A warship is on its way to collect an oil sample from the sea to establish whether it has come from missing AirAsia Flight 8501.
Dozens of planes and ships have focused their search on two patches of oil spotted in Indonesian waters this afternoon as a senior official warned the aircraft was likely at the "bottom of the sea".
As the second day's search ended at dusk, National Search and Rescue Agency chief Bambang Soelistyo said an Indonesian corvette (warship) had been sent to test the spills.
The announcement came as officials revealed that one of the pilots had been denied a request to increase altitude to avoid storm clouds minutes before the jet disappeared.
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, Flight 8501 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.
"We are making sure whether it was avtur (aviation fuel) from the AirAsia plane or from a vessel because that location is a shipping line," he said.
A satellite image of the severe weather Flight QZ8501 was flying through when it disappeared. Photo / Supplied
Earlier, Jakarta's Air Force base commander Rear Marshal Dwi Putranto said an Australian Orion aircraft had detected suspicious objects around 1120km from the point of last contact, but these were later ruled out by Indonesia's vice-president Jusuf Kalla.
"It has been checked and no sufficient evidence was found to confirm what was reported," Jusuf Kalla told a press conference at Surabaya airport where the missing plane took off.
Waters in the search area, which is roughly the size of California, are not particularly deep at between 130ft and 160ft.
Captain Iriyanto was piloting the missing flight. Photo / Facebook
The flight went missing at 6.17am local time on Sunday while travelling from Indonesia to Singapore as speculation on the cause of the disappearance centred on weather, speed and an older radar system.
Aviation experts have speculated that the flight may have encountered "black storm cells" which caused a build-up of ice on airspeed senors known as pitot tubes.
A similar scenario was blamed for the Air France disaster when Flight AF447 crashed into the Atlantic Ocean in 2009 while en route from Rio De Janeiro to Paris.
Aviation expert Geoffrey Thomas spoke to several check captains and believes the pilot of QZ8501 encountered difficult weather conditions but flew too slow in his efforts to avoid it.
Relatives of those onboard await news from the search as hopes of finding survivors fade. Photo / AP
"The QZ8501 was flying too slow, about 100 knots which is about 160 km/h too slow. At that altitude that's exceedingly dangerous," Mr Thomas said.
"Pilots believe that the crew, in trying to avoid the thunderstorm by climbing, somehow have found themselves flying too slow and thus induced an aerodynamic stall similar to the circumstances of the loss of Air France AF447 to crash in 2009.
"I have a radar plot which shows him at 36,000 feet and climbing at a speed of 353 knots, which is approximately 100 knots too slow ... if the radar return is correct, he appears to be going too slow for the altitude he is flying at."
Mr Thomas said this should not happen in an A320, so it appears as though it was related to extreme weather conditions. "He got caught in a massive updraft or something like that. Something's gone terribly wrong.
A map of the area where the Indonesian National Search and Rescue Agency is concentrating its efforts. Photo / AP
"Essentially the plane is flying too slow to the altitude and the thin air, and the wings won't support it at that speed and you get a stall, an aerodynamic stall."
The A320, while sophisticated, is not equipped with the latest radar, Mr Thomas said.
The radar used by the A320 can sometimes have problems in thunderstorms and the pilot may have been deceived by the severity of these particular ones.
The latest technology radars, which were pioneered by Qantas in 2002, can give a more complete and accurate reading of a thunderstorm, but they will not be certified for the A320 until next year.
"If you don't have what's called a multi-skilled radar you have to tilt the radar yourself manually, you have to look down to the base of the thunderstorm to see what the intensity of the moisture and the rain is, then you make a judgment of how bad it is.
"It's manual, so it's possible to make a mistake, it has happened."
The Indonesian military hunt for signs of wreckage. Photo / Getty Images
In a separate development, Earth Network, a firm that monitors weather conditions around the world, recorded a number of lightning strikes "near the path" of the plane when it disappeared on Sunday morning, the New York Times reported.
Although unlikely to have caused structural damage to the A320, lightning can affect navigation systems and flashes could temporarily disorient pilots, the paper notes.
Sudden shifts in wind direction also have the potential to force jet engines into a stall, although experts this scenario is very unlikely and point to the fact that the Airbus A320 is certified to fly up to three hours on a single engine.
AirAsia confirmed there were 155 passengers on board - including 138 adults, 16 children and one infant - and also stated there were two pilots, four flight attendants and one engineer on board.
Nationalities of passengers and crew on board are one Singaporean, one Malaysian, one British, one French, three South Koreans and 155 Indonesians.
- Daily Mail