Incident Description
An experimental airplane, undergoing a certification test flight to evaluate the flight deck audio system, was found to have evidence of an in-flight inverter fire during a post-flight inspection. The flight crew did not receive any fire warning indications, and neither the crew nor test personnel observed smoke or odors associated with burning during the flight. The inverter was located in the aft equipment bay, aft of the rear pressure bulkhead. The area was equipped with fire detection and a circuit breaker, which was not tripped. Subsequent testing confirmed the circuit breaker operated within specification limits.
Investigation Findings
Inspection of the airplane revealed hydraulic fluid below and in the inverter tray. Examination and fluid susceptibility testing confirmed the presence of Skydrol hydraulic fluid within the inverter. The inverter manufacturer reported that Skydrol intrusion, internal power circuit high voltages, operation at altitude, and drastic temperature changes may have contributed to material degradation or dielectric breakdown, resulting in the observed failure mode. Following the incident, the manufacturer tested the inverter design and confirmed it met the fluid susceptibility requirements of DO-160C for Skydrol.
A second examination by the United States Air Force Research Laboratory documented in a laboratory evaluation report stated that no root cause of failure could be found. The report noted that the majority of thermal damage was attributed to Joule heating from a sustained current overload. The examination also revealed numerous manufacturing deficiencies classified as defects per Joint Industry Standard requirements for soldered electrical and electronic assemblies, primarily violating cleanliness, conformal coating, and soldering standards. The inverter manufacturer's FAA-approved manufacturing requirements differ from those noted by the laboratory and are based on internal design standards and practices, as well as in-house workmanship standards derived from good industry practice and IPC-A-610C requirements.
Design and Regulatory Aspects
The inverter was part of a cabin outlet system considered non-essential under 14 CFR Part 25, Subpart F. Advisory Circular 25-10 states that circuit protective devices should be provided such that a fault in a nonrequired system does not cause failure of a required system. AC 25-16 supplements this, indicating that where internally faulted components will not cause automatic disconnection before causing excess temperatures potentially resulting in fire, adequate backup protection beyond aircraft circuit breakers should be provided. It also states that each electrical device installation should be assessed for its potential to generate fire or smoke under failure conditions.
The inverter was a TSO-C73 certified article. The applicable TSO addresses short circuit and overload conditions to the extent that inverters continue producing power through these conditions but does not require circuit protection when these conditions are encountered. A design analysis of the inverter as installed in the cabin outlet system to consider failure conditions and corresponding hazards such as fire or smoke generation, in accordance with 14 CFR Part 25.1309, had not been performed by the aircraft manufacturer before the incident. As a result, additional requirements for internal inverter circuit protection were not mandated. After the incident, the aircraft manufacturer performed a design analysis, and the inverter manufacturer modified the design to incorporate fuses on power input lines to interrupt electrical power in the event of a short circuit or overload failure condition inside the inverter.