WPR20FA051

The improper design of the audio interface system, which caused an electrical short circuit that led to a ground fire.

After starting the airplane’s engine for a planned flight, the pilot began to smell smoke, which became stronger as he continued with flight preparations. The pilot decided to terminate flight preparations and have a mechanic examine the airplane. After shutting down the engine, the pilot saw smoke rising from the armrest area of the right center passenger seat. By the time the mechanic arrived, the smoke had become dense, and the airplane cabin became engulfed in flames a few minutes later. Postaccident examinations revealed that the source of the fire was likely an audio interface card, which was part of a system that enabled occupants to connect their non-aviation headsets to the airplane’s intercom and entertainment system. The card was installed in the side panel of the airplane in the armrest area where smoke was first observed. Evaluation of the card revealed that its design allowed an excessive flow of current to pass through a current-limiting resistor in its power-protection circuit when its input filter capacitors or reference diode failed in a short-circuit condition. Under this scenario, the flow of current resulted in the resistor having to dissipate more power than it was rated for, resulting in a buildup of heat. The design called for the use of either a wire-wound or thick-film current-limiting resistor. Testing revealed that, under the short-circuit condition, designs with both types of resistors would result in heat sufficient to create thermal damage. The thick-film design would typically fail in an open circuit a short time later and return to a normal temperature; however, the wire-wound resistor would continue to produce heat and eventually an open flame. Compounding the electrical design flaw was the physical installation of the cards, which were encased in a heat-shrink sleeve and then wrapped with nylon cable ties to standoffs on the inside of the panel walls. The use of a heat-shrink sleeve to cover the boards meant that most of the components were in direct contact with the sleeve when shrunk. The contact was even more pronounced against the input filter capacitors because they were mounted near the board’s edge. Additionally, because the units were held to the airframe with nylon cable ties that wrapped around the boards, the components were subject to external forces, from contact with the ties, during installation and throughout their lifetime. The manufacturer of the input filter capacitors specifically stated that stress after installation could result in a crack that might cause a short circuit. Because four input filter capacitors were used, all of which were installed in parallel, the chance of failure and short circuit increased by a factor of four. The capacitor manufacturer further stated that, if the circuit being used caused an electrical shock, smoke, or fire when a capacitor is shorted, a fail-safe device, such as a fuse, would prevent secondary accidents. The audio cards were powered via a 5-ampere circuit breaker used by the airplane’s audio panel. Although this power rating was appropriate for the audio panel and all audio cards, the current drawn by an individual audio card short circuit was not sufficient to cause the circuit breaker to trip. The audio cards did not incorporate any internal or external secondary fuse. Postaccident flammability testing indicated that the cards would fail in an open circuit and self-extinguish after igniting except for one card, which was equipped with a wire-wound resistor and continued to produce an intermittent flame as long as power was supplied. As a result, the flame continued to burn and created hot flaming drips. The accident airplane was equipped with audio cards that used a wire-wound resistor. It is likely that, when the pilot engaged the airplanes electrical power via the master switch, one of the cards mounted in the right side panel failed in short-circuit, creating the flame that eventually consumed the airplane cabin. A Federal Aviation Administration airworthiness directive resulted in the removal of all audio interface cards from all equipped Cirrus airplanes. The manufacturer then redesigned the audio cards and increased the resistance and power rating for the current-limiting resistor such that, under a short-circuit condition, the resistor’s power rating could not be exceeded. The manufacturer also replaced the input filter capacitors with a more robust, single capacitor and encased the unit in a plastic box filled with potting compound.

Audio Interface System The airplane was equipped with an audio interface system, which enabled occupants to connect their non-aviation headsets to the airplane’s intercom and entertainment system using 3.5-millimeter jack sockets mounted in the armrests on the cabin panel walls. The system consisted of seven headset audio and five microphone interface cards, which were mounted throughout the airplane to the back of the cabin side panels and connected to each jack. The cards were powered by the airplane’s 28-volt DC essential bus via the 5-ampere “COM2, AUDIO PANEL” circuit breaker mounted in the pilot’s circuit breaker panel. The cards did not incorporate any internal or external secondary fuse. The audio and microphone cards were designed for Cirrus by an engineering contractor and manufactured by outside suppliers. The cards comprised a conventional laminated printed circuit board fitted with surface-mounted electronic components. The assemblies were connected to their respective wiring harnesses with thermoplastic “Micro-Fit” connectors. The units were encased in a heat-shrink sleeve and were then wrapped with nylon cable ties to standoffs inside the panel walls (see figure 3). Figure 3. Audio cards and (smaller) microphone card inside the panel wall (Source: Cirrus). The audio cards incorporated internal voltage regulation by using a 5-volt linear voltage regulator. The regulator was protected by a transient-voltage-suppression (TVS) diode configured in parallel with four input filter capacitors, and power was supplied through a current-limiting resistor. The microphone cards did not include a voltage regulator but used the same diode, capacitor, and current-limiting resistor design. Audio Interface System Design Evaluation The input filter capacitors were ceramic, and they had a capacitance of 4.7 microfarads and a rated voltage of 50 volts. The capacitor manufacturer’s reference data stated the following: After mounting a capacitor on a printed circuit board, do not apply any stress to the capacitor that causes bending or twisting to the board.... [Bending or twisting] may cause the capacitor to crack. Cracked capacitors may cause deterioration of the insulation resistance, and result in a short. If the circuit being used may cause an electrical shock, smoke or fire when a capacitor is shorted, be sure to install fail-safe functions, such as a fuse, to prevent secondary accidents. This series [of capacitor] are not safety standard certified products. A heat-shrink sleeve covered the circuit boards, allowing most of the board components to be in direct contact with the sleeve after it was shrunk. The four input filter capacitors were mounted near the board’s perimeter, where the shrink wrap would tighten as it reached the board’s edge (see figure 4). Additionally, because the units were held to the airframe with nylon cable ties that wrapped around the board, the components (in particular, the capacitors near the edge) were potentially subject to forces exerted by contact with the cable ties. Figure 4. Audio card with (top) and without (bottom) heat-shrink sleeving and with the input filter capacitors circled. The Cirrus design specifications indicated that each card should use either a thick-film or a wire-wound current-limiting resistor with a resistance of 100 ohms. The thickfilm resistor had a power rating of 1.5 watts, and the wire-wound resistor had a power rating of 1 watt. The current drawn by a 100-ohm resistor placed across a 28-volt DC source would be 0.28 amps, resulting in a power consumption of 7.84 watts, which exceeding the rating of the resistor by a factor of seven. Audio Card Design Certification The SF50 airplane model was designed according to the requirements of 14 CFR Part 23, and the Federal Aviation Administration (FAA) issued Cirrus a type certificate in October 2016. According to Cirrus, the audio card design was reviewed by a designated engineering representative and Cirrus engineers. The cards were included as part of the design for the original type certificate. The airplane sustained extensive thermal damage, with fire consuming the cabin roof and the cabin contents from the aft wall of the CAPS parachute enclosure to the engine inlet nacelle. Fire consumed the lower right walls of the cabin down to the wing root, with only composite cloth remaining. Most of the left wing and lower left exterior skins of the cabin were not damaged by the fire. The right wing was intact but sustained thermal damage to the upper skin from the root outboard to about 4 ft before the tip. The airplane remained on its main landing gear and was able to support its weight without significant structural deformation. The tail assembly aft of the pressure bulkhead, along with the engine and engine compartment were essentially undamaged (figure 5). Figure 5. Accident airplane after fire was extinguished. Remnants of the various electrical wire harnesses along the lower right side of the cabin structure were examined. The insulating sleeve material for all conductors had burnt, leaving brittle copper wire remnants. Multiple areas of the harnesses exhibited molten and globule-like breaks to their copper conductors. The molten damage was consistent with electrical arcing and was located in four separate areas from the forward edge of the windshield to the center of the right wing, adjacent to passenger seat No. 5. Additionally, copper-like material residue was found melted into the inner side of the right sidewall skin next to the No. 5 seat. All 12 interface cards were located within the wreckage, and all displayed varying degrees of thermal damage such that most of their components were obscured by melted debris and charred plastic. The thermal damage appeared more pronounced on the audio cards for passenger seats Nos. 4, 5, and 7. These cards were mounted on the right aft side of the airplane, below the armrest and cupholder for seat No. 5, and in the area where smoke was first observed. The audio cards for passenger seat Nos. 4, 5, and 7 were provided to the NTSB’s Materials Laboratory for x-ray examination. X-ray images revealed that the audio cards for seat Nos. 5 and 7 were fitted with a wirewound current-limiting resistor. For seat No. 5, the input filter capacitors and the operational amplifier integrated circuit 1 had detached, and the current-limiting resistor remained attached to the board and the copper track of the input capacitors. Most of the audio card components for seat No. 7 were intact, but the copper tracks connecting the resistor to the TVS diode and the track connecting the ground line were missing, leaving only partial track remnants. The audio card for seat No. 4 sustained thermal damage that resulted in all its components detaching from the circuit board along with the debonding of most of its copper tracks. The current-limiting resistor had detached and was not located, as such, it could not be determined if a wire-wound type had been installed (see figure 6). Figure 6. X-ray images of card Nos. 5 (left), 7 (center), and 4 (right) with location of currentlimiting resistor circled. On December 27, 2019, about 1155 Pacific standard time, a Cirrus Design Corporation SF50 (Vision Jet), N52CV, was substantially damaged after catching fire at Santa Monica Municipal Airport, Santa Monica, California. The pilot was not injured. The airplane was operated as a Title 14 Code of Federal Regulations (CFR) Part 91 flight. The pilot had planned to depart about 1215 on a flight to Carlsbad, California. Upon arrival at the airplane, the pilot began performing preflight checks and completed a walkaround. The pilot determined that the airplane was “in good condition,” so he continued to follow the preflight checklists. After starting the engine, he began to smell smoke. The smell became progressively stronger, so the p