If there was a minor mechanical failure — or even something more serious like the shutdown of both of the plane's engines — the pilots likely would have had time to radio for help. The lack of a call "suggests something very sudden and very violent happened," said William Waldock, who teaches accident investigation at Embry-Riddle Aeronautical University in Prescott, Ariz.
Instead, it initially appears that there was either a sudden breakup of the plane or something that led it into a quick, steep dive. Some experts even suggested an act of terrorism or a pilot purposely crashing the jet.
"Either you had a catastrophic event that tore the airplane apart, or you had a criminal act," said Scott Hamilton, managing director of aviation consultancy Leeham Co. "It was so quick and they didn't radio."
No matter how unlikely a scenario, it's too early to rule out any possibilities, experts warn. The best clues will come with the recovery of the flight data and voice recorders and an examination of the wreckage.
Airplane crashes typically occur during takeoff and the climb away from an airport, or while coming in for a landing, as in last year's fatal crash of an Asiana Airlines jet in San Francisco. Just 9 percent of fatal accidents happen when a plane is at cruising altitude, according to a statistical summary of commercial jet airplane accidents done by Boeing.
Capt. John M. Cox, who spent 25 years flying for US Airways and is now CEO of Safety Operating Systems, said that whatever happened to the Malaysia Airlines jet, it occurred quickly. The problem had to be big enough, he said, to stop the plane's transponder from broadcasting its location.
One of the first indicators of what happened will be the size of the debris field. If it is large and spread out over tens of miles, then the plane likely broke apart at a high elevation. That could signal a bomb or a massive airframe failure. If it is a smaller field, the plane probably fell from 35,000 feet intact, breaking up upon contact with the water.
"We know the airplane is down. Beyond that, we don't know a whole lot," Cox said.
The Boeing 777 has one of the best safety records in aviation history. It first carried passengers in June 1995 and went 18 years without a fatal accident. That streak came to an end with the July 2013 Asiana crash. Three of the 307 people aboard that flight died. Saturday's Malaysia Airlines flight carrying 239 passengers and crew would only be the second fatal incident for the aircraft type.
"It's one of the most reliable airplanes ever built," said John Goglia, a former member of the U.S. National Transportation Safety Board.
Some of the possible causes for the plane disappearing include:
— A catastrophic structural failure of the airframe or its Rolls-Royce Trent 800 engines. Most aircraft are made of aluminum which is susceptible to corrosion over time, especially in areas of high humidity. But given the plane's long history and impressive safety record, experts suggest this is unlikely.
More of a threat to the plane's integrity is the constant pressurization and depressurization of the cabin for takeoff and landing. In April 2011, a Southwest Airlines Boeing 737-300 rapidly lost cabin pressure just after takeoff from Phoenix after the plane's fuselage ruptured, causing a 5-foot tear. The plane, with 118 people on board, landed safely. But such a rupture is less likely in this case. Airlines fly the 777 on longer distances, with much fewer takeoffs and landings, putting less stress on the airframe.
"It's not like this was Southwest Airlines doing 10 flights a day," Hamilton said. "There's nothing to suggest there would be any fatigue issues."