PERTH, April 8 ― Search crews cut the patrol zone for the missing Malaysian Air jet by almost two-thirds today after detecting pings similar to those from a plane’s black boxes, the biggest breakthrough in a month-old mystery.

The signals raised investigators’ hopes that they could target a specific area with a sonar-equipped robot submarine to scan the Indian Ocean floor for wreckage. They’re racing the clock as beacons on the cockpit-voice and flight-data recorders near the end of their batteries’ 30-day lifespan since Flight 370 disappeared on March 8.

“It’s pretty amazing that they narrowed it down and actually heard something with no debris on the surface,” said Chris Portale, finance director of pinger maker Dukane Seacom, a unit of Hollywood, Florida-based Heico Corp. “You’re searching a very large area.”

Today’s surveillance in the Indian Ocean will cover 77,580 square kilometres (29,950 square miles), with as many as 11 planes and 14 ships taking part, Australia’s Joint Agency Coordination Centre said. Yesterday’s search spanned 216,000 square kilometres.

Among the vessels in the search is Australia’s Ocean Shield, which detected a pair of signals ― one lasting two hours and 20 minutes and the other for 13 minutes ― this weekend as it pulled a US Navy towed pinger-locator through the water. The ship lost contact after hearing the first signal and then made a turn and reacquired the shorter one.

‘Most promising’

“This is a most promising lead,” retired Air Chief Marshal Angus Houston, who leads the JACC, said yesterday. “It’s probably the best information we’ve had.”

The Ocean Shield has a submersible, the Bluefin-21, ready for launch once the search zone is refined, Houston said. Water depths in the area exceed about 4,500 metres (14,800 feet), and extend down to more than 5,000 metres in parts, Houston said.

Detecting a pinger signal for more than two hours suggests that what the Ocean Shield picked up was more than a false alarm, said John Fish, a principal at Bourne, Massachusetts-based American Underwater Search & Survey Ltd. who has been involved in several efforts to find aircraft that crashed in oceans.

“That speaks volumes,” Fish said in an interview. “That is exactly what you would expect to find.”

False signals tend to be shorter in duration and difficult to replicate, he said. The fact that the ship detected it a second time also indicates that it may be nearing the crash site, he said.

Dark waters

Zeroing in on the source of the pings is critical, because a search by the Bluefin-21 will be time-consuming in pitch-black waters on the ocean bottom. The Bluefin-21’s sonar uses reflected sound waves to detect any anomalies on the seabed, and if something unusual is found, the sub will surface and be fitted with a camera.

“In very deep oceanic water, nothing happens fast,” Houston said. “It could take some days before the information is available to establish whether these detections can be confirmed as being from MH370.”

The side-scan sonar carried by the Bluefin-21 is the same technology that was used to find the remains of Air France Flight 447, which crashed in the Atlantic Ocean in 2009 in 3,900 meters of water. The Bluefin’s operational depth is 4,500 meters, raising the prospect that any Flight 370 wreckage may be too deep for the vehicle, Houston said.

‘Less hostile’

The region now being explored in the tropics due west of Australia’s Pilbara iron-ore mining region is more favorable to search operations than the area in the southern Indian Ocean where the hunt was initially focused, in the so-called Roaring Forties latitudes known for rough seas and strong winds.

“It’s much, much less hostile than the previous location but it’s much, much deeper,” Charitha Pattiaratchi, a professor of oceanography at the University of Western Australia, said by phone from Perth.

The Malaysian Airline System Bhd. plane disappeared en route to Beijing from Kuala Lumpur with 239 people on board, triggering a search that is now the longest ever for a modern jetliner. Flight 370 was deliberately steered off course to a path that ended in the Indian Ocean, Malaysian Prime Minister Datuk Seri Najib Razak has said.

That makes the recovery of the jet’s recorders pivotal to determining what happened and why.

Locator beacons on the black boxes have non-rechargeable lithium batteries. The power cells like those on Flight 370’s pingers usually last three to five days longer than the 30-day specification at full signal power, according to manufacturer Dukane Seacom.

Fading signal

After that, the signal will fade as the batteries weaken and then go dead within days, Dukane Seacom President Anish Patel said last week in an interview.

Fruitless air and sea patrols across hundreds of thousands of kilometres of remote Indian Ocean waters underscored the challenges in finding the remains the Boeing Co. 777-200ER wide- body jet. Hearing the pings helps searchers put greater scrutiny on an area while still leaving enormous difficulties. The pulses from the jet’s beacons don’t pinpoint a location, just that the units are nearby.

The range of the pings is a mile, according to manuals from Honeywell International Inc., the maker of the black boxes. That may make the signals hard to pick up even if an underwater microphone is over the correct location.

‘Move around’

“It takes a fair bit of effort once you’ve picked up the signal,” said Justin Manley, director of business development for pinger manufacturer Teledyne Benthos, a unit of Thousand Oaks, California-based Teledyne Technologies Inc. “Now you need to move around to triangulate where it’s coming from and start to home in on it.”

While a Chinese ship also reported hearing an underwater pulse this weekend, that signal was detected about 600 kilometers (373 miles) away, according to a map on JACC’s website. The towed pinger-locator from the US Navy, which has decades of experience in submarine warfare and detection, is more sophisticated than China’s equipment.

“The audible signal sounds to me just like an emergency locator beacon,” Houston said of the findings from Ocean Shield. “We are very encouraged that we are getting closer to where we need to be.”

Any sound at that depth won’t be from a biological source, said Peter Marosszeky, director of consultant Aerospace Developments Pty. and an investigator into a 1989 incident in which a cargo door blew off a United Airlines Inc. flight over the Pacific Ocean.

“It’s a bit of a breakthrough,” Marosszeky said from Sydney. “It certainly raises my expectations.”

There are still significant challenges to locating any wreckage, Houston said, including the imprecise nature of the pings, the depth of the ocean, and the amount of charge in the beacon batteries.

The batteries “must be getting close to the end of life,” Houston said. “We’re already one day past the advertised shelf life. We hope it’s going a bit longer.” ― Bloomberg