But the inquiry believes it started in the vicinity of two pallets, loaded in the aft main deck, which were carrying dangerous goods.
The blaze spread “rapidly”, says Korean investigation authority ARAIB, and could not be suppressed.
Control over the flight deteriorated and neither of the two crew members survived after the aircraft – operating from Seoul to Shanghai on 28 July 2011 – crashed into the sea.
While the precise cause of the fire could not be established, the inquiry points out that two pallets close to the rear freight door had been carrying lithium-ion batteries as well as highly-flammable and corrosive liquids.
Just 3min after the flight was handed from Incheon to Shanghai area control, the crew requested an emergency descent from 34,000ft to 10,000ft, citing a main-deck fire.
Data relayed by the ACARS communication system on the 747 shows that the initial fire alert was triggered in main deck zone 11, just ahead of the rear freight door.
Several more alerts were transmitted, from other fire sensors in the main deck, over the subsequent 2min.
The ACARS data also indicates that multiple aircraft systems began to fail in quick succession.
Within 5min of the emergency declaration the system recorded alerts for flight-control systems, the satcom data unit, air conditioning packs, yaw damper and the auxiliary power unit.
This system deterioration continued and, over the following few minutes, control of the crippled 747 gradually ebbed away from the crew.
Critical flight systems were lost and the pilots – who stated that the aircraft was shaking “violently” – were unable maintain altitude. The crew told Jeju approach control that they were going to attempt a ditching, but there was no further contact.
(David Kaminski-Morrow - Flightglobal News)