The most dangerous parts of a flight are take-off and landing. Rarely do incidents happen when a plane is cruising seven miles above the earth.
So the disappearance of a Malaysia Airlines jet well into its flight Saturday morning over the South China Sea has led aviation experts to assume that whatever happened was quick and left the pilots no time to place a distress call.
It could take investigators months, if not years, to determine what happened to the Boeing 777 flying from Malaysia's capital city of Kuala Lumpur to Beijing.
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 the US.
Instead, it initially appears that there was either a sudden break-up 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 aeroplane 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.
Just nine per cent of fatal accidents happen when a plane is at cruising altitude, according to a statistical summary of commercial jet aeroplane 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.
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 (10.6km) intact, breaking up upon contact with the water.
"We know the aeroplane is down. Beyond that, we don't know a whole lot," Cox said.
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 aluminium 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.
- Bad weather. Planes are designed to fly though most severe storms. However, in June 2009, an Air France flight from Rio de Janeiro to Paris crashed during a bad storm over the Atlantic Ocean. All 228 passengers and crew aboard died. The pilots never radioed for help. But in the case of Saturday's Malaysia Airlines flight, all indications show that there were clear skies.
- Pilot disorientation. Curtis said that the pilots could have taken the plane off autopilot and somehow went off course and didn't realise it until it was too late. The plane could have flown for another five or six hours from its point of last contact, putting it up to 4800km away. This is unlikely given that the plane probably would have been picked up by radar somewhere.
- Failure of both engines. In January 2008, a British Airways 777 crashed about 300 metres short of the runway at London's Heathrow Airport. As the plane was coming in to land, the engines lost thrust because of ice build-up in the fuel system. There were no fatalities.
- A bomb. Several planes have been brought down including Pan Am Flight 103 between London and New York in December 1988. There was also an Air India flight in June 1985 between Montreal and London and a plane in September 1989 flown by French airline Union des Transports Ariens which blew up over the Sahara Desert.
- Hijacking. A traditional hijacking seems unlikely given that a plane's captors typically land at an airport and have some type of demand. But a 9/11-like hijacking is possible, with terrorists forcing the plane into the ocean.
- Pilot suicide. There were two large jet crashes in the late 1990s that investigators suspected were caused by pilots deliberately crashing the planes.
- Accidental shoot-down by some country's military. In July 1988, the United States Navy missile cruiser USS Vincennes accidentally shot down an Iran Air flight, killing all 290 passengers and crew. In September 1983, a Korean Air Lines flight was shot down by a Russian fighter jet.