Search for missing plane heads underwater as black box falls silent

An unmanned submarine will be sent to try to find wreckage from Malaysia Airlines MH370 after signals suspected to be from the aeroplane's black box fell silent. [Special coverage]

In a media briefing on Monday afternoon, Retired Air Chief Marshal Angus Houston said no acoustic trace of the 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 experts have determined that the Australian Defence vessel Ocean Shield will cease searching with the towed pinger locator later today and deploy the autonomous underwater vehicle, Bluefin-21, as soon as possible."

Retired Air Chief Marshal Angus Houston says the vehicle will first search an area of about 40 square kilometres.

"The deployment of the autonomous underwater vehicle has the potential to take us a further step towards visual identification since it offers a possible opportunity to detect debris from the aircraft on the ocean floor," he said.

"It will take the autonomous underwater vehicle two hours to get down to the bottom of the ocean.

"It will then be on task for 16 hours. It will then take two hours to return to the surface and four hours to download and analyse the data collected.

"As I have said before, aircraft wreckage needs to be visually identified before we can say with certainty that this is the final resting place of MH370."

Retired Air Chief Marshal Houston, charged with leading the recovery effort, says an oil slick has also been spotted in the vicinity of the search area, however it is unclear whether it is from the plane.

"I can report Ocean Shield detected an oil slick yesterday evening in her current search area," he said.

"A sample of about two litres has been collected and we are a number of days [away] before it can be landed ashore and conclusively tested.

"I stress the source of the oil is yet to be determined but the oil slick is approximately 5,500 metres down-wind and down-sea from the vicinity of the detections picked up by the towed pinger locator on Ocean Shield."

Retired Air Chief Marshal Houston says he is not confident about finding plane debris on the ocean surface.

"The chances of any floating material being recovered have greatly diminished and it will be appropriate to confer with Australia's partners to decide the way ahead later this week," he said.

No signals for almost a week

The last of the four acoustic signals detected by Ocean Shield were received last Tuesday night, fuelling an expectation that the unit's batteries have now run out.

"Despite the lack of further detections, the four signals previously acquired taken together constitute the most promising lead we have in the search for MH370," Retired Air Chief Marshal Houston said.

"We need to pursue this lead as far as possible.

"Analysis of the four signals has allowed the provisional definition of a reduced and manageable search area on the ocean floor."

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

The centre of the search area is about 2,200 kilometres north-west of Perth.