Indonesia army soldiers walk near a Hercules C-130 as they wait for good weather before continuing with search operations for passengers onboard AirAsia flight QZ8501, at Iskandar airbase in Pangkalan Bun district, Indonesia, December 31, 2014. [Photo/Agencies]

BAD WEATHER

The plane 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.

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 inquiry, 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. One possibility is that the plane's instruments iced up, giving the pilots inaccurate readings.

The Indonesian captain, a former air force fighter pilot, had 6,100 flying hours under his belt and the plane last underwent maintenance in mid-November, said the airline, which is 49 percent owned by Malaysia-based budget carrier AirAsia .