What happened
On 23 February 2010, a De Havilland Canada DHC-6 300 Twin Otter, registration C-FAKB, departed Birmingham Airport on a ferry flight destined for Dubrovnik, Croatia. During the cruise at FL170, the flight crew observed flickering lights on the left generator caution panel. Shortly after, the crew noticed a faint smell and a visible orange glow appearing between ceiling panels in the passenger cabin.
As the aircraft prepared for descent, the crew observed significant electrical abnormalities, including a full-scale deflection on the left generator load meter and a high reading on the right generator load meter. To manage the situation, the pilots isolated the electrical systems and diverted to London Gatwick Airport. During the final approach, the crew noted a heat blister and smoke emanating from the cabin panels. Upon landing, the right engine continued to run despite the fuel being shut off, indicating that the aircraft battery was still powering the starter/generator. The airport fire service attended the scene and disconnected the battery to mitigate fire risks.
The investigation
The AAIB investigation focused on the power distribution and generator control box located in the aircraft roof. Examination revealed that an electrical fire had nearly consumed the left reverse current relay (RCR). The heat from the fire had damaged the cabin trim and several electrical control wires.
Investigators discovered that the RCRs installed in C-FAKB were part number A-700A, an obsolete component not officially approved for use on the DHC-6 series. While maintenance records suggested the units had been overhauled, the investigation found that the process actually involved only electrical testing rather than a full overhaul. Furthermore, testing of the right RCR (removed in Calgary prior to the flight) showed that its contact resistance was significantly higher than the maximum permitted value, indicating potential for overheating.
Findings
- The fire originated in the left RCR, which caused extensive damage to the power distribution box and adjacent wiring.
- A high current flow of at least 400 amps through the left starter/generator was the most likely cause of the fire.
- The left RCR was an unapproved, obsolete part number (A-700A).
- The electrical load meter readings indicated that the right generator was supplying excessive current to the left system.
- The heat shields around the RCRs had been breached by arcing, allowing the fire to spread to the cabin trim and control cables.
Safety action
Following the investigation, several safety recommendations were made to Transport Canada, including:
- A review of the design and effectiveness of heat shields for RCRs on DHC-6 aircraft.
- Measures to ensure only approved RCRs are fitted to the aircraft.
- A review of maintenance requirements to consider regular overhauls of the RCRs.