A crew member on an Indonesian Maritime Surveillance looks out the window during a search for AirAsia's Flight QZ8501, north of Bangka island December 30, 2014. [Photo/Agencies]

NO DISTRESS CALL

The plane, which did not issue a distress signal, disappeared after its pilot failed to get permission to fly higher to avoid bad weather because of heavy air traffic.

It was travelling at 32,000 feet (9,753 metres) and had asked to fly at 38,000 feet. When air traffic controllers granted permission for a rise to 34,000 feet a few minutes later, they received no response from the aircraft.

Online discussion among pilots has centred on unconfirmed secondary radar data from Malaysia that suggested the aircraft was climbing at a speed of 353 knots, about 100 knots too slow, and that it might have stalled.

Investigators are focusing initially on whether the crew took too long to request permission to climb, or could have ascended on their own initiative earlier, said a source close to the probe, adding that poor weather could have played a part as well.

A Qantas pilot with 25 years of experience flying in the region said the discovery of the debris field relatively close to the last known radar plot of the plane pointed to an aerodynamic stall, most likely due to bad weather. One possibility is that the plane's instruments iced up in a tropical thunderstorm, giving the pilots inaccurate readings.

The lack of a distress call indicated the pilots may have realised too late they were in trouble and were too busy struggling to control the aircraft to issue a call, the Qantas pilot said.

The Indonesian pilot, a former Air Force jet fighter pilot with 6,100 flying hours under his belt, was experienced and the plane last underwent maintenance in mid-November, said the airline, which is 49 percent owned by Malaysia-based budget carrier AirAsia.