Submarine completes first dive in search for debris

A Bluefin-21 autonomous underwater vehicle (AUV) has completed its first dive. (file photo)

An unmanned submarine has been forced to cut short its first search mission looking for debris from missing Malaysia Airlines flight MH370 in the southern Indian Ocean.  [Special coverage]

The Joint Agency Coordination Center (JACC) said the Bluefin-21 sub was deployed last night from Australian vessel Ocean Shield on a mission that lasted six hours.

The search ended sooner than expected when it exceeded its depth limit of 4,500 meters and an inbuilt safety feature brought it back to the surface, the JACC said in a statement.

"The six hours of data gathered by the Autonomous Underwater Vehicle is currently being extracted and analysed," the center said.

Authorities say it takes two hours for Bluefin-21 to reach the ocean floor.

Bluefin-21 will be redeployed later today and up to 11 aircraft and 11 ships will also take part in a visual search for signs of MH370.

Yesterday, search coordinator Retired Air Chief Marshal Angus Houston said no acoustic trace of the plane's black box had been detected for six days.

"We haven't had a single detection in six days. So I guess it's time to go underwater," he said.

The black box, which refers to two orange boxes that record flight data and cockpit chatter, may provide answers about what happened to the plane.

The sub was launched after authorities gave up hope of relocating underwater signals from MH370's flight recorders.

The recorders are powered by batteries to send out a pulse, but the batteries were expected to have expired last week.

Ocean Shield yesterday ceased searching for the signals using its towed pinger locator.

The flight MH370 vanished off radar screens on March 8 en route from Kuala Lumpur to Beijing with 239 people on board, including six Australians.

The center of the search area is about 2,200 kilometers north-west of Perth.