The search for a vanished Malaysian airliner in the southern Indian Ocean went submarine (underwater) on Friday, to scour the treacherous waters to hunt for the black box 'pinger' of Malaysia Airlines MH370.

Nevertheless, the aerial search for the Boeing 777-200ER was still ongoing, said Joint Agencies Coordination Centre (JACC) chief coordinator, Australian Air Chief Marshall Angus Houston.

He said the Australian Navy ship, 'Ocean Shield' and the British Navy's 'HMS Echo' would converge along a 240km track in the southern Indian Ocean.

"Using the Towed Pinger Locator (TPL) from the United States Navy on Australian Defence Vessel 'Ocean Shield' and a similar capability on 'HMS Echo', the two ships will search a single 240km track, converging on each other," he said at a press conference here Friday.

This will be the first time a sub-surface search is being conducted in the search to detect the signal from Flight MH370's black box.

Meanwhile, Joint Task Force 658 Commander, Commodore Peter Leavy, in a statement here Friday said the two ships and their towed-pinger equipment would be operating at significantly reduced speed to search depths of three thousand metres or more.

“There has not been any change in the search,” he said, adding that no hard evidence was found to date, thus resulting in the decision to search a sub-surface area based on the predicted data analysis.

“While the preference for search operations is to use physical evidence and then drift modelling to determine a smaller sub-surface search area, the search track is considered to be the best estimate possible for an area likely to contain the...aircraft.

“The equipment on Australian Defence Vessel (ADV) 'Ocean Shield' and 'HMS Echo' can only operate effectively at reduced speed, around three knots," he added.

Leavy said the search using sub-surface equipment needed to be methodical and carefully executed in order to effectively detect the faint signal of the pinger.

Yesterday, Malaysian Prime Minister Datuk Seri Najib Tun Razak, who was on a two-day working visit to Perth, Australia discussed the underwater search for Flight MH370, with his Australian counterpart, Tony Abbott.

During the visit, Najib personally thanked Abbot over Australia's spontaneous reaction to willingly answer Malaysia's request for the search.

He also had a first-hand look at the search being conducted from the western Australian city for the missing Malaysian plane, whose flight path was believed to have ended in the southern Indian Ocean.

Flight MH370, carrying 227 passengers and 12 crew, left the KL International Airport at 12.41am on March 8 and disappeared from radar screens about an hour later, while over the South China Sea.

It was to have arrived in Beijing at 6.30am on the same day.

A multinational search was mounted for the aircraft, first in the South China Sea and then, after it was learned that the plane had veered off course, along two corridors - the northern corridor stretching from the border of
Kazakhstan and Turkmenistan to northern Thailand, and the southern corridor, from Indonesia to the southern Indian Ocean.

Following an unprecedented type of analysis of satellite data, United Kingdom satellite telecommunications company Inmarsat and the UK Air Accidents Investigation Branch (AAIB) concluded that Flight MH370 flew along the southern corridor and that its last position was in the middle of the Indian Ocean, west of Perth, Australia.

Najib then announced on March 24 - 17 days after the disappearance of the Boeing 777-200ER aircraft, that (the flight path of) Flight MH370 "ended in the southern Indian Ocean".