Investigators reduced the search area in the Indian Ocean for a missing Malaysian Air jet by almost two-thirds, a day after announcing ships detected pings similar to those from a plane’s black boxes.

The Ocean Shield, which detected a pair of signals this weekend as it pulled a U.S. Navy towed pinger locator through water, heard no further pings, said retired Air Chief Marshal Angus Houston, who heads Australia’s Joint Agency Coordination Centre. Hearing another transmission consistent with signals from black-box recorders is essential before deploying an unmanned submarine to scour the ocean bed for debris.

“If we can get more transmissions, we can get a better fix on the ocean floor,” Houston told reporters at the Royal Australian Air Force Base Pearce, near Perth. “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.”

Australia’s Ocean Shield detected two signals — one lasting two hours and 20 minutes and the other for 13 minutes. That’s raised hopes investigators could target a specific area with a sonar-equipped robot submarine to scan the ocean floor for wreckage. They’re racing the clock a month after the plane disappeared as beacons on the cockpit-voice and flight-data recorders near the end of their batteries’ 30-day lifespan. Flight 370 vanished March 8.

‘Very Noisy’

“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. (HEI/A) “You’re searching a very large area.”

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

Sending too many ships and vessels to the locations where the pings were detected would make the area “very noisy,” Houston said. “In terms of the environment, we can’t have too many ships in the area because when you’re dealing with these transmissions, you need utter silence,” he said.

No False Signal

The Ocean Shield has a submersible, the Bluefin-21, ready for launch once the search zone is refined, Houston said yesterday. Water depths in the area exceed about 4,500 meters (14,800 feet), and extend down to more than 5,000 meters 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 ship detecting it a second time also indicates that it may be nearing the crash site, he said.

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.

Deliberately Steered

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

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.

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.

Fading Signal

Fruitless air and sea patrols across hundreds of thousands of kilometers of remote Indian Ocean waters underscored the challenges in finding the remains the Boeing Co. 777-200ER wide-body jet.

The range of the pings is a mile, according to manuals from Honeywell International Inc. (HON), 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. The pulses from the jet’s beacons don’t pinpoint a location, just that the units are nearby.

“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.

While a Chinese ship also reported hearing an underwater pulse this weekend, that was detected about 600 kilometers (373 miles) away, according to a map on JACC’s website.

“We just need to let Ocean Shield continue its work with the towed pinger to try and find another transmission from whatever is down there,” Houston said.