WPR13FA294

***This report was modified on August 18, 2015. Please see the docket for this accident to view the original report.*** The operator's improper installation of a supplemental electrical power supply, which caused a short circuit due to inadequate vibration and abrasion protection, which resulted in chafing of the wires, which contacted one another, short-circuited, and caused an onboard fire. Contributing to the accident were the installation of the supplemental electrical power supply system without the supervision of an FAA-certified mechanic, the lack of an onboard means for fire suppression, and the pilot's inability to see the low-conspicuity power lines across the landing approach path until it was too late to take evasive action.

***This report was modified on August 18, 2015. Please see the docket for this accident to view the original report.***  A company that designed and manufactured airborne radar units, primarily for military applications, was using the accident airplane (as well as one other airplane) for some of the airborne development and testing of the externally mounted radar equipment. The pilot was in the left seat and a test engineer, who was not a pilot, was in the right seat. The airplane was equipped with a supplemental electrical power supply system that the company had designed and manufactured to provide electrical power for the radar systems and support equipment on the test flights. Most of the supplemental power supply system was located in the combined baggage area and the area vacated by the removal of the rear seats. In its installed position, the supplemental power supply system was not intended or able to be reached by the pilot or engineer during flight. About 2 hours into the radar test flight, the test engineer smelled smoke in the airplane. The pilot attempted to locate the source of the smoke and observed an open flame on the supplemental power supply. Because the fire was out of the pilot's or engineer's reach and the airplane was not equipped with a fire extinguisher, the pilot decided to land as soon as possible. During the attempted emergency landing on a road, the airplane struck power lines suspended above the road and then impacted the ground. Detailed examination of airplane and power system components revealed that the fire involved several wires that connected directly to the power system battery and that the fire had spread to the airplane floor carpet. The supplemental electrical power supply system components included, in part, an automobile-type 12-volt direct current battery, which was encased in a covered, plastic box, and a company-designed and -manufactured hard-plastic power distribution box. The power distribution box was stacked on top of the battery box, and they were secured in place by a ratcheting cargo strap system. In that configuration, two 12- to 14-gauge plastic-insulated wires, one red and one black, were situated and pressed between the top cover of the battery box and the bottom of the power distribution box, and then routed into the distribution box via a single grommeted hole in the bottom of that box. The installation had no provisions for separating or protecting the two wires, and the evidence was consistent with the wires abutting or crossing one another while pressed between the two boxes. The high-vibration environment of the test airplane caused relative motion between the two boxes and/or the boxes and the wires. That relative motion, combined with the pressure exerted by the boxes on the wires, abraded the insulation of those wires, which then allowed their conductors to contact one another. Because the black wire was connected directly to the negative battery terminal and the red wire was electrically connected to the positive battery terminal, contact of those conductors yielded a direct electrical short. The wires were rated to carry a maximum current of about 45 amperes, and the battery-rated output was 750 amperes. The short circuit resulted in a significant overcurrent in the wires, which caused excessive heating, additional insulation failure, smoke, and fire. Although the pilot did not recall all of the details of the event, the evidence indicated that the fire produced a significant amount of soot and heavy particulate matter, and possibly other physiological irritants.  The wire installation was not in accordance with Federal Aviation Administration (FAA) maintenance guidance, which advised that wire insulation be protected against chafing or abrasion because damage can result in a short circuit. The appearance of the supplemental electrical power supply, particularly its intercomponent wiring, was consistent with that of test-bench equipment, designed to be operated in a stationary environment with minimal or no vibration. No guidance or other documentation regarding the physical installation of the system components in the airplane or the security and protection of the associated wiring was located. A company technician, who was not an FAA-certified aircraft mechanic, reported that he accomplished the original installation of the supplemental power supply a few weeks before the accident; the power supply had accumulated about 13 hours of operation since its installation. The investigation was unable to determine if, how many times, or by whom, the power supply or its components might have been adjusted, moved, removed, and/or reinstalled. It could also not be determined whether the company-contracted aircraft mechanic had provided any installation guidance or whether he had examined, changed, or otherwise contacted or disturbed the original installation, because that mechanic did not respond to requests for information. The installation and arrangement of the affected wires were not in compliance with acceptable practices for aircraft, and the installation presented a serious hazard to flight safety due to the high potential for insulation abrasion and failure, with the resultant unintended electrical path(s).

An onboard open-flame fire occurred in the mid-aft cabin of the airplane. The fire involved the operator's supplemental power supply, the synthetic material carpet on the airplane cabin floor, and the synthetic material cargo tiedown straps. The fire produced a significant amount of smoke and dark, airborne soot and/or plastic particulate matter, and melting or burning plastic also produces fumes which are physiological irritants. The fire did not damage or affect any of the engine or flight controls, or adversely affect the structural integrity of the airplane. The fire and the involved equipment were physically beyond the reach of the test engineer and the pilot. The airplane was not equipped with a fire extinguisher. For those reasons, neither the pilot nor the test engineer was able to conduct any activities to suppress or extinguish the fire. The fire did not continue to burn after impact. The departure airport, U77, was a non-towered airport; pilot radio communications to coordinate their movements were conducted on a dedicated common traffic advisory frequency (CTAF). Aside from the normal radio transmissions on CTAF to depart U77, the pilot did not use the airplane radios to communicate any air traffic-related information, nor was he required to. During the flight, he and/or the test engineer occasionally communicated via radio with IMSAR personnel in support of the test. The pilot stated that while he was establishing the airplane on the downwind leg for his emergency landing on the road, he made one or more brief "mayday" transmissions on the standard emergency frequency, which was received by and responded to by a passing commercial airline flight. The pilot relayed his N-number, situation, location, and intentions to the airline flight crew, who then relayed the information to an air traffic control facility. The Cessna pilot then focused on flying and landing the airplane, and did not make any subsequent radio communications. IMSAR Equipment IMSAR was a designer and manufacturer of synthetic aperture radar systems that were primarily intended to be used on airborne platforms, primarily by the Department of Defense (DoD), and often as externally-mounted pods. The company used its two Cessna airplanes as low-cost platforms for portions of the airborne development and testing of the radar equipment. In order to provide for the electrical power needs of the various radar systems and support equipment being tested or used on the Cessnas, and also to enable those devices to be electrically isolated from the airplanes' electrical systems, IMSAR designed and manufactured two similar supplemental power supply systems, one for each of its airplanes. For both airplanes, the rear seats had been removed, and the bulk of the power supply system was located in the continuous space of the airplane baggage area and the area vacated by the removal of those seats. The supplemental power supply system was designed to provide 12VDC, 28VDC, and 120VAC to the radar and support equipment on the subject airplane. Primary power supply system components included an automobile-type 12VDC battery, an "isolation unit/automatic charging relay," a battery switch, a network modem/router, an inverter, a "remote switch box," a cockpit-mounted circuit breaker panel, and a "power distribution box." The system was partially connected to the airplane electrical system. The power distribution box provided multiple airplane-independent power outlets for 12VDC and 28VDC. The box consisted of a hard, stiff plastic shell with a hinged cover, which encased a step-up transformer, a bus block, and other electronics. Opening of the box cover provided access to the multiple 12VDC and 28VDC ports for the radar equipment. The system was powered by its 12VDC battery, which was charged by the airplane electrical system as needed. The inverter was used to provide 120VAC power to laptop computer(s) used by the onboard test engineer. The remote switchbox was to be used in flight to control power to the various 12VDC and 28VDC outlets, and the cockpit-mounted circuit breaker panel was to be used to control electrical power between the airplane and the supplemental power system. The remote switchbox and the circuit breaker panel were the only two components that could, or were intended to, be readily reached by the test engineer during flight. The radar units were typically carried on pods which attached to hardpoints on the wing, and were connected to the supplemental power supply and cockpit monitoring and control equipment by an IMSAR-designed and fabricated wire harness. The airplane baggage door was also modified with an external mount rack for other radar equipment. Refer to the public docket of this accident for additional details. Wire Insulation Damage Protection According to the Aviation Maintenance Technician Handbook (FAA H-8083-31) "Wires and wire groups should be protected against chafing or abrasion in those locations where contact with sharp surfaces or other wires would damage the insulation, or chafing could occur against the airframe or other components. Damage to the insulation can cause short circuits, malfunction, or inadvertent operation of equipment." An operating piston-engine general aviation airplane is a high-vibration environment, which provides the mechanism for potential mechanical damage to improperly secured or improperly protected wiring. In order to minimize or prevent mechanical damage, wires should be supported by suitable clamps or other devices at appropriate intervals to preclude relative motion and chafing. Wires should also be prevented from being pinched or crushed, which can damage or fail the insulation. Equipment Design and Fabrication Requirements The radar equipment and the supplemental power supply systems were designed and fabricated at the IMSAR facility by IMSAR personnel. Because most of the radar systems were intended for DoD airborne applications, they were designed and manufactured to the applicable specifications and industry standards for the vibration environment of the intended application. In contrast, the supplemental power supply was an internal IMSAR product that was to be flown on IMSAR owned and operated aircraft, and was subject only to IMSAR design and fabrication standards and requirements. The appearance of the supplemental power supply was consistent with that of test-bench equipment, designed to be operated in a benign, stationary environment, with minimal or no vibration. The power supply design and fabrication information that IMSAR provided to the investigation did not contain any specifics or references regarding any vibration envelope requirements, or any wire insulation abrasion resistance requirements. Equipment Installation Information According to the radar technician who fabricated the supplemental power supply, he conducted the initial installation of the system in the airplane a few weeks before the accident. The technician was not a FAA certified aircraft mechanic. No guidance or other documentation regarding the physical installation of the system components in the airplane, or the security and protection of the associated wiring was provided by the operator. Correlation with the airplane flight logs indicated that the installation occurred either just before May 31, 2013, or during the period between May 31 and June 14, 2013. Most of the large power supply system components were not permanently affixed to the airplane, but typically tended to remain installed in the airplane between flights. While the component arrangement inside the airplane remained essentially constant, how many times, or by whom it might have been adjusted, moved, or removed and reinstalled could not be determined. Because the FAA-certified aircraft mechanic who was contracted by IMSAR to assist in certain modifications to the airplane did not make himself available to the NTSB investigator,