Flight History
The SAAB 2000, registration HB-IZG, operated by Crossair as flight SWR3874, departed Zurich-Kloten at 09:20 UTC on 1 February 2001 for a scheduled service to Luxembourg. Approximately four minutes after takeoff, while passing flight level 70, the master warning "AVIONIC SMOKE" activated. The flight crew observed light smoke and an odor they associated with an electrical system fire or overheating. They immediately donned oxygen masks. A cabin attendant confirmed no smoke was present in the passenger cabin. The smoke in the cockpit then dissipated briefly, but the crew decided to abort the flight and requested priority for an approach back to Zurich-Kloten.
During the approach, the commander informed a cabin attendant that landing would occur shortly and advised her to mentally prepare for a possible emergency. The attendant misinterpreted this as an instruction to prepare for an emergency landing and briefed passengers according to the cabin emergency checklist. The aircraft landed without incident on runway 16 at 09:41 UTC. Firefighting vehicles were standing by and accompanied the aircraft during rollout. Passengers and crew deplaned normally at the stand.
Immediately after arrival, a mechanic found that the avionic smoke detector circuit breaker had tripped. He removed the forward galley to access the avionics compartment and checked the smoke detector, which was unusually warm, with an estimated external temperature above 50 °C.
Findings
The smoke detector, manufactured by Fenwall (USA), serial number 4197, had been installed in the aircraft since its production in 1995 and had not caused any issues prior to this incident. Examination of its printed circuit board revealed three locations with evidence of excessive heat development, visible as circular marks around the solder points of resistors. For comparison, temperature measurement strips installed on smoke detectors in two other SAAB 2000 aircraft showed that normal operating temperatures were below 40 °C.
The avionics compartment on the SAAB 2000 is located immediately behind the commander's seat on the left side of the aircraft. It is separated from the cockpit and has a separate cooling air circuit. The wall facing the cockpit includes a flexible grommet designed for inserting a fire extinguisher nozzle during firefighting; this grommet was not completely airtight.
All other equipment in the avionics compartment functioned normally, and no hidden defects were found in any device that could have produced smoke in the weeks following the incident.
Cause
The serious incident was caused by an internal malfunction of the smoke detector in the avionics compartment, resulting in localized overheating of a printed circuit board. The smoke produced then spread into the cockpit through an opening.