PERTH, Australia (AP) - The search area for the missing Malaysian jet has proved too deep for a robotic submarine which was hauled back to the surface of the Indian Ocean less than halfway through its first seabed hunt for wreckage and the all-important black boxes, authorities said on Tuesday.
Search crews sent the U.S. Navy's Bluefin 21 deep into the Indian Ocean on Monday to begin scouring the seabed for the missing Malaysia Airlines Boeing 777 after failing for six days to detect any signals believed to be from its black boxes.
But just six hours into its planned 16-hour mission on the sea bed, the unmanned sub exceeded its maximum depth limit of 4,500 meters (15,000 feet) and its built-in safety feature returned it to the surface, the search coordination center said in a statement.
The data collected by the Bluefin on Monday was analyzed after it returned to the surface and nothing of interest was found, the U.S. Navy said in a statement. Search crews were hoping to send it back under water on Tuesday, if weather conditions permit.
Search authorities knew that the primary wreckage from Flight 370 was likely lying at the limit of the Bluefin's dive capabilities. Deeper diving submersibles have been evaluated, but none is yet available to help.
The Bluefin was programmed to hover 30 meters (100 feet) over the seafloor as it moved through the search area, but ended up reaching its maximum depth, triggering the safety feature that returned it to the surface, the U.S. Navy said.
A safety margin would have been included in the sub's program to protect the device from harm if it went a bit deeper than its 4,500-meter limit, said Stefan Williams, a professor of marine robotics at the University of Sydney.
"Maybe some areas where they are doing the survey are a little bit deeper than they are expecting," he said. "They may not have very reliable prior data for the area, so they have a general idea. But there may be some variability on the sea floor that they also can't see from the surface."
Meanwhile, officials were investigating an oil slick about 5,500 meters (3.4 miles) from the area where the last underwater sounds were detected, said Angus Houston, the head of a joint agency coordinating the search off Australia's west coast.
Crews have collected an oil sample and are sending it back to Perth in western Australia for analysis, a process that will take several days. Houston said it does not appear to be from any of the ships in the area, but cautioned against jumping to conclusions about its source.
The Bluefin can create a three-dimensional sonar map of any debris on the ocean floor. But the search in this area is more challenging because the seabed is covered in silt that could potentially cover part of the plane.
"What they're going to have to be looking for is contrast between hard objects, like bits of a fuselage, and that silty bottom," Williams said. "With the types of sonars they are using, if stuff is sitting up on top of the silt, say a wing was there, you could likely see that ... but small items might sink down into the silt and be covered and then it's going to be a lot more challenging."
The search moved below the surface after crews picked up a series of underwater sounds over the past two weeks that were consistent with signals from an aircraft's black boxes, which record flight data and cockpit conversations. The devices emit "pings" so they can be more easily found, but their batteries only last about a month and are now believed to be dead.
Australian Prime Minister Tony Abbott raised hopes last week when he said authorities were "very confident" the four strong underwater signals that were detected were from the black boxes on Flight 370, which disappeared March 8 during a flight from Kuala Lumpur, Malaysia, to Beijing with 239 people on board, mostly Chinese.
But Houston warned that while the signals are a promising lead, the public needs to be realistic about the challenges facing search crews in the extremely remote, deep patch of ocean.
"I would caution you against raising hopes that the deployment of the autonomous underwater vehicle will result in the detection of the aircraft wreckage," Houston said Monday. "It may not."
The submarine takes 24 hours to complete each mission: two hours to dive to the bottom, 16 hours to search the seafloor, two hours to return to the surface, and four hours to download the data.
The black boxes could contain the key to unraveling the mystery of what happened to Flight 370. Investigators believe the plane went down in the southern Indian Ocean based on a flight path calculated from its contacts with a satellite and an analysis of its speed and fuel capacity. But they still don't know why.
On Tuesday, Malaysia's defense minister, Hishamuddin Hussein, pledged to reveal the full contents of the black boxes, if they are ever found.
"It's about finding out the truth," he told reporters in Kuala Lumpur. "There is no question of it not being released."
Up to 11 planes and as many ships were on Tuesday scouring a 62,000 square kilometer (24,000 square mile) patch of ocean about 2,200 kilometers (1,400 miles) northwest of Perth, hunting for any floating debris.
But the weeks-long surface search was expected to end in the next two days. Officials haven't found a single piece of debris confirmed to be from the plane, and Houston said the chances that any would be found have "greatly diminished."