United Arab Emirates investigators have been unable to pinpoint the precise reason for the auto-ignition of a batch of lithium batteries, the resulting fire from which brought down a UPS Boeing 747-400F.
It confirms previous findings that the blaze started on the forward main deck, in a zone located 12.5-15.5m (41-51ft) behind the nose. The area is immediately below truss assemblies supporting flight-control cables.
The UAE's General Civil Aviation Authority says it determined with "reasonable certainty" that the location of the fire was in an area of cargo that contained lithium batteries.
Investigators have not determined exactly when the fire ignited. The probe says the batteries, "for reasons which cannot be established", entered a state of energetic failure and thermal runaway, resulting in auto-ignition. Smoke detectors did not pick up the initial signs of fire. The inquiry notes that pallets with rain covers "can contain smoke until a large fire has developed".
Based on cargo pallet and container testing by the US National Transportation Safety Board, says the inquiry, a "large catastrophic fire" which "cannot be contained" can develop in the space of 10min.
Investigators have not determined how long the UPS 747 fire was burning before the pilots received the first alarm, about 22min into the flight to Cologne, but says it possibly ignited 10-15min beforehand.
"Less than three minutes after the first warning to the crew, the fire resulted in severe damage to flight control systems and caused the upper deck and cockpit to fill with continuous smoke," the inquiry says.
At least three shipments of lithium batteries were identified as being in pallet positions consistent with the origin of the fire. The inquiry says these should have been declared as dangerous cargo during loading in Hong Kong, but states: "There were no declared shipments of hazardous materials on board the accident flight."
Neither pilot survived the crash, which occurred as the aircraft attempted to return to Dubai on 3 September 2010.