Surabaya: The plane sought permission to climb above threatening clouds. Air traffic control couldn't say yes immediately - there was no room.
Six other airliners were crowding the airspace, forcing AirAsia Flight 8501 to remain at a lower altitude.
Minutes later, the jet carrying 162 people was gone from the radar without ever issuing a distress signal.
The plane is believed to have crashed into Indonesia's Java Sea, but broad aerial surveys today turned up no firm evidence of the missing Airbus A320-200.
Searchers spotted two oily patches and floating objects in separate locations, but no one knew whether any of it was related to the plane that vanished Sunday halfway into what should have been a two-hour hop from Surabaya, Indonesia, to Singapore. Officials saw little reason to believe the flight met anything but a grim fate.
Based on the plane's last known coordinates, the aircraft probably crashed into the water and "is at the bottom of the sea," Indonesia search-and-rescue chief Henry Bambang Soelistyo said. Still, searchers planned to expand their efforts onto land on Tuesday.
The last communication from the cockpit to air traffic control was a request by one of the pilots to climb from 32,000 feet to 38,000 feet because of the rough weather. The tower was not able to immediately comply because of the other planes, said Bambang Tjahjono, director of the state-owned company in charge of air traffic control.
When planes confront storms, they generally veer left or right, said Sarjono Joni, a former pilot with a state-run Indonesian carrier. A request to climb would most likely come if the plane were experiencing heavy turbulence, he said, and heavy traffic is not unusual for any given airspace.
The twin-engine, single-aisle plane was last seen on radar four minutes after the final communication from the cockpit.