A de Havilland Canada DHC-6 Twin Otter 300 was on a flight from Birmingham (United Kingdom) for a flight to Dubrovnik, Croatia. It was the 4th leg of the series of delivery flights from Calgary, Canada to the Maledives in February 2010. At 23.36 local time, the aircraft had lifted off from the runway at Birmingham. The captain and co-pilot were the only occupants.
Burned electrical equipment after removal of ceiling panels (Source; aaib.gov.uk)
The cruise flight level was initially planned to be FL170 and after an uneventful climb the crew settled in for the cruise at FL170.
A few minutes after establishing cruise flight the captain observed the left "GENERATOR" caution light flicker twice. The crew discussed the situation and opted to open the DC Bus Tie to split the electrical system in two.
5 minutes later a faint smell was observed by the captain, but nothing abnormal was observed. A short while later a dim orange flickering glow was observed between two ceiling panels on the righthand side of the cabin. An emergency was declared and after going through several options it was decided to divert the London Gatwick where they could land on Runway 26L.
Heatshield from the fire area (source; aaib.gov.uk)
While diverting to London Gatwick the crew observed;
Left generator load indication near zero
RIght generator load indication nearly at full scale (maximum indication ~400 amp)
Simplified electrical schematic (source aaib.gov.uk)
It was unclear to the crew what was wrong, and the Emergency Check List (ECL) did not contain a procedure for this situation. Based on the indications the crew decided to shut down the right generator (this generator had also caused an issue before their departure from Calgary)
The "GENERATOR" caution light remained off, with the generator switched off. To conserve power all unnecessary electric equipment. To avoid an oxygen fed fire the supplementary oxygen was switched off, and also the left generator was switched off, leaving only the battery as a power source. While on the approach to Gatwick the crew was under the impression the glow between the ceiling panels had slightly reduced. After landing the aircraft was stopped once clear of the runway and the crew commenced with the shutdown checklist, the left engine shut down once the fuel supply was shut, but the left engine kept running at 12% to 15% rpm. Once the battery Master Switch was set to off, the engine shut down. Fire Fighters boarded the aircraft and determined that the temperature in the area where the crew as the orange glow was slowly rising, even with the aircraft fully powered down. Once the battery was disconnected the temperature started to drop, and once the temperature had dropped the firefighters were satisfied that there no longer was a risk for a fire. An investigation was launched by the Air Accidents Investigation Branch (AAIB), They determined that the cause of the fire was in a relay box (Reverse Current Relay). Due to heat damage, the AAIB could not determine the exact cause. The AAIB report is available by clicking here. The aircraft was ferried to an MRO in Switzerland for repairs on the 4th of March 2010. It subsequently continued to the Maldives, where it entered service with its new owner.
The aircraft in service after the repairs with its new owner, on floats ©Edward Alsford Photo