N52CV

HISTORY OF FLIGHTOn 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.

AIRCRAFT INFORMATIONAudio 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.

AIRPORT INFORMATIONAudio 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...