Incident Overview
While descending from cruise altitude, the flight crew observed smoke in the cockpit and declared an emergency. The copilot identified the source as a red-hot push button selector indicator (PBSI) switch for the flight idle baulk test. The copilot discharged a portable fire extinguisher onto the switch, after which the smoke ceased.
Maintenance Findings
Following the incident, maintenance personnel removed the PBSI switch. Examination revealed the switch was scorched and melted. Additionally, adjacent wiring exhibited heat damage.
Manufacturer Service Bulletins
The manufacturer had previously released two service bulletins addressing PBSI switch malfunctions linked to water ingress into the cockpit near the switches. At the time of the incident, the operator had not implemented these voluntary service bulletins.