Why did the plane or pilot not send out any distress signal?
In a crisis, the drill for pilots is to fly, navigate, communicate - in that order.
The cockpit crew of Malaysia Airlines Flight MH370, which went missing en route from Kuala Lumpur to Beijing, did not send out a distress signal. This suggests that they faced a catastrophic problem, said observers.
Nearly 100 per cent of the time, pilots are able to regain control of the aircraft and call for help, said Captain Mok Hin Choon, president of the Air Line Pilots Association-Singapore. He did not comment on the incident but spoke in general terms.
He said: "Even if there is total engine failure at 35,000 feet and you're not able to restart the engines, the plane can continue to glide and you have about 15 to 20 minutes minimum to do what you have to, send out the distress signals and make an emergency landing."
Captain Manmohan Singh, a pilot turned flight instructor, said: "Depending on the severity of the situation, it is possible - though rare - that pilots don't even get past the first stage of flying.
"When you're not even able to control the aircraft, that's when planes fall from the sky."
Presumably, this was what happened to MH370, other experts said.
Even if pilots do not call for help, today's planes are capable of sending out their own distress signals. But not all airlines have the systems to monitor these.
Mr Michael Daniel, a retired United States Federal Aviation Administration official who has investigated several air accidents, said it is compulsory for airlines to have communication systems to reach cockpit crew if needed.
But it is not mandatory for them to monitor real-time flight data. Most global carriers would have such systems, but it is not known if MAS had this.
Why can't the ringing cellphones of MH370 passengers be used to track the plane's location?
Nearly 20 families managed to get a ringtone on the phones of the passengers days after the plane disappeared, according to reports. But experts said hearing a ringtone does not mean the call has gone through.
"Callers may hear a ringback tone as programmed by the network operator while the network is trying to connect to the phone - whether the phone is online or not," said Mr Clement Teo, Singapore telco analyst for US-based market research firm Forrester.
The ringing may last a few seconds depending on whether it is recognised as a local or overseas call. "It does not mean that the phone is ringing on the other side," he said.
Even if the cellphone is ringing, it may not mean that it is ringing on the aircraft. The ringing could be due to the common practice of travellers forwarding calls to another number before boarding their plane.
Several telecom network experts who declined to be named said cellphones work only if they are not under water and are near a cell tower, which typically covers a radius of 500m to 1km.
While smartphones are known for their poor battery life - typically up to 24 hours - they can run for several days after a full charge if they are set to flight mode.
For tracking by cell tower to be possible, the phone must be powered to transmit signals to the cellular network. Another way to track cellphones is via the Global Positioning System (GPS).
"GPS tracking is possible on certain smartphones even when they are set to flight mode," said security expert Aloysius Cheang, Asia-Pacific managing director of global computing security association Cloud Security Alliance.
There are so many satellites in space. Why have none of them captured the location or movement of MH370?
China has deployed 10 satellites in the search for MH370, while commercial satellite firm DigitalGlobe has roped in 25,000 volunteers online to look at its high-resolution pictures.
But experts say that while satellites can deliver high-resolution pictures and make out items as small as 1sqm, using them to find a lost plane may be challenging.
Even so, can the historical data of satellites show the direction the plane took after it lost contact with air traffic controllers?
This would be hard as it was flying at night, said Dr Santo Salinas, a senior research scientist at the National University of Singapore's Centre for Remote Imaging, Sensing and Processing.
What about using satellites to look for the aircraft or debris from a crash?
This is possible, said experts, but only if it is known roughly where to look.
In the tropics, cloud cover can also obscure a satellite's field of vision.
Where a plane goes down is another issue. It is harder to find something on land, said Dr Salinas, as cities, forests, houses with metallic roofs and so on will show up with different brightness. The ocean, on the other hand, is mostly uniform, which makes it easier to spot objects that stand out.
Even after satellites have taken pictures, the images still need to be scanned by human eyes. "It's like looking for a needle in a haystack," said Dr Salinas.
Long searches in past accidents
Five notable commercial aircraft incidents that took up to 10 days for their wreckage to be recovered:
JUNE 1, 2009: Air France Flight 447 plunged into the Atlantic Ocean while flying from Brazil to France. There were 228 people on board the Airbus A330.
A day after the aircraft went missing, Brazilian officials confirmed that a 5km trail of wreckage discovered off the country's north-eastern coast was that of Flight 447.
But the main wreckage, including the plane's black boxes, was recovered only two years after the accident.
JAN 1, 2007: Adam Air Flight 574, a Boeing 737-400, went down near Indonesia's Sulawesi island. The domestic flight had 102 people on board.
A local fisherman found a piece of the plane's tail at sea, 10 days later. The plane's recorders were retrieved seven months after the crash.
SEPT 2, 1998: Swissair Flight 111, a McDonnell Douglas MD-11 aircraft with 229 people on board, dived into waters near Nova Scotia, Canada, while flying from the United States to Switzerland.
While the plane debris was confined mainly to an area of about 70m by 30m, it took five days to find the flight data recorder.
DEC 19, 1997: SilkAir Flight 185, carrying 97 passengers and seven crew, went down while flying from Indonesia to Singapore. The Boeing 737-300 crashed into the Musi River near the Indonesian city of Palembang. No one survived.
A search and rescue team found two aircraft pieces near the river the following day. The flight data recorder was found only eight days later.
AUG 12, 1985: Japan Airlines Flight 123, a Boeing 747, crashed into a mountain ridge in central Japan shortly after take-off, killing 520 of the 524 people who were flying from Tokyo to Osaka.
Rescuers could reach the crash site only the following morning because of poor weather and nightfall.
Other fragments like pieces of the tail were found a few days later, some 200km away. The incident remains one of the deadliest commercial aviation accidents.
Sources: AP, Reuters, SIA, Transportation Safety Board of Canada, The Los Angeles Times
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