Malaysia Airlines Flight 370 disappeared Saturday somewhere over Southeast Asia. Authorities don't know where the plane is or what happened to it. Here are answers to frequently asked questions about this baffling disappearance:
Where is the plane?
As evidence grows that the plane could have flown for hours after losing contact with air traffic control, the search area too has grown to include the Gulf of Thailand, South China Sea, Strait of Malacca and the Andaman Sea and the Indian Ocean.
It could be anywhere from India to Australia. If that's the case, only the luckiest of breaks would turn up the missing plane. "This area is tremendously large. This is an impossible task," said Peter Goelz, former National Transportation Safety Board managing director. "They've got to narrow it down more."
Did it go off course?
Possibly. Some accounts have placed the aircraft hundreds of miles from its expected flight path to Beijing, and authorities have expanded the search area to include that possibility. But they're also still searching along the planned flight path. A big problem is that the plane's identifying transponder stopped working. Though it would have remained visible to radar, that is more difficult to follow. Still, experts are reviewing radar and satellite data in hopes of finding the plane.
What is a transponder?
It's a radio transmitter in the cockpit that works with ground radar. When it receives a radar signal, it returns a code that identifies the aircraft, its position, altitude and call sign. Air traffic controllers use the signals to determine a plane's speed and direction.
Why did it stop working?
That's one of many million-dollar questions. The transponder is situated between the pilots and can be turned off with a twist of the wrist, but former airline captain Mark Weiss said that, because of the vital information the transponder provides, it's highly unlikely a pilot would turn it off. Without the cockpit voice recorder and flight data recorder, it's difficult to say who was in the cockpit and what happened, Weiss said. Other experts give conflicting opinions: One says the circumstances point to someone -- perhaps a hijacker -- deliberately turning the plane around; another says a catastrophic power failure could explain the anomalies.
What about the plane's data and voice recorders?
Searchers would love to find them. The recorders would hold a trove of information on what was going on in the cockpit and technical data about what the plane was doing and how it was performing. The problem is that investigators have not been able to detect where they might be. So, until searchers can narrow their efforts, they won't help.
Was anything else sending data on the plane?
Authorities believe "pings" from the plane were transmitted to satellites for four to five hours after the transponder stopped sending signals, a senior U.S. official told CNN. The pings from the plane's Aircraft Communications Addressing and Reporting System, or ACARS, suggest that the plane flew to the Indian Ocean, the official said. But the reports on this latest lead have conflicted: Malaysian authorities earlier said earlier that nothing on the plane was transmitting after 1:07 a.m. Saturday.
Didn't the Wall Street Journal report that the plane had sent out engine data for hours?
Yes. The newspaper later corrected its story, saying that data leading investigators to believe the plane had flown for up to five hours actually came from the plane's satellite-communication link. Malaysian officials denied the newspaper's initial report, and a senior aviation source with extensive knowledge of the matter told CNN's Richard Quest that the newspaper's account was wrong. The source told Quest the plane was not sending engine data, as the newspaper had originally reported.
Could the plane have landed somewhere?
One theory U.S. officials are considering, according to that Wall Street Journal report, is that someone might have taken the plane to be used for some other purpose later. So it's theoretically possible that the plane could have landed at a remote air strip where it's being hidden. But there are some big holes in that theory. The 777 is a big plane. It requires, at minimum, nearly a mile to land. And, says Quest, there's the matter of getting it someplace without setting off alarm bells. "You can't just fly a Triple 7 and not have a radar trace," he said. One senior U.S. official, citing information Malaysia has shared with the United States, told CNN that "there is probably a significant likelihood" that the aircraft is on the floor of the Indian Ocean.
Couldn't a pilot just "fly under the radar"?
Again, theoretically. As a tool intended to keep track of what's going on in the sky, radar data don't extend all the way to the ground. Military pilots are trained to take advantage of this when they need to sneak into a country undetected. But those aircraft are also equipped with terrain-evading radar and other features intended to help fighter and helicopter
pilots hug the ground, noted aviation consultant Keith Wolzinger of the Spectrum Group. Understandably, Boeing doesn't offer those features on its commercial airliners. "Airline pilots are not trained for radar avoidance," said Wolzinger, himself a former 777 pilot. "We like to be on radar." Also, unlike military craft, civilian airliners don't have gear to detect when they've been spotted on radar. So any efforts to fly undetected would be rudimentary.
How does the search work?
Authorities break huge swaths of Earth into small grids. Then, planes or ships scour the grids to eliminate them as candidates for the crash site. As of Friday, 57 ships and 48 aircraft from 13 countries were involved in the search. The grids are massive and make up 35,000 square miles of land and sea, including the southern tip of Vietnam, South Thailand, about half of Peninsular Malaysia and parts of the South China Sea, Gulf of Thailand, Strait of Malacca and Andaman Sea.
What about those Chinese satellite photos?
On Wednesday, China released satellite images from a spot in the South China Sea that appeared to show large objects floating in the water Sunday, a day after the disappearance. Search crews checked the location and found no trace of wreckage. China later said that releasing the photos was a mistake and that the images weren't related to the plane, according to a Malaysian official.
If the plane crashed into the water, would there be anything floating this many days later?
Not large pieces of the plane, according to Steve Wallace, the U.S. Federal Aviation Administration's former director of accident investigation. But "it's certainly possible that substantial pieces of lightweight debris, not aircraft structure, could be found floating six days after the aircraft struck the water," he said. That could include things like life jackets and seat cushions, he said.
Could 'crowdsourcing' help find the plane?
Ostensibly. Colorado firm DigitalGlobe has one of the most advanced commercial satellite networks, and its images of the Strait of Malacca and Gulf of Thailand can capture details as small as a baseball field's home plate, the longest side of which is 17 inches. Volunteers can flag anything they find interesting, but so many answered the call this week that the firm's website crashed. (The website appeared to be up on Friday.) Also, there's the vast size of the search area.
Is this the first time a plane has vanished?
No. Perhaps no disappearance proved as vexing as Air France 447, which went down after departing Rio de Janeiro on June 1, 2009. It took four searches and almost two years before the bulk of the wreckage and majority of bodies were recovered. The voice and data recorders weren't found on the ocean floor until May 2011.
How does a plane disappear?
There's no simple answer here, especially when you consider the massive amounts of technology on a state-of-the-art jetliner, which includes UHF and VHF radios, automatic beacons, GPS and computer communications systems. It doesn't help that Flight 370's flight path is unclear and that the search areas include vast waters and sparsely populated jungles and mountains.
Were the pilots experienced?
Yes. Fariq Ab Hamid, 27, joined Malaysia Airlines in 2007 and was first officer on the flight. He has 2,763 flying hours and was transitioning to the Boeing 777-200 after finishing training in a flight simulator. The pilot, Capt. Zaharie Ahmad Shah, 53, has 18,365 flying hours. He joined the airline in 1981.
There are reports that the first officer let passengers in the cockpit on another flight. Is that legal?
Jonti Roos has told several media outlets that Fariq invited her and a friend into the cockpit for a 2011 flight from Phuket, Thailand, to Kuala Lumpur, Malaysia, from takeoff to touchdown. Though this would be a violation of U.S. regulations put in place after the September 11 attacks, the legality would vary from country to country. Upon learning of Roos' assertion, Malaysia Airlines said it was "shocked," and former FAA chief of staff Michael Goldfarb said such behavior "violates every code of conduct."
Is foul play possible? Hijackers? Terrorism?
The CIA and FBI aren't ruling it out, but authorities aren't ruling out much, at this point. It's highly suspicious that the plane may have turned around. Those suspicions are further fueled by the loss of communications with the plane, considering the aircraft had "redundant electrical systems" that would have had to be disabled. Robert Francis, former vice chairman of the U.S. National Transportation Safety Board, said his first thought upon hearing the circumstances of the flight's mysterious disappearance was that it blew up, but even an explosion would not be proof of terrorism.
What about those passengers with stolen passports?
Interpol says it has identified the men as Iranians Pouri Nourmohammadi, 18, and Delavar Seyed Mohammad Reza, 29, and Malaysian investigators say neither of them has any apparent connection to terrorist organizations.
Stolen passports don't necessarily indicate terrorism. In fact, passengers flew without having their travel documents checked against its lost-and-stolen passport database more than a billion times in 2013, according to Interpol. Among the reasons someone might use a stolen passport: to immigrate to another country, to export goods without being paying taxes or to smuggle stolen goods, people, drugs or weapons.
What about pilot error?
Certainly possible. That's what the investigation showed happened with the 2009 Air France flight, though there was an element of mechanical failure as well. In that case, though, there was also inclement weather -- not the case with Flight 370. As of Wednesday, nothing suggests that pilot error played a role in the flight's disappearance.
So, could mechanical failure explain it?
It's one of the stronger possibilities. The absence of a debris field could suggest that the pilot made an emergency landing on water and the plane then sank intact, but there is still the mystery of the distress signal. There wasn't one. However, aviation consultant Kit Darby has said that it's possible there was a power failure, and during the hour of backup power the pilot was attempting to return to "the airports and a region he knows." There's also the possibility that the tail or a wing tore from the fuselage. This particular Boeing had suffered a clipped wingtip in the past, but Boeing repaired it. Another possibility is that a window or door failed, which would cause the temperature inside the plane to drop to 60 degrees below zero, creating a freezing fog and giving crew members only seconds to don oxygen masks before becoming disoriented and then incapacitated.
Could it have been hit by a meteor?
There was a meteor in the area at takeoff, but this seems to be atop a list of strange conspiracy theories popping up in the absence of empirical data explaining the plane's disappearance. Given what little is known about the flight path, it seems like a long, long shot that a meteor is to blame.
What about reports that passengers' cell phones continued operating after the flight's disappearance?
The answer to the question about meteors and conspiracy theories applies here, too. When phones are disabled or turned off -- which would presumably happen after a plane crash -- calls to those cell phones go directly to voice mail. Friends and loved ones of the missing passengers, however, reported ringing when they called. Technology industry analyst Jeff Kagan says a call would connect first to a network before trying to find the end user, and the ringing sound callers hear masks the silence they would otherwise hear while waiting for the connection to be made. "If it doesn't find the phone after a few minutes, after a few rings, then typically, it disconnects, and that's what's happening," he said.