An Australian P3 Orion aircraft has detected another possible signal from a black box recorder in the search for missing Malaysia Airlines flight MH370.

Retired Air Chief Marshal Angus Houston, who is heading the search, says the aircraft detected the signal near the Australian Navy ship Ocean Shield on Thursday.

He says further analysis is needed but the signal may be from a man-made source.

The search area has been narrowed from 75,000 square kilometres to about 58,000 square kilometres after four signals were detected in an area less than 40 kilometres apart by a US Navy towed pinger locator deployed by Ocean Shield.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 2 minutes 59 seconds 2 m 59 s Search closing in on MH370

The fifth signal was detected by the aircraft picking up transmissions from a listening device buoy deployed near the ship.

"Whilst conducting an acoustic search this afternoon a RAAF AP-3C Orion aircraft has detected a possible signal in the vicinity of the Australian Defence vessel Ocean Shield," Retired Air Chief Marshal Houston said in a statement on Thursday.

The centre of the search area lies approximately 2,280 kilometres north-west of Perth.

Air Chief Marshal Angus Houston says it may be just a matter of days before the search closes in on MH370's "final resting place".

"I think we're looking in the right area, but I'm not prepared to confirm anything until such time as somebody lays eyes on the wreckage," he said.

Time running out for black box search

The black boxes record cockpit data and may provide answers about what happened to the plane, which was carrying 227 passengers and 12 crew when it vanished on March 8 while flying thousands of kilometres off its Kuala Lumpur to Beijing route.

The batteries in the black boxes have already reached the end of their 30-day expected life, making efforts to locate them on the murky ocean floor all the more critical.

"We are still a long way to go, but things are more positive than they were some time ago," Australian Transport Safety Board chief commissioner Martin Dolan said.

Up to 12 military aircraft, three civil aircraft and 13 ships are involved in the search effort which has found no plane debris so far.

Efforts are now focused on two areas - aircraft and ships in the area about 2,240 kilometres north-west of Perth and a smaller area about 600 kilometres closer to the WA city.

What is the Bluefin-21? An autonomous underwater vehicle (AUV) designed for deep-sea surveying.

An autonomous underwater vehicle (AUV) designed for deep-sea surveying. It has a "swappable payload". It will first use sonar in the search and will be refitted with cameras if something is detected.

It has a "swappable payload". It will first use sonar in the search and will be refitted with cameras if something is detected. It's 5m long and weighs 750kg. Has an endurance of 25 hours underwater at a speed of 3 knots, with a top speed of 4 knots.

It's 5m long and weighs 750kg. Has an endurance of 25 hours underwater at a speed of 3 knots, with a top speed of 4 knots. It has a depth rating of 4,500m, meaning it will be at its limit in the Indian Ocean search zone.

It has a depth rating of 4,500m, meaning it will be at its limit in the Indian Ocean search zone. Bluefin Robotics says its AUV can also be used for archaeology, oceanography, mine countermeasures, and unexploded ordnance.

The Ocean Shield picked up the acoustic signals around the smaller zone and dozens of acoustic sonobuoys were dropped there on Wednesday.

Each of the sonobuoys carries a listening device called a hydrophone, which is dangled about 305 metres below the surface and is capable of transmitting data to search aircraft via radio signals.

"That does provide a lot of sensors in the vicinity of the Ocean Shield without having a ship there to produce the background noise," said Australian Navy Commodore Peter Leavy, who is heading the Australian search operations.

Once the final search area has been identified an autonomous underwater vehicle, the Bluefin-21 could be deployed to look for wreckage on the sea floor.

Experts say it will still be a difficult task.

"Working near the bottom of the ocean is very challenging because this is uncharted territory: nobody has been down there before," University of NSW oceanographer Erik van Sebille said.

Air Chief Marshal Houston agrees, warning the exceptionally deep water will mean any recovery effort of the missing plane will be a challenge.

Silty conditions on the sea bed could also complicate recovery efforts.

"Silt is a material that tends to be very deep and something falls into it and it's sort of swallowed up by the sort of silty, sandy material that lies on the ocean bottom," he said.

"So that will complicate things when we get into a visual search."

ABC/Reuters