Perth (Australia) (AFP) – A mini-sub hunting missing Flight MH370 was set to sweep the Indian Ocean seabed again Tuesday after cutting short its first mission, as Malaysia vowed to reveal any ‘black box’ data found.

The unmanned submarine equipped with sonar gear was deployed Monday night from the Australian ship Ocean Shield, which has spearheaded the hunt for the Boeing 777 that vanished on March 8 with 239 people aboard.

But the dive by the Bluefin-21 detected nothing of interest before it automatically aborted the mission after breaching its maximum operating depth, the U.S. Navy said in a statement.

The Australian agency coordinating the search said the Bluefin-21 “exceeded its operating depth limit of 4,500 meters (15,000 feet) and its built-in safety feature returned it to the surface.”

The unmanned Autonomous Underwater Vehicle was undamaged and set for a second sonar sweep during the day, weather permitting, officials said.

U.S. Navy Captain Mark Matthews said the vehicle had exceeded programmed operational limits and automatically resurfaced.

“In this case the vehicle’s programmed to fly 30 meters over the floor of the ocean to get a good mapping of what’s beneath,” he told CNN from Perth.

“It went to 4,500 meters and once it hit that max depth, it said ‘This is deeper than I’m programmed to be’, so it aborted the mission.”

Officials said the crew would now refine the task to cope with the depth encountered.

“To account for inconsistencies with the sea floor, the search profile is being adjusted to extend the sonar search for as long as possible,” the US Navy statement said.

Australia’s Joint Agency Coordination Center chief Angus Houston announced Monday officials would end three weeks of listening for signals from the plane’s black boxes and launch the submarine operation.

The mini-sub would conduct a sonar survey of the silty ocean floor for 16 hours at a time in hopes of finding some wreckage from the Malaysia Airlines flight which vanished en route from Kuala Lumpur to Beijing.

The US navy estimated it would take the Bluefin-21 from six weeks to two months to scan the entire search area, which has been deduced using satellite data and the detection of electronic pulses linked to black box recorders which were last heard a week ago.

Houston has described the detections as the best lead in the hunt for the plane, and added Monday that an oil slick had also been sighted in the search area.

It would take several days to test a sample of the oil ashore, but Houston said he did not think it was from one of the many ships involved in the hunt.

The cause of the plane’s disappearance, after being diverted hundreds of miles off course, remains a mystery. No debris has been found despite an enormous search involving ships and planes from several nations.

It is 39 days since the plane vanished, presumably crashing into the southern Indian Ocean, and the batteries powering the black box tracker beacons had a life of only around 30 days.

Ocean Shield detected four signals linked to the black boxes, but the last ping came on Tuesday last week and officials suspect the batteries are now dead.

Houston has stressed the enormous difficulties of working at great depths in such a remote location and cautioned about the difficulties of finding the black boxes.

If they are ever found, Malaysia’s Transport Minister pledged Tuesday to make public any data recovered, as the government battles widespread criticism over the transparency of its investigation.

“It’s about finding the truth. And when we… find out the truth, definitely we have to reveal what’s in the black box,” Hishammuddin Hussein told reporters.

“So there is no question of it not being released.”

The Malaysian government has been tight-lipped about its ongoing investigation into the disappearance of the jet, adding to the anger and frustration of relatives.

It has come under fire for a seemingly chaotic initial response, while the scarcity of official information on MH370 has prompted questions over its transparency.

Hishammuddin said at the weekend that Malaysia’s attorney general had been sent abroad to confer with the International Civil Aviation Organisation and determine which country would have custody of the black box, if it is ever found.

But he shrugged off the importance of the custody issue on Tuesday.

“I don’t think it’s important who gets custody as far as I’m concerned,” he told reporters.

Malaysian authorities insist they are hiding nothing but need to be cautious on commenting on ongoing investigations.

Hishammuddin also said an “international investigation team” that Malaysia plans to set up to probe MH370’s disappearance would be transparent and operate in accordance with international standards.

AFP Photo/Malaysian Maritime Enforcement