Analysis of the fatal UPS Boeing 747-400 freighter fire has been unable to determine whether a diversion to Doha, rather than the longer turnback to Dubai, would have altered the outcome.
United Arab Emirates investigators looking into the on-board fire - caused by the combustion of lithium batteries - examined the captain's decision to pursue a return to Dubai, a track of 185nm (340km), having been informed that Doha was closer at 100nm.
Neither pilot survived the 3 September 2010 accident. The captain was incapacitated and the first officer's attempt to land the crippled aircraft single-handed at Dubai was unsuccessful, and it crashed just outside of the city.
Checklists for a main-deck fire instruct the crew to plan a landing at the nearest available airport.
At the point of the diversion decision the aircraft would have needed 20min to reach Doha runway 15, says the UAE General Civil Aviation Authority, which would have put a Doha landing some 6-7min ahead of the time of the crash.
But the inquiry points out that the crew was familiar with Dubai and did not have Doha's charts or instrument landing system frequency immediately available. The aircraft's flight-management computer also reprograms automatically to the departure airport in case of a turnback within 400nm.
"There is no direct information as to why the crew elected to choose Dubai verses Doha," says the inquiry. "However, it is likely that at the time of the initiation of the turnback, the crew was not yet aware of the full extent of the fire and its effects."
The analysis notes that a Doha diversion would not necessarily have been successful. It says that the progressive failure of systems on the 747 - including elevator, speed-brake and oxygen supply problems - would have generated a similar level of control difficulties for the crew.
Investigators also found that the landing-gear failed to extend when the first officer selected the undercarriage lever, and this would have complicated a Doha approach. The aircraft was still heavily fuelled, fully-laden, and "in all probability" would have made a gear-up landing, says the inquiry.
But the GCAA states that the turnback created a "major difficulty" by taking the aircraft out of VHF radio range with Bahrain area controllers. Smoke prevented the first officer from retuning the radio frequency and communications had to be relayed via other aircraft.
Diverting to Doha would have enabled the crew to maintain direct contact with Bahrain, while radar surveillance and co-ordination would have been simplified.
"Analysis of the diversion to Doha and the likely outcome is speculative as the crew incapacitation, and smoke and fumes in the cockpit, would have prevailed," says the inquiry, adding that the 747's control was "seriously compromised".
It says: "The likely outcome of the diversion to Doha is therefore inconclusive, although the communication and task saturation issues experienced by the remaining pilot would have been negated by a Doha diversion."