What happened
On 21 July 2011, a Bombardier DHC-8-402 Dash 8, registration G-ECOF, was performing a commercial passenger flight from Newquay to Edinburgh. While cruising at FL240, approximately 25 nm south-west of Carlisle, the crew received a master warning and a smoke warning light indicating smoke in the forward baggage compartment.
Treating the warning as a genuine threat, the pilots implemented emergency checklists. These actions included de-powering various electrical systems, which resulted in the loss of the autopilot, the transponder, the CVR, the FDR, and the co-pilot's flight displays. The procedure also caused the aircraft to depressurise. Although the crew discharged fire suppressant into the forward compartment and the warning light eventually extinguished, the smoke light later illuminated permanently, leading the crew to fear a persistent electrical fire.
Upon landing at Edinburgh, the aircraft vacated the runway and the crew ordered an evacuation. Passengers exited through the available doors onto a taxiway. While most passengers exited safely, some experienced difficulty jumping from the rear doors due to the 1.6 m height, and one passenger sustained a minor injury to her hip.
The investigation
The AAIB investigation focused on determining the cause of the intermittent smoke warning. Investigators examined the aircraft's electrical systems and the smoke detector components. No actual fire or smoke was found in the forward baggage compartment during the inspection.
Technical analysis of the smoke detector connector revealed that several blanking pins were missing from the socket. This allowed moisture to enter the connector. Laboratory testing of the smoke detector and fire control amplifier showed that when subjected to humidity and thermal cycling, the components produced false warnings. Specifically, corrosion was found on the detector plug and the female connector socket.
Findings
- The smoke warning was spurious, triggered by an intermittent short-circuit in a smoke detector connector.
- Moisture ingress occurred because blanking pins were missing from the unused holes in the smoke detector socket, allowing humidity to reach the pins.
- Corrosion was present on the smoke detector plug and the connector socket, which facilitated the electrical short-circuit.
- The crew's decision to follow the checklist was based on treating the warning as a real threat, despite no visible smoke in the cabin.
Safety action
Following the incident, the aircraft operator initiated an inspection programme for all similar aircraft in its fleet to ensure that blanking pins are correctly installed in smoke detector connectors and to assess the condition of the pins.