What happened
On February 1, 2001, at 09:20 UTC, a SAAB 2000, registration HB-IZG, operated by Crossair, departed Zurich-Kloten for Luxembourg as flight SWR 387 and 74. Approximately four minutes into the climb, while passing through flight level 070, the crew received an "AVIONIC SMOKE" master warning. The pilots immediately noticed light smoke and a smell indicative of an electrical fire or overheating, prompting them to don oxygen masks.
While a cabin attendant confirmed that no smoke was visible in the passenger cabin, the smoke in the cockpit persisted long enough for the pilot to briefly remove his mask to inform passengers before re-donning it for the remainder of the flight. The crew decided to abort the flight and requested priority for an immediate landing at Zurich. During the approach, a communication misunderstanding occurred when the captain advised a cabin attendant to prepare for an emergency landing, which the attendant interpreted as an instruction to initiate the cabin emergency checklist. The aircraft landed safely at Zurich-Kloten at 09:41 UTC, accompanied by emergency services.
The investigation
Following the event, a maintenance technician noted that the avionics smoke detector circuit breaker had tripped. Upon inspecting the avionics compartment, the technician found the smoke detector was unusually hot, with external temperatures estimated to be over 50 °C.
Investigators examined a Fenwall smoke detector (serial number 4197) that had been installed in the aircraft since 1995. Examination of the detector's printed circuit board (PCB) revealed circular heat marks around three resistor solder points. To establish a baseline, investigators measured temperatures on similar aircraft of the same model, finding that normal operating temperatures remain below 40 °C. The investigation also focused on the structural interface between the avionics compartment and the cockpit, noting a flexible cuff designed for fire extinguisher access.
Findings
- The primary cause of the smoke was the overheating of a printed circuit board within the smoke detector due to an internal malfunction.
- Excessive heat at the resistor solder points caused the paraffin-like protective coating on the PCB to evaporate, generating smoke.
- The smoke was able to enter the cockpit through a gap in the flexible cuff that separates the avionics compartment from the flight deck.
- The circuit breaker tripped due to high current draw, which ultimately stopped the overheating process.
- The flight crew's response, including the immediate use of oxygen masks and the decision to return to the airport, was appropriate.