In the last communication with air traffic control six minutes before it vanished off radar, one of the pilots asked permission to turn left and climb from 32,000ft to 38,000ft due to the adverse weather.
However, the request could not immediately be granted because another plane was in the airspace at 34,000ft, said Bambang Tjahjono, director of the state-owned company in charge of air-traffic control.
By the time clearance could be given, Air Asia Flight QZ8501 had disappeared, he added.
The developments will bring further anguish to relatives of the 162 passengers and crew who are desperately clinging to hope they may find survivors.
The Airbus A320-200 lost contact en route from Surabaya in Indonesia’s east Java to Singapore on Sunday after the crew requested a change of flight plan due to stormy weather in the third crisis for a Malaysian carrier this year.
Indonesian Air Force spokesman Hadi Tjahjanto said the search was now concentrated on an oil patch spotted off Belitung island, across from Kalimantan on Borneo island in the Java Sea.
Dozens of planes and ships have focused their search on two patches of oil spotted in Indonesian waters as a senior official warned the aircraft was likely at the ‘bottom of the sea’.
Singapore has sent a team to assist with the search and rescue operations.
Suggestions that the plane crashed on land, even in a remote area, were dismissed by Neil Hansford, owner of Hong Kong-based Strategic Aviation Solutions, who said it would have been impossible for the plane to have crashed on land without people knowing.
‘It was carrying a lot of fuel on board and there would have been an almighty explosion,’ he said.
‘It would have shaken the locals out of the trees.’
A warship is on its way to collect an oil sample from the sea to establish whether it has come from missing Air Asia Flight QZ8501.