What happened
On 19 October 2018, a Cirrus SR20, registration G-GCDA, was being collected from Turweston Aerodrome following its annual inspection. The pilot performed all standard pre-flight procedures, including engine start and power checks, without incident. However, as the aircraft lifted off the runway, smoke began emerging from a switch panel located near the primary instrument display.
The pilot notified the airfield radio and performed a brief circuit to land the aircraft safely. Upon landing, the pilot deactivated all electrical equipment and opened the cabin door to dissipate the smoke. Despite the shutdown, the pilot observed that the avionics appeared to remain powered. Seeking help from an engineer, the crew removed the engine cowling and disconnected the main battery, but smoke continued to emit from the switch panel. Upon removing the panel, it was discovered that a small fire had ignited the insulation material behind the unit.
The investigation
Investigators examined the 'bolster' switch panel and identified that the fire originated within the circuit board behind the system master switches. They found that a copper circuit track connected to the bat 2 master switch had melted due to extreme heat, which also damaged surrounding components and nearby insulation.
The investigation focused on the aircraft's dual battery system, specifically the modification used for the Cirrus Airframe Parachute System (CAPS). This modification included an external diode installed across the relay terminals to prevent voltage spikes. It was noted that the No 2 battery relay had been replaced shortly before the flight to address a charging issue. The investigation looked into whether the diode had failed or been disturbed during this recent maintenance.
Findings
- The fire was caused by an electrical overload on a circuit track that was not designed to handle the resulting current.
- A fault or disturbance involving a diode in the battery wiring harness allowed the No 2 battery load to pass through the relay coil and into the aircraft wiring.
- The overheating of the diode was evident, though the exact timing of its failure remained unclear.
- The diode assembly was difficult to notice due to its unremarkable appearance and awkward location within the harness.
Safety action
- The aircraft manufacturer has updated the parts catalogue, wiring manual, and electric CAPS service bulletins.
- A new design for the harness assembly, which includes an additional fuse to prevent such overloads, has been released for use in new aircraft.
- The manufacturer is planning the release of new CAPS kits and has made the fused harness available as a field-retrofit part for existing aircraft.