What happened
On 10 January 2020, a DHC-8-402 Dash 8, registration G-PRPH, was undergoing a routine turnaround at Leeds Bradford Airport. During the process, the crew noted a strong odor resembling a burning clutch in the cockpit and forward cabin, though the smell dissipated quickly and was initially attributed to nearby ground vehicle activity.
Following pushback at 1956 hrs, the crew successfully started the right engine. However, upon switching off the Auxiliary Power Unit (APU), the 'APU STARTER' light remained illuminated despite the switches being in the off position. As the pilot performed emergency drills from the Quick Reference Handbook, additional caution lights appeared on the primary flight displays and engine displays.
In an attempt to resolve the issue, the crew connected a ground power unit (GPU) to reset the system. During this procedure, the aircraft batteries began to discharge rapidly, causing three flight deck display screens to lose power. Simultaneously, ground personnel reported seeing smoke rising from the rear of the aircraft. Following advice from fire crews, the commander ordered a rapid disembarkation. All 51 passengers and 4 crew members exited the aircraft via the forward left door without injury.
The investigation
The investigation focused on the electrical malfunction and the subsequent smoke production. Maintenance engineers examined the aircraft and discovered heat damage on the APU starter/generator. To rectify the issue, the starter/generator and the K26 contactor—the component responsible for connecting the starter/generator to the right main bus—were replaced.
Findings
- The investigation established that the APU starter/generator had overheated.
- The overheating was likely caused by a failure of the K26 contactor, which may have resulted in the main contacts welding together.
- Such a failure would have kept the starter/generator powered whenever the right main bus was active.
- While the manufacturer has implemented inspection procedures for these contacts during C checks, it was not possible to verify if this specific aircraft had undergone such an inspection during its most recent check.