What happened
While descending from cruise altitude, the flight crew observed smoke entering the cockpit. Due to the presence of smoke, the crew declared an emergency.
The copilot identified the source of the smoke as a red-hot Push Button Selector Indicator (PBSI) switch, specifically the one used for the flight idle baulk test. To mitigate the situation, the copilot discharged a portable fire extinguisher directly onto the switch, which successfully stopped the smoke production.
The investigation
Following the incident, maintenance personnel removed the PBSI switch for examination. The inspection revealed that the switch was scorched and melted, and the adjacent wiring had sustained heat damage.
Investigation into the component's history showed that the manufacturer had previously released two service bulletins addressing malfunctions of the PBSI switch. These bulletins noted that such failures were associated with water ingress into the cockpit area near the switches. At the time of this incident, the operator had not complied with these voluntary service bulletins.