The absence of a Mayday call and sudden loss of radio signals has fuelled rumours about what caused the Malaysia Airlines plane to disappear.
Three days into the mysterious vanishing of flight MH370, Malaysian officials investigating believe the flight probably disintegrated 35,000 feet in the air and scattered debris without a concentrated pattern, making it difficult to spot the wreckage.
Malaysian and Vietnamese authorities said they had not received any radio signal from the plane to suggest it was in trouble.
A Mayday call would have taken just seconds in the form of radio or transponder code. Experts say only in a rare sudden emergency would it not have been possible to transmit one.
Dr Eric Wong Tsun-tat of Polytechnic University's department of mechanical engineering said it would take just five to 10 seconds to send a Mayday alert.
"Even under very urgent circumstances, a pilot or the co-pilot will still have the chance to issue one to air traffic controllers, or to their own company," said Wong, a former engineer at the Hong Kong Civil Aviation Department.
If not able to do so verbally over radio, pilots can also send out distress signals via transponder code. The transponder code for a hijacking is "7500", while a general distress code is "7700".
Watch: All theories considered for missing jet: Malaysia civil aviation
Professor Tian Yafei, a radio expert at the Beijing University of Aeronautics and Astronautics, said judging by the complete loss of radio signals, a sudden disintegration caused by a powerful force such as explosion should not be ruled out.
Tian said most radio communications use low frequencies that can reach very far to ground stations or satellites, so even if the plane dropped near to sea level, it would still have a chance to send out some signals.
Only an "explosion powerful enough to tear the plane into pieces and destroy all communication devices at once" could lead to the sudden, complete radio silence, he said.
He pointed to the 1988 Lockerbie bombing disaster of Pan Am Flight 103, a Boeing 747, where the crew failed to send an emergency signal and lost all contact after an explosion in the cargo bay caused the plane to disintegrate.
But others scholars remain sceptical of the hypothesis.
Wong said the absence of any sort of debris raised doubts about the mid-air disintegration theory. He said it was possible the aircraft may have crashed into the sea in one piece before breaking apart.
Professor Cheng Wei, who studies plane structure robustness at BUAA, said the explosion theory was "almost impossible".
The aircraft, a Boeing 777, was a modern, solidly built jumbo jet designed to withstand an internal explosion, he said. A bomb, be it in a carry-on luggage or cargo bay, could make a hole in the airplane but would not lead to immediate fragmentation.
"To completely disintegrate a 777, the whole plane must be filled with high explosives," Cheng said, adding that a jet plane's hull was built for high internal pressure, and the force of an explosion would be quickly dispersed from the hole.
But Tian said it was equally possible that the plane had been hijacked and all radio devices turned off on purpose.
More mysterious, however, is the fact that disintegration or not, none of it was detected on radar, despite being in range of most air traffic control.
In part, the disappearance strikes parallels with doomed Air France AF447, which disappeared from radar and crashed into the south Atlantic in 2009. No distress signal was issued.
David Newbery, a pilot and accredited aviation accident investigator, said it was still too early to speculate with theories. He pointed to the AF447 incident, in which debris did not surface until five days after the crash, while the wreck was not found until two years later.
"Right now, all the facts tell us is that the plane is no longer flying," said Newbery. "They must first find out where they're supposed to look."
Warren Chim Wing-nin, of the Institute of Engineers Aircraft Division, said it would be hard to locate the wreckage as even the emergency location transmitter (ELT) had failed to activate. The transmitter, located in the upper aft tail area of every aircraft, is supposed to switch on automatically following a crash and beam out signals for 24 hours.
"The search will be tough as they aren't even able to achieve this first step. Without the ELT, it's like looking for a needle in a haystack."