What happened
On 23 October 2013, a DHC-8-402 Dash 8, registration G-JECJ, was operating an en-route flight when the crew began receiving several cautions and warnings on the Central Warning Panel. As the flight progressed, the frequency of these warnings increased, and the crew observed failures in both the cockpit and cabin lighting. In response to the deteriorating electrical situation, the aircraft diverted to Manchester Airport, where the crew performed an uneventful landing.
The investigation
Following the incident, the aircraft manufacturer conducted a detailed examination of the electrical components. The investigation focused on the right starter/generator and its associated Generator Control Unit (GCU). The manufacturer's analysis revealed that the brushes and the collector of the DC generator were significantly worn and damaged. However, testing of the Generator Line Contactor (GLC) and the DC GCU showed no faults.
Technical analysis determined that the loss of contact between the brushes and the armature occurred while the system was supported by the aircraft battery. This specific failure mode meant that the generator power quality protection circuits did not detect the issue. Because no fault was detected by these automated systems, the GCU and EPCU did not reconfigure the electrical system as they would during a standard power quality failure.
Findings
- The primary cause of the electrical instability was severe wear and damage to the DC generator brushes and collector.
- The failure was not detected by automatic protection circuits because the battery was providing backup power at the time of the contact loss.
- The crew could have identified the issue by monitoring for zero generator output current on the electrical load meter page or by observing abnormal positive discharge current from the battery.
- The electrical load on the right DC bus was being sustained by the main aircraft battery, leading to its progressive discharge.