PERTH - Ships listening for underwater signals from Flight MH370 have logged no more "pings" and will spend several more days trying to pinpoint a crash site before a mini-sub is launched to scour the seabed, searchers say.
A month to the day since the Boeing 777 vanished with 239 people on board, time is running out to detect further signals as the batteries in beacons on the jet's black box data recorders reach their expiry date.
Transmissions picked up by Australia's Ocean Shield naval ship - consistent with those from aircraft black boxes - had raised hopes that a robotic submersible would soon be sent down to look for debris.
But search chief Angus Houston clarified yesterday that while the pings were an exciting development, further transmissions were needed before deploying the mini-sub.
"We need to continue that (search) for several days to the point at which there is absolutely no doubt that the pinger batteries will have expired," Houston said.
"Until we stop the pinger search we will not deploy the submersible."
He said no further transmissions had been detected in the remote search area off western Australia which could help pinpoint where the jet might have crashed.
It went missing en route from Kuala Lumpur to Beijing on March 8 and the search is now focusing on a 600km arc of the remote southern Indian Ocean.
Houston had indicated that the time was nearing for the Ocean Shield, which is criss-crossing the area to try to home in on the signals again, to launch the US-made autonomous underwater vessel (AUV) Bluefin-21.
But he spelt out a more definitive timeframe yesterday, saying the 5m-long submersible sonar device would not be put into the water until the pinger search had ended.
"If we go down there now and do the visual search it will take many, many, many days because it's very slow, very painstaking work to scour the ocean floor," he said.
Once in the water, if the mini-sub detects something unusual with its sonar, it can be brought to the surface and sent down again equipped with a video camera to provide visual evidence of a crash.