N52CVCirrus Design SF50 · 2017

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 pilot decided to terminate flight preparations and have a mechanic examine the airplane. The pilot completed the engine shutdown about 10 minutes after turning on the airplane’s master switch.

The pilot then opened the main cabin door and saw smoke rise from the armrest area of the No. 5 (right center) passenger seat (see figure 1). A mechanic with a fire extinguisher arrived at the airplane within a few minutes, by which time the smoke had become dense and was streaming out of the cabin door. A few minutes later, flames began to emerge from the cabin, after which the cabin became completely engulfed in fire (see figure 2).

Figure 1. Smoke rising from armrest (Source: Pilot).

Figure 2. Flames engulfing airplane cabin (Source: Pilot).

The airplane was equipped with a crash-hardened recoverable data module (RDM) flight recording device, which was installed above the forward cabin footwell. The RDM recorded critical airplane systems and flight parameter information at 1-second intervals. Review of the RDM data indicated that electrical power to the airplane was turned on about 1138:00 and that the engine was started 3 minutes later. At 1148:30, the engine was shut down; the RDM stopped recording a few seconds later. The voltage of the three electrical buses remained constant throughout the startup and engine run phases, and no voltage drops were noted.

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.

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.

Other Damaged Audio Cards

During the investigation of this accident, Cirrus became aware of two earlier model SF50 airplanes (serial Nos. 4 and 5) that appeared to have damaged audio cards. Examination revealed that the two damaged cards both used the wire-wound current-limiting resistor. Additionally, a short circuit was measured across the input filter capacitors of both cards. Further examination of the cards revealed that the current-limiting resistor had overheated and burned through the heat-shrink sleeve (figure 7). Cirrus also found that a microphone card for another SF50 airplane (serial No. 31) had been returned after the current-limiting resistor burned in a similar manner.

Figure 7: Audio card exhibiting burnt resistor (Source: Cirrus).

In response to these findings, on February 7, 2020, Cirrus issued Alert Service Bulletin SBA5X-23-03, which provided instructions to disconnect and remove the audio and microphone card assemblies and return them to Cirrus. On February 14, 2020, compliance with this bulletin became mandated through FAA Emergency Airworthiness Directive 202003-50. All the cards were removed from the SF50 fleet and returned to Cirrus; none of them exhibited similar heat damage.

Updated Card Design

As a result of the accident, Cirrus redesigned the audio interface card. The circuit design remained essentially the same except that the current-limiting resistor was changed to a 1,ooo-ohm, 3-watt resistor that had a flame-proof coating. Therefore, under the same capacitor short-circuit condition, the resistor’s power consumption would be 0.784 watts, which was 3.5 times lower than its power rating. Additionally, the four surface-mounted input filter capacitors were replaced by a single epoxyencased 15-microfarad tantalum capacitor. The card was fitted in an injection-molded plastic box and encased with a potting compound.

Because the original audio and microphone cards were the subject of an airworthiness directive, the updated design was approved directly by the FAA rather than a designated engineering representative. The installation of the updated audio card was authorized through Cirrus Service Bulletin SB5X-23-04, which was issued on August 18, 2021.

Other Related Event

The NTSB investigated an accident involving a Cirrus SR22 (accident number CEN11FA267). The investigation found that the airplane’s RDM had stopped working about 9 months earlier. Examination revealed that the unit’s power supply module had an input voltage protection design that used a TVS diode and current-limiting resistor (similar to the design of the audio cards). The investigation determined that the TVS diode had failed, resulting in a short circuit that created an overcurrent condition in the resistor. The overcurrent condition resulted in the resistor’s failure in an open circuit after it burned through the surrounding conformal coating and damaged the printed circuit board.

The airplane was equipped with a crash hardened Recoverable Data Module (RDM), a flight recording device installed above the forward cabin footwell. The RDM recorded critical airplane systems and flight parameter information at 1 second intervals. Review of the RDM data indicated that electrical power to the airplane was turned on at 1138, and that the engine was started 3 minutes later. At 1148:30, the engine was shut down, and a few seconds later the unit stopped recording. The voltage of the three electrical busses remained constant through the startup and engine run phase, and no voltage drops were noted.

Flammability Testing

After this accident, Cirrus performed flammability tests to determine if an audio card could ignite an interior panel and a flame could propagate. The tests involved installing audio cards, which had been modified to simulate the capacitor short-circuit condition, into a section of the interior side panel. The panel was a production version, the interior side was constructed from a glass-fiber composite panel, and a carbon fiber rib was constructed with a PVC foam core.

Multiple cards were tested using both the thick-film and wire-wound resistor types. The tests involving the cards with the thick-film resistors exhibited local heating and burnthrough of the heat-shrink sleeve but did not result in any flame. The cards with wire-wound resistors displayed greater heating properties, with the resistor from one card glowing, erupting in flames, and igniting the panel material. Although the burn propagation speed for the panel material was slow (6 minutes to travel 2 inches), the card continued to produce a flame until power was disconnected. Additionally, the card emitted flaming drips during the test, most of which extinguished within 1 second, but one flaming drip extinguished after 8 seconds.

Manufacturing Assessment

The NTSB submitted two of the audio cards recovered from the returned aircraft in the field to the Air Force Research Laboratory/Materials Integrity Branch for a manufacturing assessment. Both cards were manufactured by different suppliers.

According to Cirrus documentation, the cards were to be assembled in accordance with IPC (Institute to Printed Circuits) standards J-STD-001. Requirements for Soldered Electrical and Electronic Assemblies, Class 2 and IPC-A-610, Acceptability of Electronic Assemblies, Class 2.

IPC class 2 defects were observed on both cards; thus, they did not meet all the applicable design and acceptability requirements described by the standards. The defects included incorrectly bent component leads, insufficiently wetted solder junctions, conformal coating presence in areas required to be free of such coating, conformal coating loss of adhesion bubble (void), and flux residue remnants.

Design Review

Cirrus undertook a design review of all in-house designed electrical systems across the entire Cirrus Aircraft fleet to determine if any of the systems used the same power regulation design. Only one other system was found to have similar input properties, the “thumbwheel checklist” circuit in SF50-series airplanes. The circuit card, which was directly mounted to the thumbwheel switch assembly in the control column, was not coated with heat shrink or subject to the same mounting forces as the audio cards. Testing by Cirrus revealed that, with a shorted capacitor, the current-limiting resistor would fail in an open circuit within 5 seconds but would not result in an open flame.