What happened
On a flight departing Oshawa Municipal Airport for Kingston, Ontario, the pilot and one passenger of a Cessuna 152, registration C-GCSJ, encountered an electrical fire shortly after departing the Oshawa control zone. While flying east toward Bowmanville, the occupants detected a burning smell followed by sparks emerging from beneath the lower left instrument panel.
The pilot immediately initiated a return to Oshawa and notified air traffic control. During the emergency, the pilot deactivated all electrical systems, including the master switch. Small flames were observed igniting the plastic instrument panel. The passenger utilized a fire extinguisher to suppress the flames, though the resulting smoke and extinguishing agent significantly obscured visibility within the cockpit. To clear the fumes, the crew opened both side windows. The aircraft landed safely at Oshawa, with the pilot sustaining one minor burn to the leg.
The investigation
Investigators examined the electrical components and the physical state of the cockpit. While the wiring harness, circuit breaker, and landing light circuit were found to be appropriately rated for the electrical load, the landing light switch itself was identified as the source of the failure.
Evidence showed that the switch was being used beyond its design capacity. Furthermore, the switch had accumulated a layer of dust and engine oil residue that had leaked from an oil pressure line located directly above it. This accumulation, combined with the electrical load, led to arcing at the contacts. This arcing caused heat levels to rise until the switch casing melted, compromising its structural integrity and allowing the internal contacts to shift or fall away.
Findings
- The landing light switch was unsuitable for the electrical load it was controlling.
- Arcing and oxidation within the switch caused excessive heat, which melted the switch casing and destabilized the internal contacts.
- The arcing caused the accumulated oil residue to flash, which ignited the surrounding dust and subsequently the plastic instrument panel.
- The fire was sustained by the combustible nature of the plastic panel, producing smoke that could potentially cause crew incapacitation if not for the window ventilation.
Safety action
Following the investigation, the TSB issued safety advisories to Transport Canada regarding the risks of using AC-rated switches in DC landing light circuits and the need to review emergency procedures for smoke in the cabin. In response, Cessna Aircraft Company updated its service manuals and issued a special inspection requirement. This mandate requires the inspection and replacement of landing and taxi light switches during 100-hour or annual inspections, with a recurring replacement interval every five years.