What happened
On September 22, 2018, a Cirrus SF50 Vision Jet, registration N474CG, was operating a training flight from Mainz-Finthen, Germany, to Zurich, Switzerland. Following landing at Zurich Airport, the aircraft taxied to the General Aviation apron. During the taxi phase, the crew noticed a faint, unusual odor inside the cabin. Once the aircraft had come to a stop and the engine was shut down, the smell intensified, which the student pilot described as an electrical burning odor.
Upon exiting the aircraft, the student pilot observed white smoke emerging from the nose gear bay and an access panel on the right side of the forward fuselage. The student also noted significant heat radiating from the access panel. The pilot-in-end noticed that despite turning off all electrical consumers, Battery 1 was still discharging a current of 56 Amperes. To mitigate the situation, the pilot deactivated Battery 1, which subsequently reduced the smoke production. Later, while the aircraft was being moved into a hangar, airport personnel observed an unusual odor and a gelatinous substance leaking from the nose gear bay.
The investigation
SUST examined the aircraft's electrical system and the air conditioning compressor unit. Investigators analyzed data from the aircraft's data logger, which confirmed a high discharge current from Battery 1 to the main bus during the period in question. Physical inspection of the air conditioning compressor controller revealed heavy soot deposits and significant fire damage near the electrical connections, including evidence of a white burn and a metallic melt bead on the positive terminal.
To determine the cause, investigators performed tests on a functional controller. They established that while loose connections were unlikely, the possibility of moisture ingress was a viable theory. Experimental setups demonstrated that introducing a saline solution between the printed circuit board (PCB) and the heat sink caused a leakage current and eventually led to a short circuit and fire, mirroring the damage found on the aircraft's component.
Findings
- The primary cause of the incident was an electrical short circuit within the air conditioning compressor controller.
- The short circuit was likely triggered by the ingress of water or moisture into the controller, which compromised the electrical insulation between the PCB and the heat sink.
- The compressor failed to operate during the flight, though the electrical arcing and fire development likely occurred during the final phase of the flight or after landing.
- The crew's failure to immediately notify airport authorities of the smoke and heat presented a safety risk to ground personnel.
- The crew also failed to follow the engine shutdown checklist regarding the securing of the Cirrus Airframe Parachute System (CAPS) safety pin.