Lack of electronic pings means MH370 black-box batteries may finally be dead.

After a week of optimism over four underwater signals believed to be coming from the missing Malaysian plane, the sea has gone quiet and Australia's leader is warning that the massive search will likely be long.

No new electronic pings have been heard since April 8, and the batteries powering the locator beacons on the jet's black box recorders may already be dead. They only last about a month, and that window has already passed.

Once officials are confident no more sounds will be heard, a robotic submersible will be sent down to slowly scour for wreckage across a vast area in extremely deep water.

Watch: MH370: Two more pings heard from Indian Ocean


Searchers for Malaysia Airlines flight 370 are still trying to narrow the search area before deploying the undersea drone.

But there have been no confirmed acoustic detections from the plane's black boxes in the past 24 hours.

Up to 11 military aircraft, one civil aircraft and 14 ships yesterday were searching an area of up to 57,506sq km about 2200km northwest of Perth, where it is believed the plane crashed after mysteriously vanishing en route from Kuala Lumpur to Beijing with 239 people on board on March 8.

Australian Defence Vessel Ocean Shield is continuing focused sweeps with a towed pinger locator to try to locate further signals, supported by British ship HMS Echo. Military AP-3C Orions are continuing their acoustic search of the area.

Watch: MH370: Ship hears 'signal'; unclear if jet-related

"This work continues in an effort to narrow the underwater search area for when the Autonomous Underwater Vehicle is deployed," the Joint Agency Co-ordination Centre said.

Prime Minister Tony Abbott on Saturday said pings from the surface search area have helped authorities narrow down the over-sea area. But fears remain the black box batteries will run out.

"When we think we've got everything we can through this means, we will deploy a submersible.

"By that stage we hope we will have narrowed the search area on the sea bed to as little perhaps as a square kilometre."

He said success wasn't certain, and it might still take weeks or months.

Meanwhile, Fariq Abdul Hamid, the co-pilot of the missing jet, reportedly switched on his mobile phone above Penang - just before the plane vanished.

The New Straits Times claimed Hamid turned off his phone before the plane took off. He then "reattached" the phone mid-air, as the plane was passing within reach of the Penang control tower - but did not make any contact.

Watch: MH370: Signals come from black box

The newspaper described the mid-air activation of the phone as "a desperate call".

Hamid last used his phone to send a WhatsApp message around 11.30pm on March 7 - just before he boarded the aircraft for his six-hour flight to Beijing. Two hours previously he had made his final call, which was to a "regular contact".

An unnamed source told the paper: "The telecomms tower established the call that he was trying to make. On why the call was cut off, it was likely because the aircraft was fast moving away from the tower and had not come under the coverage of the next one."

A second source said that connection to the phone had been "detached" before the plane took off.

"This is usually the result of the phone being switched off," they said.

"At one point, however, when the aeroplane was airborne, between waypoint Igari and the spot near Penang [just before it went missing from radar], the line was 'reattached'. A 'reattachment' does not necessarily mean that a call was made. It can also be the result of the phone being switched on again."

Many airlines insist crews turn their mobile phones off while airborne. But some pilots leave their phones on - either intentionally, to surreptitiously read emails; or by mistake.

"If it was suddenly switched on mid flight, then it does suggest something untoward was occurring," said Alastair Rosenschein, an aviation expert and former British Airways pilot. "But it's not unusual for a phone to be left on innocently, by mistake, and then come into signal area.

"There has been so much uncorroborated material on this flight that it is very difficult to determine fact from fiction or speculation. This could be yet another red herring."