Smoke in the cockpit and an inability to maintain altitude forced a UPS Boeing 747-400 crew to attempt a return to Dubai before the aircraft crashed near the airport, killing both pilots.

Evidence that a fire on board the 747 preceded the 3 September accident has been disclosed by the United Arab Emirates' General Civil Aviation Authority.

The cockpit-voice recorder was retrieved six hours after the crash but investigators are still trying to locate the flight-data recorder, the authority states.

UPS' flight 6 to Cologne/Bonn departed Dubai International Airport at 18:53. But less than 25min after take-off, at 19:15, information relayed from the Bahrain flight information region stated that the jet was intending to return to Dubai.

 © AP/Press Association Images
Neither crew member survived the crash which destroyed the 747

The GCAA says the aircraft had "smoke in the cockpit" and was "unable to maintain altitude".

United Arab Emirates air traffic control cleared the aircraft to land while it was some 40km (22nm) from Dubai.

But the GCAA says the aircraft was "high on the approach" - still at 8,500ft while 24km from the airport - and it overflew the airport "very high" before conducting a right turn.

It says the crew was informed that all runways at Dubai were available to the aircraft, but the 747 headed southwest and "rapidly lost altitude" before controllers lost radar contact at 19:42, 49min since departure.

The impact point, between Emirates Road and Al Ain Highway, is unpopulated and lies to the southwest of Dubai International Airport and north of the Al Minhad military air base.

US investigators from the National Transportation Safety Board are assisting the UAE authorities in the inquiry.

Source: Air Transport Intelligence news