The hunt for a missing Malaysian plane suffered another setback on Wednesday when a second seabed search by a mini-submarine was cut short due to "technical" troubles after the first also aborted in very deep water.
Australia's Joint Agency Coordination Centre (JACC) issued a brief statement which spoke of an unspecified "technical issue" with the unmanned Bluefin-21 sonar device.
The first mission which began on Monday night aborted automatically after breaching the machine's maximum operating depth of 4,500 metres (15,000 feet).
But there was no explanation for what caused the interruption of the second mission, which began on Tuesday night, or how long it lasted.
Phoenix International workers Chris Minor (R) and Curt Newport inspect the US Navy’s Bluefin 21 autonomous underwater vehicle (AUV) before deployment in the southern Indian Ocean to look for the missing Malaysia Airlines flight MH370 in this handout picture released by the Australian Defence Force. (Reuters)
"The Autonomous Underwater Vehicle, Bluefin-21, was forced to resurface this morning to rectify a technical issue," JACC said.
"Bluefin-21 was then redeployed and it is currently continuing its underwater search."
Before the device was put in the water for the third time, data had been downloaded from the vehicle while on the deck of the Australian vessel Ocean Shield, which has led the search for the Malaysia Airlines Boeing 777 that vanished on March 8 with 239 people aboard.
"Initial analysis of the data downloaded this morning indicates no significant detections," JACC said.
The Bluefin's first mission, cut short after just six of an intended 16 hours mapping the seabed with sonar, had also drawn a blank.
After more than three weeks of hunting for black box signals, the autonomous sub was deployed for the first time on Monday night.
The US navy explained that the Bluefin-21 had automatically aborted its first mission after six hours upon breaching its maximum operating depth.
JACC added that it had "exceeded its operating depth limit of 4,500 metres and its built-in safety feature returned it to the surface".
The sub was undamaged and had to be re-programmed, said US Navy captain Mark Matthews.
"In this case the vehicle's programmed to fly 30 metres over the floor of the ocean to get a good mapping of what's beneath," he told CNN from Perth after the aborted dive.
"It went to 4,500 metres and once it hit that max depth, it said 'This is deeper than I'm programmed to be', so it aborted the mission."
Two months to scan area
Questions were raised about how deep the seabed may be in the search area.
JACC chief Angus Houston has stressed that the AUV cannot operate below 4,500 metres and that other vehicles would have to be brought in to cope with greater depths.
"There are vehicles that can go a lot deeper than that," he said Monday. "They are usually much larger vehicles; they do recovery as well and obviously those sorts of possibilities will be looked at... they are being looked at as we speak.
"But a lot will depend on the outcome of what we find when we go down and take a look," said Houston, who has not revealed what has been seen so far.
He had announced on Monday the end of listening for signals from the plane's black box flight recorders and the launch of the submarine operation.
The mini-sub is supposed to conduct a sonar survey of the silty ocean floor for 16 hours at a time, looking for wreckage from the Malaysia Airlines flight which mysteriously disappeared en route from Kuala Lumpur to Beijing.
The US Navy has estimated it would take the Bluefin-21 "anywhere from six weeks to two months to scan the entire search area".
In this photo provided by the Australian Defense Force an autonomous underwater vehicle is deployed from ADV Ocean Shield in the search of the missing Malaysia Airlines Flight MH370 in the southern Indian Ocean. (AP Photo)
The area has been narrowed down using satellite data and the detection of electronic pulses from the black box which were last heard more than a week ago.
Houston has described those 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 kilometres off course, remains a mystery. No debris has been found despite an enormous search involving ships and planes from several nations.
The visual search for debris also continued Wednesday, JACC said, with as many as 14 aircraft and 11 ships involved over an area of 55,151 square kilometres (21,288 square miles) more than 2,000 kilometres (1,250 miles) northwest of Perth.