What happened
On 9 December 2007, a Beech King Air 350, registration F-GVLB, was conducting a commercial passenger flight from Galway, Ireland, to Paris Le Bourget, France. While cruising at Flight Level 330, the crew noticed an acrid smell of burning electrical insulation within the cockpit.
In response to the smell, the commander began a descent to Flight Level 120 and initiated the standard smoke removal procedures. Although no smoke was initially visible, the smell returned alongside visible smoke in the cockpit. The crew declared a 'MAYDAY' to Air Traffic Control and diverted the aircraft to Cardiff Airport. The aircraft landed safely and without further incident, and while airport emergency services inspected the plane, no active fire was found.
The investigation
The investigation focused on the source of the electrical burning smell and smoke. It was determined that the issue originated in the right-side circuit breaker panel, which is positioned directly beneath the direct vision panel on the co-pilot's side window.
Prior to the flight, the aircraft had been parked outdoors at Galway for two days during which period it had rained. Examination of the aircraft revealed that water had managed to bypass the seal of the direct vision panel, allowing moisture to run down onto the circuit breakers.