What happened
On 26 May 1999, a Fokker F27 Mark 500, registration G-JEAG, was performing a passenger flight from Belfast when the crew encountered smoke in the cockpit. Shortly after departing Belfast, the captain noted an odor that was initially thought to be related to a previous fuel spill. As the aircraft climbed through FL80, visible smoke and a distinct smell appeared near the left panel switches.
In response to the smoke, the flight crew declared a smoke emergency, donned their protective masks, and executed standard memory drills. The commander ordered the cockpit heater to be deactivated to help identify the source of the smoke. Following the shutdown of the heater, the visible smoke dissipated, though the odor remained. As a precautionary measure, the flight was diverted back to Belfast, where the aircraft landed safely with no injuries to the 3 crew members or 15 passengers.
The investigation
The investigation, based on the pilot's accident report and the operator's findings, focused on the electrical and heating systems. Initial inspections of the electrical wiring and engine ground runs yielded no further signs of smoke or defects. Because the issue could not be immediately replicated, the fault was initially recorded as an Acceptable Deferred Defect.
Subsequent checks on 4 June revealed that the problem was linked to the cockpit heating unit. Upon replacing the heater, the issue was resolved. The investigation also identified that a burnt-out element within the heater was the source of the burning smell and smoke.