What happened
On 23 October 2013, a DHC-8-402 Dash 8, registration G-JECJ, was operating a commercial passenger flight from Edinburgh to Brussels at flight level 250. The crew initially noted a pusher system failure and followed the appropriate checklists, deciding to continue the flight. However, the situation rapidly deteriorated as cabin ceiling lights began to extinguish progressively from the front of the aircraft.
Shortly thereafter, the flight crew experienced a cascade of electrical issues, including elevator feel and pitch trim cautions. The co-pilot's Multi Function Display (MFD) and Primary Flight Display (PFD) both failed, and the crew received further warnings regarding the TCAS and Yaw Damper. As the aircraft descended, the cockpit lights, the No 2 Audio Radio Control Display Unit (ARCDU), and even the crew's emergency torch failed.
Observing that the No 2 generator showed zero load, the crew declared a PAN call and diverted to Manchester Airport. During the approach, the crew noted a zero reading for hydraulic system 2 contents, leading them to configure the aircraft for landing early. Despite the extensive list of approximately 25 cautions and failures, the aircraft landed safely on Runway 2 and taxied to a remote stand using a 'Follow Me' vehicle because the crew could not switch radio frequencies.
The investigation
The AAIB examined the aircraft's electrical generation system, which utilizes three 28v DC starter/generators. The investigation focused on why the aircraft failed to automatically reconfigure its electrical buses following the loss of the No 2 generator's output.
Analysis of the flight recorder and crew accounts suggested that while the No 2 starter/generator or its Generator Control Unit (GCU) had likely malfunctioned, a secondary failure prevented the automatic connection of the right DC bus to the left DC bus. This caused the right DC bus to draw power from the main aircraft battery, leading to a progressive discharge of the battery and the subsequent loss of various electrical services.
Findings
- The primary cause of the electrical degradation was a failure of the auxiliary contacts within the K2 contactor to signal the Electrical Power Control Unit (EPCU).
- This failure prevented the automatic closing of the bus tie contactor, meaning the right DC bus was not tied to the left DC bus.
- The operator's inspection of the K2 contactor revealed severe pitting on the auxiliary contacts, which likely prevented the necessary signal from being sent to the EPCU.