BEIJING — Investigators looking for Malaysia Airlines Flight 370 have put away their towed pinger locator and are about to call off searches for surface debris. Now, it's all up to a little yellow robotic submarine to find the missing Boeing 777 in an area bigger than the city of Los Angeles.

Technicians aboard the Australian ship Ocean Shield on Monday afternoon deployed the Bluefin-21 underwater autonomous vehicle in the Indian Ocean, sending it almost three miles down to the seabed and using its side-scanning sonar arrays to look for wreckage from the plane. 

“It is time to go underwater,” retired Air Chief Marshal Angus Houston, who is coordinating the search from Perth, Australia, said in announcing the new phase of operations. 

Unless the robot sub gets lucky, the process could take a while: The U.S. Navy, which lent the Bluefin-21 to the search team, said mapping the area where the plane most likely disappeared could take six weeks to two months. 

The 16-foot, 1,650-pound sub moves at a walking pace and will be searching an area of about 600 square miles. Houston said the first day's work was set to cover about 15 square miles, but the vehicle automatically returned to the surface after just six hours because it exceeded its maximum operating depth of 2.8 miles. Searchers planned to send it back out Tuesday.

"The whole key on these searches is you have to be methodical and persistent, and they can take quite a bit of time," said David Kelly, president and chief executive of Bluefin Robotics, the Quincy, Mass., company that makes the vessel.

The Bluefin-21 — which costs $4 million to $6 million, depending on options — typically operates on a 24-hour cycle. It takes two hours each way to get to the seafloor and back and can search for 16 hours. Once it surfaces, it requires four hours to download the data gathered and prepare the machine for its next dive.

While it's combing through the pitch-black waters, hovering about 150 feet above the seabed as it scans a half-mile-wide swath, the Bluefin-21 uses sonar to gather data that will yield a high-resolution, 3-D map of the seafloor.

If something noteworthy is detected, Kelly said, the sonar can be swapped out for high-definition cameras. When the cameras are being used, the sub hovers just 15 feet off the seafloor and takes a series of overlapping pictures that provide a composite image.

The depth at which the Bluefin-21 will work — 2.8 miles — is hard even for oceanographers to fathom: That's as deep as Mt. Whitney is tall. Horizontally, it's equal to the distance traveled driving along Wilshire Boulevard from Rodeo Drive in Beverly Hills to the La Brea Tar Pits.

Kelly said the Bluefin-21 recently was tested at that depth in Hawaiian waters because Phoenix International, the company that owns and operates it on behalf of the Navy, was checking some “upgraded capabilities.”

While the robotic sub does its duties, search personnel aboard the Ocean Shield and other vessels will have no idea whether it's found anything. There is limited acoustic communication back to the surface ship — relaying basic information such as depth and power remaining — but Kelly said the connection is intermittent and at a "low bandwidth."

"You give it a track and it goes," said Charitha Pattiaratchi, a professor of oceanography at the University of Western Australia. "It doesn't transfer any imagery or anything while it's down there."

The decision to deploy the Bluefin-21 came on day 38 of the search for the plane, which vanished March 8 with 239 people aboard en route from Kuala Lumpur, Malaysia, to Beijing. Searchers operating more than 1,300 miles off the western coast of Australia had hesitated to deploy the sub, hoping to further narrow the search area based on acoustic signals transmitted by beacons on the plane's two black boxes.

On four occasions, starting April 5, a pinger locator towed by the Ocean Shield picked up transmissions from what investigators strongly suspect were one or both of the black boxes. But with no new transmissions in the last six days, investigators have apparently concluded that the batteries on those devices — which are supposed to last 30 days — have run out.

Meanwhile, Houston indicated that the so-far unsuccessful search for debris on the ocean surface — with lookouts scanning the waters from the air and from ships — would probably be called off in the next two to three days.

"The chances of any floating material being recovered have greatly diminished, and it will be appropriate for Australia and its partners to decide the way ahead later this week," Houston said.

Van Gurley, a retired U.S. Navy captain and senior manager at Metron Inc., a Virginia-based scientific consulting firm that helped find the wreckage of Air France Flight 447 after it crashed in the Atlantic in 2009, described the move as a "tough call to make."

"I'm of the opinion that if they haven't found surface debris field by now, they're not going to. Then it becomes almost a political decision: At what point do you suspend that operation? Given that you haven't found anything on the ocean bottom yet, and the families are eager for new information," he said.

"It sounds like Angus Houston said some words to prepare everybody that they're going to, in a couple days, shut down all the air and surface search operations."

Gurley noted that the search for Flight 370 is dramatically different from the quest for the Air France jet. In the latter case, investigators found surface debris, including bodies, within a week of the crash and had communication with the airplane until about 4½ minutes before impact.

However, investigators did not have any detected signals from the plane's black boxes, and repeated searches with underwater vehicles and sonar scanning equipment turned up no debris on the ocean floor for nearly two years. Finally, a Metron analysis helped identify a tract that had been searched only by a towed pinger locator and not by an underwater vehicle similar to the Bluefin-21. Within a week, the underwater wreckage had been found.

Gurley said the Air France search was "much simpler" than the Malaysia Airlines case, but the area of focus this time is "much much smaller" than the 5,000 square miles covered in the 2009 crash.

If the Bluefin-21 does find the wreckage, salvage operations could take several months to get underway. Vessels operating in the search area, Gurley said, can only look at the bottom of the sea, not pick anything up.

"It requires a whole different set of gear," he said. "It's another couple of months to start being able to retrieve and bring things up from the ocean bottom."

julie.makinen@latimes.com