What happened
On 12 December 2012, an Airbus A319-112, registration HB-IPX, was operating a scheduled flight, LX 1250, from Zurich (LSZH) to Stockholm (ESSA). During the flight, the aircraft experienced a series of electrical disturbances. The sequence began with a brief voltage drop on a sub-busbar, which triggered the activation of the essential transformer rectifier (ESS TR).
Between 13:13 and 13:30 UTC, the aircraft experienced intermittent power supply fluctuations between the two transformer rectifiers. This instability resulted in several cockpit alerts, including warnings for the brake system, blower, and extract fan. Subsequently, transformer rectifier 1 (TR 1) failed, forcing the aircraft to rely on TR 2 for the supply of DC BUS 1 and the battery bus. Despite this reconfiguration, multiple voltage-lowering events continued to occur.
At 13:57 UTC, the automatic tripping of circuit breaker AB11 de-energized bus 103XP, leading to the loss of various systems, including cockpit temperature control and certain control panel lighting. Further disruptions occurred during the descent and approach, including brief failures of the AC ESS BUS and various navigation displays (PFD and ND) on the commander's side. During the landing at Stockholm Arlanda, the left engine thrust reverser failed to function due to these electrical issues.
The investigation
The Swiss Transportation Safety Investigation Board (STSB) examined the aircraft's electrical power supply system, including the transformer rectifiers, circuit breakers, and various busbars. Investigators analyzed the flight data recorder, cockpit voice recorder, and the post-flight report from the aircraft's centralized fault display system (CFDS).
Technical tests were performed on the transformer rectifier, contactors, and circuit breakers. The investigation also looked into the aircraft's configuration changes and the absence of specific indications, such as the thrust reverser failure, during the flight. A detailed visual inspection was conducted at a suspected loose contact point, but no irregularities were found at that specific location.