Occurrence
On a flight from Oshawa Municipal Airport to Kingston, Ontario, a Cessna 152 (registration C-GCSJ, serial number 15284043) departed at approximately 1500 eastern daylight time. While proceeding eastbound near Bowmanville, Ontario, the pilot and passenger detected an electrical burning odor. About 10 seconds later, sparks were observed from beneath the lower left instrument panel. The pilot immediately turned back toward Oshawa and advised the tower of the situation.
Emergency Response
The passenger discharged the cockpit fire extinguisher, quickly extinguishing the small flames. However, the extinguishing agent and smoke reduced visibility. Both side windows were opened, clearing the smoke. The aircraft landed without further incident. The pilot suffered a minor burn to the leg.
Investigation Findings
Examination revealed that the landing light switch was not capable of handling the electrical load of the landing light circuit. Over time, the switch arced at its contacts, accelerating deterioration. Heat from arcing melted the switch casing, causing the contacts to fall out. Exposed contacts arced, igniting oil residue that had seeped from the oil pressure gauge line. This ignited accumulated dust, which in turn ignited the plastic instrument panel, sustaining the fire.
The 15-amp circuit breaker and wiring harness were appropriate for the circuit. The switch had been coated with dust and oil residue from the oil pressure line. The resulting fire produced gases that could be harsh to inhale; opening the windows likely prevented incapacitation.
Safety Actions
The Transportation Safety Board of Canada issued Aviation Safety Advisories to Transport Canada regarding landing light switch failure and smoke-in-cabin emergency procedures. Cessna revised service manuals for its 100-series aircraft, adding inspection and replacement requirements for landing and taxi light switches at 100-hour or annual intervals and every five years thereafter.
Probable Cause
The landing light switch was beyond its design capability for the circuit it controlled. Excessive heat from arcing weakened the switch, allowing contacts to fall out. Arcing ignited oil residue and dust, and the fire was sustained by the plastic instrument panel.