What happened
On 19 September 2013, a BAe 1s46-300, registration EC-LOF, was performing a commercial cargo flight from Exeter Airport to Liege Airport. While climbing through flight level 220, the crew observed the FREIGHT DOOR UNLOCKED warning light illuminate on the Master Warning Panel.
Shortly after this initial indication, the pilots heard loud, intermittent, and unidentified noises coming from the main cargo compartment. Due to the combination of the warning light and the audible noises, the crew became concerned that the aircraft might have sustained structural damage. Consequently, the pilots declared an emergency with Air Traffic Control and requested a diversion to the nearest suitable airfield. The aircraft subsequently diverted to London Gatwick Airport, landing without further incident.
The investigation
An engineering investigation was conducted to determine the cause of the warnings and noises. The investigation established that the two issues were entirely separate events. The investigation found that the freight door warning was the result of a latch proximity switch that was out of adjustment.
Regarding the noises heard in the cargo area, investigators determined that a flexible duct within the air conditioning bay had become detached. This duct normally provides a connection between the right air-conditioning pack and a non-return valve on the pressure bulkhead. It was concluded that the movement of the pipe or the cycling of the valve likely caused the loud sounds heard by the crew.