What happened
On August 20, 2002, an Embraer EMB 145 LU operated by Swiss International Air Lines, registration HB-JAT, was performing a scheduled flight from Düsseldorf to Zurich. While flying through the RILAX holding pattern, the co-pilot detected an electrical odor and smoke within the cockpit. In response to the developing situation, the flight crew declared an emergency and requested an immediate landing.
The crew donned oxygen masks to protect themselves from the fumes. During the approach to runway 16, the captain periodically removed his mask to reassess the situation, noting that the odor did not appear to be intensifying. The cabin crew confirmed that no smoke or odor was present in the passenger cabin. The aircraft landed safely at Zurich Airport, and passengers were able to disembark normally.
The investigation
Following the incident, the Swiss Transportation Safety Investigation Board (SUST) initiated an inquiry. Investigators, working alongside Swiss and SR Technics, performed an extensive examination of the aircraft's electrical systems, wiring, and cockpit components. No traces of fire or damage were found on the aircraft's electrical lines or ventilation systems. A ground run of the engines also revealed no signs of smoke or odor.
During a period when the aircraft was connected to an external power source in the hangar, an engineer detected an electrical smell near the co-pilot's station. This led to the discovery of a flashlight in its holder that was still switched on. The lens and the holder's cover showed clear signs of overheating and melting.
Findings
The investigation determined that the source of the smoke was a DME Corporation EF-2C flashlight that had been left in the ON position when placed back into its storage bracket. Because the aircraft's electrical system continuously recharges the flashlight's battery via the SHED BUS 2, the battery did not deplete, allowing the bulb to remain lit for several hours.
As the bulb remained active, the heat became trapped between the lens and the holder's cover, reaching temperatures as high as 230 °C. This caused the plastic components of the lens and the holder to melt and emit a pungent odor. While the manufacturer had previously issued a service letter suggesting a lower-wattage bulb to mitigate heat, the investigation found that the heat buildup in the enclosed holder was the primary driver of the melting process.
Safety action
Following the incident, the operator inspected all flashlights in its Embraer 145 fleet, discovering two other units with deformed plastic lenses. To prevent recurrence, the operator's engineering department implemented a modification to drill a hole in the holder covers, allowing heat to escape and making it easier for crews to see if a light is left on. Additionally, Embraer issued a global Service Letter (SNL 145-33-0011) in June 2003 to alert other operators to this potential hazard.