What happened
On February 24, 2015, a Bombardier DHC-8-400 operated by EUROLOT SA was performing a scheduled flight from Gdańsk to Amsterdam. While flying over German airspace, the crew observed the activation of the Master Caution light shortly before the top of descent.
An investigation of the Crew Warning Panel revealed that the failure of the number 2 engine AC generator had caused a cascade of secondary alerts. Specifically, the crew identified failures related to the left AC bus, the left Transformer Rectifier Unit (TRU), the left pitot heat, the number 1 engine air inlet heat, and the stick pusher system.
Following established checklists for the left AC bus and stick pusher system failures, the crew disconnected the autopilot and continued the flight using manual controls. After landing in Amsterdam, the aircraft was powered down and restarted to troubleshoot the issue. Although the initial engine test showed no recurrence of the fault, the captain decided to return the aircraft to Gdańsk. During the return leg, the left AC bus failure alert reactivated approximately 40 minutes before arrival in Gdańsk. The flight was completed without further incident, and the aircraft was handed over to maintenance personnel.
The investigation
The PKBWL examined the aircraft's electrical systems and maintenance records. Technical troubleshooting was performed on the aircraft to identify the root cause of the intermittent bus failure. The investigation focused on the sequence of electrical warnings and the subsequent impact on flight control systems, such as the stick pusher and pitot heating.
Findings
- The primary cause of the incident was a faulty K1 relay within the AC electrical system.
- The failure of the number 2 engine AC generator was a consequence of this relay malfunction.
- The activation of the left AC bus failure led to the loss of several secondary systems, including the stick pusher and pitot heat, due to the loss of power on that specific bus.
Safety action
- Maintenance personnel replaced the faulty K1 relay in accordance with the manufacturer's technical documentation (card 24-21-21). Following successful testing, the aircraft returned to service.