Investigators have been unable to determine precisely why a cargo container on board a Boeing 737-400 freighter was not secured, causing it to shift in flight during a service to Edinburgh.
The West Atlantic aircraft (G-JMCZ) had been inbound from London Stansted on 26 April.
It was transporting 8.9t of cargo with 10 of its 11 main-deck bays loaded with containers. Only bay A, the most forward, was empty.
The jet had just started its descent to Edinburgh when the 695kg container in bay B, immediately behind bay A, shifted forwards and struck the bulkhead wall.
According to the UK Air Accidents Investigation Branch, the crew heard a “dull double bang” and initially thought the right-hand engine had surged.
But with the engine indications normal, the pilots considered the possibility of a cargo shift.
The aircraft’s centre-of-gravity remained within limits and there were no limitations in control, and the crew opted to continue to Edinburgh.
When the cargo was unloaded, personnel found the bay B container occupying bay A. “It is likely that the locks on the [container] loaded in bay B were not secured as required,” says the inquiry.
But the cargo had already been unloaded by the time the crew was notified, so investigators were unable to ascertain whether the locks had not been engaged or had malfunctioned.
“Given the lack of space within the main deck, it is not possible for anyone to check the security of the load or the position of the locks once the cargo is loaded,” the inquiry says.
While the captain – or a designated crew member – is responsible for cargo security, an individual crew check of each container or pallet as it is loaded would be difficult to achieve.
The inquiry believes either some, or all, of the locks on the bay B container were not secured and it moved forward when the 737 pitched down for descent.
Some minor damage was inflicted on the bulkhead but the cargo-loading system was not affected. Neither pilot was injured.
After the incident the carrier sought to lower the risk presented by a single empty bay, issuing guidance that bay B should be void rather than bay A. The UK Civil Aviation Authority also established a flight operations liaison group for medium-sized operators to share best practice.
