CHI03IA066 · Bombardier BD 100-1A10

The experimental airplane returned from a test flight with evidence of an in-flight inverter fire discovered during a post flight inspection. The test flight was part of a new airplane certification test program and was to evaluate the airplane's audio design/flight deck audio system. The flight crew did not receive any fire warning indications in-flight and neither they or the test crew members noted any 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 within which the inverter was located was equipped with fire detection and an airplane circuit breaker for the system. The airplane circuit breaker for the inverter was not tripped. Subsequent testing demonstrated that the circuit breaker operated within specification limits.

Inspection of the airplane revealed the presence of hydraulic fluid below and in the inverter tray. Examination of the inverter 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 changes in temperature may have contributed to material degradation or deterioration or even possible dielectic breakdown, which resulted in the failure mode observed. Following this incident, the manufacturer reported that the inverter design was tested and confirmed that it met the fluid susceptibility requirements of DO-160C for Skydrol. A second examination of the inverter, performed by the United States Air Force Research Laboratory, was documented in a laboratory evaluation report. The failure analysis section of this report states that no root cause of failure could be found and noted that the majority of the thermal damage to the inverter was attributed to Joule heating from a sustained current overload. The examination also revealed the existence of numerous manufacturing deficiencies that would be classified as defects per Joint Industry Standard requirements for soldered electrical and electronic assemblies. The majority of these defects violated cleanliness, conformal coating, and soldering. The inverter manufacturer's FAA approved manufacturing requirements differ from those noted by the Air Force Research Laboratory and are based on its own internal design standards and practices as well as in-house workmanship standards derived from good industry practice and requirements contained in IPC-A-610C, "Acceptability of Electronic Assemblies".

The inverter was part of a cabin outlet system considered non-essential under 14 CFR Part 25, Subpart F. Information contained in Advisory circular 25-10 regarding guidance for installation of miscellaneous, non-required equipment states. that circuit protective devices should be provided such that a fault in the nonrequired system does not cause a failure of a required system. Supplementing this guidance, AC 25-16, "Electrical Fault and Fire Prevention and Protection," states that in cases where internally faulted components will not cause automatic disconnection of their circuit protective devices before causing excess temperatures potentially resulting in fire, these electrical equipment should be provided with adequate backup protection (beyond the aircraft circuit breakers). In addition, the AC states that each installation of an electrical device should be assessed for its potential to generate fire or smoke under failure conditions. The inverter specified for this installation 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 it 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/smoke generation, in accordance with the related provisions of 14 CFR Part 25.1309, had not been performed by the aircraft manufacturer prior to this incident. As a result, additional requirements to address internal inverter circuit protection for this installation were not mandated by the aircraft manufacturer. Subsequent to this incident, a design analysis was performed by the airplane manufacturer. Correspondingly, the inverter manufacturer modified the inverter design to incorporate fuses on the power input lines to interrupt electrical power in the event a short circuit or overload failure condition develops inside the inverter.

Verbatim NTSB analysis and probable cause from the NTSB dataset