What happened
On December 22, 2000, a Fokker F27 MK050, registration SE-LIP, was operating a scheduled flight from Copenhagen/Kastrup to Linköping, Sweden. During the flight, while at approximately 17,000 feet, the flight crew detected a burning odor and a faint mist of smoke in the cockpit.
In response to the odor, the commander delegated flying duties to the co-pilot to begin troubleshooting. The crew requested clearance to descend directly toward Linköping. While the cabin attendant checked the passenger area and prepared fire extinguishers, the pilots opted not to don their oxygen masks because the odor appeared to subside.
As the aircraft approached Linkö and entered instrument meteorological conditions, several additional electrical failures occurred. These included the tripping of the DME circuit breaker, a warning regarding the left engine de-icing system, and a loss of autopilot trim functionality, which forced the co-pilot to transition to manual flight. Despite these complications, the crew landed the aircraft visually at Linköping/SAAB without difficulty. Because an emergency had not been formally declared, local search and rescue services were placed on alert by the tower controller rather than through an official distress signal.
The investigation
The Swedish Accident Investigation Authority (SHK) examined the aircraft's electrical architecture, specifically the direct current (DC) and alternating current (AC) systems. The investigation focused on the source of the smoke and the cause of the simultaneous instrument malfunctions.
Investigators discovered two separate, independent technical failures. The first involved the DC system, where a malfunction in a TRU contactor (a mechanical relay) caused the voltage to become unstable, leading to the intermittent failure of various instruments. The second failure was located in the AC system, where the investigation identified a burnt-off power cable.
Findings
- The smoke and burning smell were caused by electrical arcing in the AC-power system. A cable supplying power to the aircraft's ovens—which had been removed from the aircraft—had become energized because the circuit breaker had failed to trip. This live, free-ended cable intermittently shorted against grounded components during flight.
- The instrument failures were caused by a malfunction in the DC-power system's TRU contactor, which caused the voltage to periodically drop below the required specifications for the flight instruments.
- The crew did not declare an emergency despite the presence of smoke and multiple system failures.
- The investigation noted that a single component failure in this aircraft type's electrical system could, in a worst-case scenario, disable a significant portion of one of the two DC-power systems.