History of the Flight
On 23 February 2010, a De Havilland Canada DHC-6-300 Twin Otter, registration C-FAKB, departed Birmingham Airport at 2336 hrs for a ferry flight to Dubrovnik, Croatia. The aircraft climbed to FL170. A few minutes after leveling off, the captain noticed two brief flickers of the left generator caution light. After discussion, the commander opened the DC bus tie to electrically separate the two DC generator busbars. Approximately five minutes later, the commander detected a faint smell, but nothing abnormal was visible. The crew discussed the symptoms and decided not to reset the left generator as there had been no steady generator caution light.
Shortly after, the crew observed a dim orange flickering glow between two ceiling panels on the right side of the cabin, near the location of both reverse current relays (RCRs). The commander declared an emergency and requested ATC assistance for the nearest suitable airport. The aircraft was overhead the River Thames estuary near Manston, but Manston and Lydd airports were closed. Ostend Airport, Belgium, was 58 nm away, but the crew preferred not to fly over the sea. Southend Airport was open but had broken cloud at 200 ft aal, deemed unsuitable. After further coordination with ATC, the crew decided to approach Runway 26L at London Gatwick Airport.
During the descent, the crew shut down the right generator and switched off unnecessary electrical items. The commander also went into the cabin to turn off the supplementary oxygen. The left generator was then switched off, leaving the battery as the sole power source. On final approach, the glow behind the panels appeared reduced, but a heat blister had developed on the cabin trim, which was smoking slightly. The aircraft landed and taxied clear of the runway. The left engine shut down normally, but the right engine continued running at 12% to 15% Ng after fuel shutoff. The captain stopped the engine by selecting the battery Master Switch to off.
Airport fire crews attended and found the temperature near the RCRs slowly increasing. They disconnected the aircraft battery and remained until the fire risk subsided.
Examination of the Aircraft
An inspection revealed an electrical fire that had nearly consumed the left RCR. The fire burned through the cover of the power distribution and generator control box, located in the roof on the right side of the cabin. The cabin trim blistered but remained intact, and there was no smoke damage in the cabin. Combustion products were trapped between the fuselage and cabin trim, exiting through a roof vent.
The power distribution box was replaced, and the aircraft was flown to a maintenance organization in Switzerland for a detailed inspection of the electrical system. The maintenance organization reported that while the wiring was in poor overall condition, no obvious cause for the fire could be found. The entire aircraft wiring was replaced, and engine ground runs tested the electrical generation and distribution system satisfactorily. The aircraft then continued to the Maldives.
Reverse Current Relays
The RCRs are located within the power distribution box. The drawing for the box lists approved part numbers for the DHC-6 as A-700AP, A-700AAP (300 amps), and A-701D (400 amps). The left RCR that failed had a different part number not authorized for use on the aircraft. The Type Certificate Holder provided records of 18 RCR failures since 1974.