Missing flight MH370: Robotic sub first mission cut short

A robotic submarine deployed to search for the missing Malaysia Airlines plane in the southern Indian Ocean has had its first mission cut short.

The Bluefin-21 was sent to search the sea floor for wreckage after signals believed to be consistent with "black box" flight recorders were detected.

But the drone exceeded its operating limit of 4,500m (15,000ft) and was brought back to the surface.

It was due to return later on Tuesday if weather conditions permitted.

"To account for inconsistencies with the sea floor, the search profile is being adjusted to extend the sonar search for as long as possible," an update from the US Navy - which operates the Bluefin-21 - said.

Flight MH370 went missing on 8 March with 239 people on board. It was flying from Kuala Lumpur to Beijing when it lost contact with air traffic controllers over the South China Sea.

Malaysian officials believe, based on satellite data, that it ended its flight thousands of kilometres off course, in seas west of the Australian city of Perth.

'Operating depth'

Amid a major international search, an Australian navy vessel last week detected four acoustic signals using a US Navy towed pinger locator. Officials believe these could come from the missing plane's flight recorders.

No signals have been detected since 8 April, however, leading to fears that the recorders' batteries - which last about a month - have run out.

On Monday, Air Chief Marshal Angus Houston, who heads the agency co-ordinating the search, said it was time to deploy the Bluefin-21 to begin its search of the sea floor. It set off on its first mission on Monday night.

"After completing around six hours of its mission, Bluefin-21 exceeded its operating depth limit of 4,500m and its built in safety feature returned it to the surface," the Joint Agency Co-ordination Centre said in a statement.

"Bluefin-21 is planned to redeploy later today when weather conditions permit."

The US Navy said in a later update that no objects of interest were found when the six hours of data were downloaded and analysed.

Bluefin-21 is an almost 5m-long vehicle that can create a sonar map of the sea floor. On Monday officials said each mission was expected to last 24 hours, with 16 hours spent on the ocean floor, four hours' diving and resurfacing time, and four hours to download data.

The submersible has a safety feature that brings it to the surface if it exceeds its performance capabilities, however.

The sea where the Bluefin-21 is searching is estimated to be about 4,500m deep, but experts say there could be variations on the sea floor.

Australian officials have said previously that they are confident they are searching in the right area for the missing plane.

But ACM Houston warned on Monday that the search of the sea floor could be a long, painstaking process that might not yield results.

He said that other, larger vehicles that could go deeper than the Bluefin-21 were being looked into, but it depended on "the outcome of what we find when we go down and take a look".

Officials have no idea yet why the plane diverted so far from its intended flight path. Investigators are looking at options including hijacking, mechanical failure, sabotage and pilot action.

Recovering the flight recorders is seen as key to understanding what happened to the plane.