What happened
An investigation into the origin of an aircraft fire focused on the area surrounding the left heat exchanger cooling fan. Investigators identified significant heat damage to several components, including relays R2-54 and R2-53. Specifically, the R2-53 relay showed unique signs of distress, including holes burned through its housing and loose terminal studs, which were not present on the neighboring relays. The wire bundles located beneath the left and right heat exchanger cooling fans, as well as the ground service tie relays, also showed insulation damage, with the most intense heat concentrated directly below the R2-53 relay.
During the initial on-scene investigation, it was noted that three of the four circuit breakers associated with the left heat exchanger cooling fan had tripped. To understand why one breaker remained in the closed position, the components were sent to the Materials Integrity Branch at Wright-Patterson Air Force Base for comprehensive testing. These tests included insulation resistance, contact resistance, voltage drop, and calibration measurements. The laboratory results indicated that the single non-tripping circuit breaker was functioning according to all manufacturer specifications and showed no mechanical or electrical anomalies.
Findings
Post-incident disassembly revealed that the R2-53 relay had undergone a repair that did not meet the manufacturer's required standards. The manufacturer confirmed that the damage to the relay's housing was consistent with a phase-to-phase arc occurring between terminals A2 and B2. This internal failure of the R2-53 relay was identified as the primary cause of the fire initiation. While the fourth circuit breaker did not trip, investigators noted that such a device is designed to respond to sustained overloads; therefore, intermittent arcing or a sudden circuit opening could have prevented the breaker from reaching the thermal threshold necessary to trip.