The Indonesia AirAsia plane, an Airbus A320-200, disappeared after its pilot failed to get permission to fly higher to avoid bad weather during a flight from the Indonesian city of Surabaya to Singapore on Sunday.
Flight QZ8501 did not issue a distress signal and disappeared over the Java Sea five minutes after requesting the change of course, which was refused because of heavy air traffic, officials said.
"Based on our coordinates, we expect it is in the sea, so for now (we think) it is on the sea floor," Soelistyo, head of Indonesia's search and rescue agency, told reporters when asked about the missing plane's likely location.
A senior Indonesian civil aviation source told Reuters that authorities had the flight's radar data and were waiting for search and rescue teams to find debris before they started their investigation into the cause of the accident.
Onboard Flight QZ8501 were 155 Indonesians, three South Koreans, and one person each from Singapore, Malaysia and Britain. The co-pilot was French.
The incident caps a disastrous year for Malaysia-affiliated airlines, with Indonesia AirAsia 49 percent owned by Malaysia-based budget carrier AirAsia.
Malaysian Airlines Flight MH370 went missing on March 8 on a trip from Kuala Lumpur to Beijing with 239 passengers and crew and has not been found. On July 17, the same airline's Flight MH17 was shot down over Ukraine, killing all 298 people on board.
The AirAsia group, including affiliates in Thailand, the Philippines and India, had not suffered a crash since its Malaysian budget operations began in 2002. The group's shares in Kuala Lumpur fell as much as 12.9 percent on Monday.
Indonesian Air Force spokesman Hadi Thahjanto said two C-130 Hercules planes were focusing the search for Flight QZ8501 in areas northeast of Indonesia's Bangka island, which lies roughly halfway between Surabaya and Singapore, in the Java Sea.
An Australian P3 Orion surveillance plane had joined the search, the Australian Defence Department said.
Singapore said it had sent two navy ships to help, while Malaysia said it would send three naval vessels and a C-130.
China also offered to send aircraft and ships to help in the search, as well as any other assistance Indonesia needed.
Soelistyo said Indonesia might not have the best technology to search underwater and had accepted offers of help from the United States, Britain and France.
"An underwater search is not easy based on experience," Soelistyo said, referring to the months it took Indonesia to recover flight data recorders from a Boeing 737-400 operated by Indonesia's Adam Air which crashed off Sulawesi island in 2007, killing all 102 people on board.
Flight QZ8501 was traveling at 32,000 feet above the Java Sea and had asked to fly at 38,000 feet to avoid clouds, said Joko Muryo Atmodjo, air transportation director at Indonesia's transport ministry.
Permission was not given at the time due to traffic in the area. Five minutes later, at 6:17 a.m. on Sunday (2317 GMT Saturday), the plane lost contact with air traffic control, Atmodjo said.
Data from Flightradar24.com, which tracks airline flights in real time, showed several nearby aircraft were at altitudes ranging from 34,000 to 36,000 feet at the time, levels that are not unusual for cruising aircraft.
Pilots and aviation experts said thunderstorms, and requests to gain altitude to avoid them, were not unusual in the area where the flight disappeared.
"You can climb to push above the cloud layer to get clearance and radar readings to pick your way over the storm," said a Qantas Airways pilot with 25 years experience flying in the region.
"But the airplane's performance is directly related to the temperature outside and increasing altitude can lead to freezing of the static radar, giving pilots an erroneous radar reading."
The resulting danger is that pilots take incorrect action to control the aircraft, said the pilot, who requested anonymity because he was not authorized to speak on the record.
The Indonesian pilot was experienced and the plane last underwent maintenance in mid-November, the airline said. The aircraft had accumulated about 23,000 flight hours in some 13,600 flights, according to Airbus.
Malaysia AirAsia chief Tony Fernandes flew to Surabaya and, along with Indonesian officials, updated distraught relatives of passengers at a makeshift crisis center at the airport in Indonesia's second-largest city.
"This is my worst nightmare," Fernandes said on Twitter. "But there's no stopping", he said of the search.
Indonesia's Transportation Ministry said the government would carry out a review of AirAisa's Indonesian business unit to improve safety.
Indonesian President Joko Widodo urged his people to pray for the safety of the passengers and crew. Pope Francis, during his Sunday address at St. Peter's Square in the Vatican, said those on board were in his prayers.
Louise Sidharta was at Singapore's Changi Airport waiting for her fiancée to return from a family holiday.
"It was supposed to be their last vacation before we got married," she said.
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