Ships combing the Indian Ocean for a missing Malaysian Air jet failed to detect new pings that might point the way to the jet’s flight recorders, as investigators narrowed the search area by almost two-thirds.

Australia’s Ocean Shield heard no more sounds after picking up signals this weekend, the Joint Agency Coordination Centre said today. Detecting pulses consistent with transmissions from the black boxes is essential before sending an unmanned submarine to scour the seabed.

Investigators are racing against time to pick up the pings again because beacons on the cockpit-voice and flight-data recorders are nearing the end of their batteries’ 30-day lifespan since Flight 370’s March 8 disappearance. What they don’t know is whether they have strayed out of range or whether the pingers’ power is fading.

“The fact that they haven’t been able to replicate it isn’t surprising,” said John Fish, a principal of Bourne, Massachusetts-based American Underwater Search & Survey Ltd. “That they have a position now is pretty much a game changer compared to before the weekend.”

Australia’s Ocean Shield detected two signals — one lasting two hours and 20 minutes and the other for 13 minutes. The vessel is pulling a U.S. Navy towed pinger locator listening for transmissions from the pingers on the Boeing Co. 777-200ER.

Eight Hours

The cumbersome search process requires about eight hours for the vessel to make one pass over a search zone. That may explain why sounds haven’t been heard since the weekend, Fish said in an interview. While less probable, it’s also possible that the pingers’ batteries failed, he said. Batteries typically last longer than their 30-day certification, he said.

A ship towing the locator follows a “ladder pattern,” with each leg of the search followed by a second pass in the opposite direction “on a slightly different heading,” the U.S. Navy’s Seventh Fleet said in an e-mailed statement. The course adjustments on each pass steadily expand the patrol zone.

Today’s surveillance in the Indian Ocean was planned for 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 to the locations where the pings were detected would make the area “very noisy,” according to retired Air Chief Marshal Angus Houston, who leads the JACC.

‘Utter Silence’

“In terms of the environment, we can’t have too many ships in the area,” Houston told reporters at Royal Australian Air Force Base Pearce, near Perth. “When you’re dealing with these transmissions, you need utter silence.”

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 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, according to Fish of American Underwater Search & Survey, which has been involved in several efforts to find aircraft that crashed in oceans.

“That speaks volumes,” Fish said. “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.

Sonar Technology

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.

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 pinger maker Dukane Seacom, a unit of Hollywood, Florida-based Heico Corp. (HEI/A)

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.

Pinger Range

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 beacons don’t mark 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.

source: http://www.bloomberg.com/news/2014-04-07/subsea-pings-raise-hopes-in-search-for-missing-jetliner.html