NTSB Factual Report

HISTORY OF FLIGHT

On February 4, 2003, at 1800 central standard time, an experimental Bombardier BD-100-1A10, C-GJCV, operated by Bombardier Incorporated, returned from a test flight to Wichita-Mid Continent Airport, Wichita, Kansas, with evidence of an inverter fire within the tail cone. The airplane structure was undamaged. Visual meteorological conditions prevailed during the flight. The 14 CFR Part 91 flight was operating on a visual flight rules flight plan. The two airline transport pilots and two engineers were uninjured. The local flight originated at 1600.

The airplane returned from a test flight, number 100, which was to evaluate the audio design/flight deck audio. During the flight, an engineer in the cabin was reported to have plugged his laptop computer into a cabin outlet and noticed that there was no electrical power shortly after takeoff. No smell or indication of smoke was noted by the pilots and engineers during the flight. The airplane landed without incident. Two of those aboard noticed a smell when they exited the airplane but did not associate the smell with the airplane. Ground personnel noticed a burn odor when the airplane was then towed into a hanger where soot in and around access panels of the left hand engine pylon and aft equipment bay was discovered. Further inspection revealed evidence that the inverter exhibited evidence of a fire.

AIRPLANE INFORMATION

The BD-100-1A10 (Challenger 300), serial number 20004, received a United States Special Flight Authorization for research, development, compliance with an expiration October 17, 2003, as a part of a new aircraft certification program.

The inverter was a Avionic Instruments Inc. (AI2) 2KVA, single-phase 60 Hz, model number 2E2000-1AC-1996, serial number (S/N) JC000002, static inverter. This make and model of inverter was not installed on the other BD-100-1A10 airplanes. The inverter consisted of three 667 VA power modules in parallel to provide a 60 Hz, 2 KVA output from a nominal 28 VDC input. Two of the three power modules used were also used in model number 1E1000-1A-2017 inverters.

The inverter was installed in the aft equipment bay aft of the rear pressure bulk head between fuselage stations 685 - 724, center line 0 to right buttock line 15, and waterline 68 - 80. The inverter was attached directly to the airplane structure and was not mounted to a tray. The inverter was part of a nonessential airplane system (as defined by 14 CFR Part 25.1309 Equipment, Systems, and Installations) that supplied power to six interior cabin outlets and was not a part of any other aircraft system. A 1309 reliability and design analysis had not been performed on the system prior to the incident.

The aft equipment bay was equipped with fire sensors and an airplane 100-amp cabin inverter circuit breaker, part number MS25361-100.

Inverter S/N JC000002 was installed on October 25, 2002, and accumulated a total time of 278.2 hours. S/N JC000002 replaced inverter S/N JC000001 due to a failure of the unit on October 25, 2002. S/N JC000001 accumulated a total time of 185.9 hours at the time of failure. This unit was removed and sent to AI2 for examination which revealed there was chalk and conductive debris below the power semiconductors. Two of the power modules were working properly except for voltage breakdown from the output diodes of the DC/DC to the heat sink of the power module.

WRECKAGE AND IMPACT INFORMATION

The aft equipment bay access door was opened and the inverter displayed evidence of a fire and there was soot on and around the inverter.

The area above the inverter was burnt and had curled paint. An area of about 1 inch by 1.5 inches was burned through. The entire floorboard area contained soot. An avionics unit to the left of the inverter contained soot but did not have burn damage. When this unit was removed from its tray, there was evidence of Skydrol hydraulic fluid erosion on the tray's painted surface.

The inverter casing was wrinkled on the top and inboard sides. The upper portions had heavy soot deposit and an unknown white substance that protruded through some vent holes. There was no presence of Skydrol on the inverter. The bottom fuselage skin below and forward of the inverter contained Skydrol.

The inverter was removed from the airplane and opened by Bombardier maintenance personnel in the presence of AI2 representatives and under the supervision of the Federal Aviation Administration (FAA). The unit's two upper circuit boards were noted to be burned. The inverter was then quarantined by Bombardier and sent to the FAA Manufacturing Inspection District Office (MIDO) having oversight of AI2.

The airplane circuit breaker for the inverter powering the cabin electric outlets was not tripped. The circuit breaker was later tested during the investigation.

TEST AND RESEARCH

A query of the FAA's Service Difficulty Reporting Data, for the period from January 1, 1995 to February 7, 2003, for inverters in all aircraft make and models, yielded 243 reported occurrences relating to inverters. Of the 243 reported occurrences, 57 reported the presence of smoke. A second query of the FAA's accident/incident database was performed, for the period from January 1, 1974 to February 7, 2003. The query yielded 35 occurrences involving inverters. These database searches are included in the public docket of this report.

Technical Service Order (TSO) C73, Static Electrical Power Inverter, dated December 18, 1963, stipulates under Part 514, the minimum performance standard and specification for material parts, and appliances used in aircraft. Subpart A of this part contained the general requirements applicable to all TSOs and subpart B contains the technical standards and specifications to which a particular product must conform. TSO-C73 cites the required performance standards under environmental test conditions 12 variables which include overload capacity, input overvoltage, and short circuit condition, as follows:

Overload Capacity - The inverter shall be capable of withstanding, without damage, a current overload of at least 150 percent for a time duration of 5 minutes;

Input voltage - The inverter shall be capable of withstanding, without damage, input overvoltage for a time period of 5 minutes while supplying full rated output power;

Short Circuit Condition - The inverter shall be capable of withstanding, without damage, and output short circuit applied separately to each phase or simultaneously to all phases for a time period of one minute. Within 5 minutes after removal of the short circuit condition, the unit shall be energized and run cautiously for a period of at least 20 hours. During this period the unit shall, without degradation of performance, deliver the specified output.

TSO-C73 has no requirement for short circuit or overload protection and has not been amended since its inception.

The design and manufacturing standards followed in the manufacture of AI2 inverters were set forth by the company. The FAA Aircraft Certification Office bases their design approval on TSO-C73, and the FAA MIDO then uses the manufacturer’s standards in the conduct of its inspections.

The inverter was sent to AI2 for examination under the oversight of the FAA. According to the AI2's final investigation summary report, "An outside test lab analysis confirmed the presence of Skydrol within the inverter. 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. The inverter design was tested and confirmed that is meets the Fluid Susceptibility requirements of DO-160C for Skydrol."

"Avionic Instruments Inc. proposed the following design changes which will provide additional protection to the inverter in the event of high input current conditions and lower voltage levels within the modules there by reducing the possibility of dielectric breakdown failures. " The changes include:

Modified DC/DC converter module split secondary reduced internal voltage level by 4 times.

The addition of fuses on the input line of each module, to eliminate the continuous current draw in the event of an internal failure.

AI2 performed a qualification test procedure (QTP) to demonstrate that the model 2E2000-1AC-1996 inverter meets the following military/commercial requirements:

MIL-STD-810, Environmental Test Methods

MIL-I-46058C, Insulating Compound, Electrical (For coating printed circuit assemblies)

MIL-STD-461, Requirements fro the Control of Electromagnetic Interference and Susceptibility

FAR 25, Airworthiness Standards

MIL-HDBK-217, Reliability, Stress, and Failure Rate Data for Electrical Equipment

MIL-STD-704, Aircraft Electrical Power Characteristics

RTCA/DO-160D, Environmental Conditions and Test Procedures for Airborne Equipment

The following AI2 requirements were also followed in the QTP:

Fluid Susceptibility Test Procedure

Qualification By Similarity Report for 2E2000-1AC-1996

AI2 Quality System Manual

Environmental Stress Screening Procedure

Manufacturing Production Environmental Stress Screening (ESS) for Static Inverters

Acceptance Test Procedure for 2E2000-1AC-1996 Static Inverter

The fluid susceptibility test began March 12, 2003, with the inverter being subjected to a spray of Skydrol for a 24-hour period. On March 13, 2003, the inverter was operated "failure free" under the supervision of the FAA during the initial turn-on test. The inverter was placed into a chamber for 160 hours at 65 degrees Celsius. After removal from the chamber, the units was tuned on and operated within specification. The unit was opened and inspected and no damage was reported.

AI2 stated that it does not use MIL-STD-454, Standard General Requirement For Electronic Equipment, in design of its products. It follows its own internal design standards and practices as well as in-house workmanship standards, which are based upon good industry practice and requirement based upon IPC-A-610C, Acceptability of Electronic Assemblies.

The inverter was then sent to the United States Air Force Research Laboratory, Dayton Ohio, for a second examination performed by the laboratory. According to laboratory's evaluation report, "The failure analysis of power module 1 in a static inverter manufactured by Avionic Instruments Incorporated found no 'root cause' of failure. The majority of the thermal damage is attributed to Joule heating from a sustained current overload. The three bus bars in the inverter are an effective method for controlling inductance problems with DC/DC converter switching circuits. This arrangement, however is susceptible to shorting from conductive debris, moisture condensation or other fluid contamination since the bus bars are no conformally coated."

"A manufacturing assessment was performed on two of the power modules in the inverter that were relatively undamaged. Numerous manufacturing deficiencies that would be classified as defects per Joint Industry Standard IPC/EIA J-001C (Requirements for Soldered Electrical and Electronic Assemblies) were observed. The majority of these defects violate cleanliness, conformal coating, and soldering..."

The 100-amp cabin inverter circuit breaker was tested by Bombardier under the supervision of the FAA. The test showed that with 137 amps applied, the circuit breaker tripped after 4 minutes and 25 seconds. The specification allows up to an hour.

Advisory Circular 25-10, Guidance for Installation of Miscellaneous, Nonrequired Electrical Equipment, states under Installation Requirements, 3b(1), "Ensure that electrical equipment and terminals are protected from lint, dripping fluids and condensation."

ADDITIONAL INFORMATION

Following the incident, a 1309 reliability and design analysis was performed on the system.

The FAA, Bombardier, and AI2 were parties to the investigation. The Transportation Safety Board of Canada provided a designated representative to the investigation.

NTSB Probable Cause

The uninterrupted current overload of the cabin outlet system static inverter leading to ignition and sustained fire. The cause of the initiating overload condition could not be determined. Contributing factors were the lack of adequate current overload protection for the static inverter due to a failure to adequately assess fire/smoke hazards of the inverter in this installation and the absence of timely fire annunciation to the flight crew.