A fourth search vessel equipped with a sophisticated underwater drone is set to join the search for missing Malaysia Airlines Flight MH370, the Australian search coordinator said on Wednesday.

Months of searches have failed to turn up any trace of the Boeing 777 aircraft, which disappeared on March 8, carrying 239 passengers and crew shortly after taking off from the Malaysian capital of Kuala Lumpur, bound for Beijing.

The current phase of the search is focusing on a rugged and previously unmapped 60,000-sq-km (23,000- sq-mile) patch of sea floor some 1,600 km (1,000 miles) west of the Australian city of Perth.
Dutch engineering firm Fugro has deployed three vessels to map and then search the sea floor in the area.

The new search vessel, Fugro Supporter, is equipped with a Kongsberg HUGIN 4500 autonomous underwater vehicle (AUV), which is pre-programmed with an area to search and then released, rather than tethered to a ship via a cable.

"The AUV will be used to scan those portions of the search area that cannot be searched effectively by the equipment on the other search vessels," Australia's Joint Agency Coordination Centre (JACC) said in a statement.

More than 14,000 sq km of sea floor has been searched, JACC said, and the search of the current area could be completed as early as May, provided there are no delays.

Fugro Supporter is expected to arrive in the search area in late January.

On July 17, the same airline's Flight MH17, also a Boeing 777, was shot down over Ukraine, killing all 298 people on board.

Another Malaysian-affiliated aircraft, an Indonesia AirAsia Airbus A320-200, crashed on a flight from Indonesia to Singapore on Dec. 28 with the loss of all 162 people on board. Its wreckage has been found and its flight recorders are being examined. 

12 key clues to world's most baffling aviation mystery

The disappearance of Malaysia Airlines Flight 370 has generated dozens of theories on where it is now, from the Indian Ocean to the South China Sea, and how it vanished.

Here's a rundown.

THE WHERE

LAST CONTACT: Flight MH370 last communicated with air traffic control on March 8 east of Malaysia, and that area of the South China Sea between Malaysia and Vietnam initially was the focus of the search. Many experts assumed the plane had suffered a sudden catastrophic event because pilots didn't alert ground control before it vanished from radar screens.
However, nothing was found there or farther north in the Gulf of Thailand. Within 18 hours, Malaysian authorities said they believed the plane may have tried to turn back, and search planes and vessels were sent to the Strait of Malacca, on the other side of Malaysia.

EXPANDED SEARCH: The search moved into the Indian Ocean on Friday after U.S. officials said the plane had sent signals to satellites for hours after its last contact with air traffic control. The U.S. Navy moved one of its ships in the Strait of Malacca. India has said it is searching hundreds of small, uninhabited islands in the Andaman Sea more than 700 miles (1,126 kilometers) west of the plane's last known position.

SEISMIC REPORT: A Chinese university says it detected a seismic event in a "non-seismic zone" near the spot in the South China Sea where the plane lost contact with air traffic control. US geologists said the event was a 2.8-magnitude earthquake off the coast of Indonesia, and said a quake resulting from a plane crash was improbable.

THE WHY

'HUMAN INTERVENTION': A US official said in Washington that investigators are examining the possibility of "human intervention" and that the disappearance may have been "an act of piracy." The key evidence behind the theory is the fact that contact with the Boeing 777's transponder stopped several minutes before a messaging system on the jet stopped working. Some experts are leaning toward the theory that pilots or someone else with aviation experience deliberately veered the plane off course. They find it hard to believe that a modern jetliner like the 777 would experience a total electronic failure that left the plane unable to communicate, yet able to keep flying.

TERRORISM: Early speculation leaned toward terrorism after two men, later identified as Iranians, boarded the plane with stolen passports. Authorities later determined they were migrants seeking to travel to Europe illegally. No group has claimed responsibility for the lost plane, and experts have questioned why, if the jet was hijacked, those responsible didn't target a city or military installation.

MECHANICAL FAILURE: Experts now see this theory as unlikely following word that the plane was emitting signals for hours after it disappeared, and given the fact that no debris has been recovered from close to the plane's last known position. The plane's flight data recorder and cockpit voice recorder can transmit signals for up to 30 days, but none have been detected.

DECOMPRESSION: Experts say it's possible the plane continued to fly while a decompression left everyone on board unconscious and unable to respond, similar to what happened in the 1999 crash that killed golfer Payne Stewart and five others. A Malaysian investigator said on Friday that based on the plane's course, it appeared a skilled person was flying in the cockpit.

THE SIGNALS

WHAT THEY MEAN: The plane was sending out signals to a satellite, not unlike the way a cellphone tries to make contact with a base station even when it is off. That the plane was still sending out these messages for at least four hours after it was last spotted means it was still in one piece after it stopped communicating with the ground.

WHERE THE PLANE COULD HAVE BEEN: Experts believe the transmissions mean it was still in the air after it stopped communicating with air traffic control.

WHY SIGNALS STOPPED: A retired pilot and instructor for Boeing 777s says the plane has two transponders that emit radar data. Both can be shut off by simply turning a knob in the cockpit, keeping radar contact alive but removing key identifiers like air speed and altitude from air traffic controllers' screens. Retired Capt. Ross Aimer says under normal circumstances, there's no reason for a pilot to turn off the transponders.

THE IMAGES

CHINESE SATELLITE PICTURES: Images published on a government website showed three suspected floating objects, and search planes were dispatched to an area of the South China Sea off the southern tip of Vietnam. But several experts said it appeared very unlikely the debris was from a crashed Boeing 777.

OIL SLICKS: Oil slicks were seen Saturday in the South China Sea, raising initial speculation that the plane had gone down there. But the substance was tested and was found not to be jetliner fuel.

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