What happened
On August 15, 2003, a SAAB 2000, registered as HB-IZJ, departed Zurich Airport for a scheduled flight to Cologne. While cruising at flight level 220, the crew noticed an unusual odor resembling melting plastic. The crew attempted to identify the source by shutting down the air conditioning pack and the windshield heating, but the smell persisted.
Unable to locate the cause of the odor, the flight crew decided to abort the flight and requested priority handling from air traffic control to return to Zurich. The aircraft performed an emergency descent and landed safely on runway 14. There were no injuries among the 50 passengers and four crew members.
The investigation
The Swiss Transportation Safety Investigation Board (SUST) examined the aircraft's electrical systems and the physical condition of the forward galley. Investigators focused on the area behind the galley, which is mounted on rails to allow access to the avionics compartment. The investigation also reviewed recent maintenance history, noting a previously disconnected circuit breaker (E 28) that had been causing lighting issues, though investigators determined this was unrelated to the primary incident.
Findings
Investigators established that the incident was caused by an electrical arc occurring between two live wires and the aircraft's metallic structure. The investigation found that the wiring for the galley work lights was designed with enough flexibility to allow the galley to be moved in and out on its rails. However, it is believed that during the process of pushing the galley back into place, the cables became pinched between the galley structure and the door frame of the avionics compartment.
This pinching action damaged the wire insulation, allowing electricity to jump to the airframe. The resulting high temperatures from the arcing caused the plastic insulation to melt and vaporize, creating the distinct burning smell noticed by the crew.