Occurrence
On 24 February 2015, a Bombardier DHC-8-400 operated by EUROLOT SA was conducting a scheduled flight from Gdańsk to Amsterdam. While over German airspace (EDWW), a few seconds before commencing descent, the Master Caution warning activated. The crew identified the fault on the Crew Warning Panel as an L AC BUS caution, along with additional indications for L TRU (Transformer Rectifier Unit), PITOT HEAT 1 (left pitot heat), ENG ADPT HEAT 1 (engine air inlet heat), and PUSHER SYST FAIL (stick pusher system failure). All secondary cautions were determined to be consequences of the left AC bus fault.
Crew Actions and Return
The crew executed the "L AC BUS" (Caution Light) checklist and the subsequent "PUSHER SYST FAIL" checklist. In accordance with the latter, the autopilot was disengaged and flight continued manually. After landing in Amsterdam, the crew informed the Maintenance Control Center (MCC). The aircraft was completely de-powered. Upon repowering, an engine run-up test was performed, and the AC generator fault did not recur. By the captain's decision, the aircraft returned to Gdańsk.
During the return flight at cruising altitude, the left AC bus fault indication reappeared approximately 40 minutes before landing. The crew again performed the relevant checklists and continued the flight to Gdańsk, where a normal landing was made.
Technical Findings
The aircraft was handed over to technical services. Troubleshooting identified a failure of relay K1 in the AC electrical system. According to task card 24-21-21, the relay was replaced. After satisfactory functional tests, the aircraft was returned to service. The fault did not reoccur.
Probable Cause
The cause of the incident was a failure of a relay in the AC electrical system.