MH370: Ocean depth thwarts first sonar probe

The first mission of the underwater search for the missing Malaysia Airlines plane has been aborted 10 hours short of its scheduled time, with waters in parts of the area too deep for the vessel.

Searchers ended the hunt for acoustic signals from MH370's black box on Monday after it became clear the batteries had died, eight days after their guaranteed shelf life and six days after the last audio detections.

By evening, the search had gone underwater, with the deployment of the automated underwater vehicle (AUV) Bluefin-21.

But on Tuesday morning, the Joint Agency Co-ordination Centre said the AUV had been returned to the surface after just six hours.

It was expected to be a 24-hour mission, comprising four hours travel to and from the ocean floor, 16 hours of sidescan sonar work and four hours of downloading and analysing data.

The vessel exceeded its depth limit of 4.5km, which was the earlier rough estimate for water depths in the Indian Ocean search area, some 2200km northwest of Perth.

JACC said the Bluefin-21's built-in safety feature returned it to the surface.

US Navy Captain Mark Matthews, a salvage expert who is playing a key role in the operation, said part of the search zone was deeper than 4.5km.

'The vehicle is programmed to fly 30 metres over the floor of the ocean to get a good mapping of what's beneath and to the sides,' Capt Matthews told CNN.

'The chart we had showed between 4200 and 4400 metres depth.

'As it was contouring itself to the bottom ... it went to 4500 metres and once it hit that max depth, it said 'hey, this is deeper than I am programmed to be', so it aborted the mission to basically interface with the crew so they could refine the task.'

He said the mission was aborted 'in a very far corner' of the search area.

'So they're just shifting the search box a little bit away from that deep water and proceeding with the search.'

JACC chief Angus Houston told reporters on Monday that another much larger vessel with wreckage recovery capability would be needed if the water depths exceeded Bluefin-21's capability, saying that was 'being looked at as we speak'.

JACC confirmed last night that the air and surface search for floating debris would be completed within two to three days.

Up to nine military aircraft, two civil aircraft and 11 ships will assist in today's search.

Today the Australian Maritime Safety Authority has planned a visual search area totalling approximately 62,063 square kilometres.

The centre of the search areas lies approximately 2,170 kilometres north west of Perth.

As the painstaking seabed search for wreckage continues, a US official has confirmed that a mobile phone tower in Penang detected the co-pilot's phone searching for service about the time MH370 vanished.

CNN reports this morning that the official, who has firsthand knowledge of the MH370 investigation, said the phone signal was picked up about 400km from where the flight disappeared on March 8.

The revelation follows a weekend report from Malaysia that co-pilot Fariq Abdul Hamid had tried to make a telephone call while the plane was in flight.

But the US official told CNN said investigators had no evidence that it was the co-pilot who had tried to make the call.

The mobile phone report does 'appear to reaffirm suggestions based on radar and satellite data that the plane turned around and was likely flying low enough to obtain a signal from a cell tower', the official said.