection between the circuit card and the engine loop, the system would continue to overheat until a short occurred.
Findings
- Adjacent wire bundles in a high-vibration area of the number four engine were secured using a non-standard method.
- The improper wiring configuration was not identified by the operator's maintenance or quality control teams during installation, modifications, or routine under-cowl inspections.
- Wire chafing allowed a high-voltage power surge to reach the cockpit electronics box, causing the resistors to overheat and ignite.
Safety action
Northwest Airlines Inc. launched a fleet-wide inspection of its Boeing 747 aircraft to search for similar wiring and chafing issues, though no other instances were found. Additionally, the airline updated its 600-hour under-cowl maintenance program with a specific step to identify and correct poorly routed wiring in the affected area.