Authorities who picked up promising signals on Sunday in the search for a missing Malaysian Airlines plane have not been able to detect the signals again.
Fears the plane's emergency locator batteries have died are growing by the hour as an Australian ship towing a black box detector tries to find flight MH370.
In what was described on Monday as the “most promising” breakthrough in a month-long search for the plane, Australian authorities revealed the Australian Defence Vessel Ocean Shield had detected two acoustic events in the far north of the search zone.
But it has been more than 40 hours since the ship detected a transmission.
The spot where the signals were detected coincides with analysis from two weeks ago, which estimated where the final satellite contact with the plane occurred. It is thought this final "half-handshake" could have been the moment at which the plane ran out of fuel and plunged into the ocean.
The beacons on the plane's black boxes have a minimum battery life of 30 days and it has now been 32 days since flight MH370, with 239 people on board, vanished on March 8, en route to Beijing from Kuala Lumpur.
“There have been no further contacts with any transmissions and we need to continue that for several days, right up until there's absolutely no doubt that the pinger batteries have transpired," the head of the search's Joint Agency Co-ordination Centre, Air Chief Marshal (ret) Angus Houston, said on Tuesday.
He said an automatic underwater vehicle, the Bluefin-21, which is fitted with sonar and camera capabilities, would not be deployed until there were further transmissions that would narrow the seafloor search.
Defence Minister David Johnston said on Tuesday the detections were still “the most positive definitive lead we have”.
“We are totally focused on assisting our Malaysian friends on identifying the location of this aircraft and bringing some closure to loved ones and family aboard,” he said.
“We're out there again today seeking to enhance that contact.”
On Sunday, the pinger locator was 300 metres below sea level when it detected consecutive pings at one-second intervals. A signal was then held for more than two hours when the locator was lowered to 1400 metres below sea level. The signals got stronger and were consistent with the ship having approached and passed the source of the signals.
About five hours later, during a return swing, the Ocean Shield team again detected a separate set of pings, which lasted 15 minutes, when the pinger locator was dropped to an optimal depth of 3000 metres.
On Monday, Air Chief Marshal (ret) Houston said the ADV Ocean Shield's two separate detections were consistent with the MH370's two black boxes: a flight data recorder and a cockpit voice recorder.
However, verification about whether the acoustic events are linked to the missing plane could still be days away.
The underwater search would continue for days, until “there's absolutely no chance that the device is transmitting", he said.
Although 133 surface missions to find the plane’s wreckage have failed to turn up any solid evidence, Mr Johnson said the intensity of the search to locate debris on the surface of the ocean was not waning.
‘‘The debris search is obviously vital to us adding another important piece to this jigsaw puzzle,’’ he said.
‘‘We are currently very actively and aggressively pursuing where we think that debris field might be so as to give us further information to recalculate back where the point of entry might have been.
‘‘We have at least several days of intense action ahead of us.’’
When asked about whether the Australian search co-ordination effort was still investigating pulse signals detected by the Chinese vessel Haixun Zero One on Friday and Saturday, Mr Houston said the Ocean Shield’s was now the best lead.
‘‘Funny things happen to transmissions in water,’’ he said. ‘‘Some of the false leads we’ve had have been actually transmissions from the ship that was actually searching and its got its own transmissions back again.
‘‘In terms of the search we think the Ocean Shield transmission is probably the most promising one and we continue to prosecute that.’’
There were 14 aircraft - 11 military and three civil - and 14 ships scheduled to assist with Tuesday’s search of about 77,500 square kilometres, an area 2268 kilometres north-west of Perth.