Investigators have determined that the fire which brought down a UPS Boeing 747-400 freighter near Dubai probably started in the main cargo deck just forward of the wings.

The United Arab Emirates' General Civil Aviation Authority states that there were no declared shipments of hazardous material on the 747, but that package details identified "many" shipments of lithium batteries.

Further investigation found that at least three consignments contained lithium ion battery packs which met hazardous materials criteria and "should have appeared on the cargo manifest", the GCAA adds.

The crew of the aircraft, en route to UPS's Cologne hub on 3 September last year, advised Bahrain air traffic control of a fire warning 22min after leaving Dubai. The 747 was cruising at 32,000ft at the time and turned right to return to Dubai - a distance of some 150nm.

In an interim factual report on the accident the GCAA identifies the approximate location of the fire, in the forward main deck, below areas of ducting for the elevator and rudder controls.

Having been cleared to descend to 27,000ft the crew requested an immediate descent to 10,000ft.

"It is likely that less than 5min after the fire indication on the main deck, smoke had entered the flight deck and intermittently degraded the visibility to the extent that the flight instruments could not effectively be monitored by the crew," says the GCAA.

It adds that that flight-data recorder information shows a "movement anomaly" with the elevator, which was not responding properly to control column input.

Seven minutes after the initial alarm, during the emergency descent, the captain transferred control of the 747 to the first officer after declaring a lack of oxygen supply. Portable oxygen is stored behind the pilots' positions and the cockpit-voice recorder indicates that the captain left his seat. There was no further evidence on the recorder of further interaction from the captain for the remaining 22min of the flight.

The jet was inbound to Dubai International Airport runway 12L but overflew the airport's northern boundary at 4,500ft and 340kt.

While the flight crew were advised of the availability of Sharjah Airport, which required a left turn, heading 095 degrees. The GCAA says the co-pilot acknowledged the heading change and entered 195 degrees on the mode control panel.

It adds that the autopilot disconnected, the jet reduced speed and entered a descending right turn at 4,000ft. Several bank-angle, sink rate and ground-proximity warnings sounded before the 747 eventually crashed on a military installation 9nm south of Dubai, killing both pilots.

Source: Air Transport Intelligence news