Incident Overview
On 23 October 2020 at 0920 UTC, a Beech B200 (registration G-YVIP) was on final approach to Bournemouth Airport in clear weather at approximately 1,200 ft. Shortly after selecting approach flap, the crew noticed a yellow glow and smoke emanating from behind the commander's circuit breaker (CB) panel. The crew attempted to isolate the problem by operating the master switch gang bar, which disconnects the battery and electrical generators, but this had no effect. They subsequently declared a mayday. The first officer attempted to reach the fire extinguisher under his seat but found it difficult due to shoulder straps. After landing, the crew evacuated the aircraft on the runway. The airport fire service attended, but the smoke ceased without intervention when the aircraft was shut down. No injuries were reported among the two crew members; there were no passengers on board.
Operator's Investigation
The operator conducted an investigation and discovered evidence of moisture ingress in the commander's CB panel. The back-lighting circuit board showed signs of overheating and burning. The commander's CB panel is located beneath the left storm window, which can be opened. The storm window has a seal intended to minimize moisture leakage into the cockpit. After the incident, the operator assessed the seal as serviceable. The operator reported that there is a 'reasonable body of evidence that they are prone to water ingress, particularly when the aircraft is parked outside in the rain', and that water collecting on the outside of the window can enter the cockpit when the window is opened. An improved seal is available as an optional modification, but it had not been fitted to G-YVIP.
The operator noted that they normally keep their aircraft in a hangar when not in use, but G-YVIP was a recent addition to the fleet and anecdotal evidence indicated it had previously spent most of its time outdoors. Additionally, the operator's crews had been routinely opening the storm window to clear the cockpit of a 'fogging' agent used for sterilisation in response to the coronavirus pandemic. The operator considered this might have increased the likelihood of water ingress.
Conclusion
The operator's investigation concluded that the most likely scenario is that moisture entered the cockpit through the storm window, eventually resulting in electrical breakdown of the CB panel back-lighting circuit board.
Safety Actions Taken
Following the incident, the operator installed the improved storm window seal across their entire fleet of King Air aircraft. Additionally, whenever possible, the operator's aircraft are now parked inside the hangar and only towed outside when required.