PERTH, Australia (AP) - Search teams racing against time to find the flight recorders from the missing Malaysia Airlines jet crisscrossed another patch of the Indian Ocean on Saturday, four weeks to the day after the airliner carrying 239 people vanished.
Two ships, the Australian navy's Ocean Shield and the British HMS Echo, carrying sophisticated equipment that can hear the recorders' pings, returned to an area investigators hope is close to where the plane went down. They concede the area they have identified is a best guess.
So far, there's been no sign of the Boeing 777.
Up to 13 military and civilian planes and nine other ships were also taking part in the search.
Two pieces of Navy equipment were added to the underwater search for the missing Malaysia Airlines jet Friday, one for sound and one for visual images.
Search experts warned that the equipment moves slowly and covers a small area. Searching hundreds of thousands of square miles of Indian Ocean for wreckage from Flight 370, which has been missing since March 8, remains a difficult task.
'These are very slow, with a very limited range,' said Alan Diehl, a former investigator with the National Transportation Safety Board. 'It's been compared to looking for an object on a football field by viewing it through a soda straw.'
Despite the challenges, the search is urgent. The jet's data and voice recorders are certified to send out signals for 30 days after coming into contact with water, although they can last a couple of weeks longer, depending on conditions.
Officials have said the hunt for the wreckage is among the hardest ever undertaken, and will get much harder still if the beacons fall silent before they are found.
'Where we're at right now, four weeks since this plane disappeared, we're much, much closer,' said aviation expert Geoffrey Thomas, the editor-in-chief of AirlineRatings.com. 'But, frustratingly, we're still miles away from finding it. We need to find some piece of debris on the water, we need to pick up the ping.'
The search will get more complicated if the signal beacons fall silent.
The so-called pingers on the jet's recorders send a signal that can be picked up by sonar within 2 miles, although the distance is diminished by uneven topography on the ocean floor or if muffled by the jet's wreckage
Search planes have dropped buoys to listen for a signal. If the ocean is more than 2 miles deep at the spot where the buoys float, they could miss a signal even if they were right above the wreckage.
The advantage the two types of Navy technology provide is that they can swim deeper.
The towed pinger locator, called a TPL-25, is lowered by cable behind a ship and kept about 1,000 feet above the ocean floor, according to the Navy. The locator can hear a signal about 1 mile away and to a depth of 20,000 feet.
The ship towing the locator can generally move 3 to 6 mph to search about 150 square miles per day, the Navy said. Search planes coordinated by Australia have been covering 100,000 square miles, an area as large as Oregon.
Another piece of Navy equipment is the Bluefin-21, an unmanned underwater vehicle 16 feet long and 21 inches in diameter, weighing 1,650 pounds. It is certified to go 14,763 feet deep.
The submersible can travel up to 5 mph along a path the technicians program, in order to scan the ocean floor with sonar and camera. The Navy hopes to scan about 40 square miles per day with the submersible.
The locators work optimally in a small search area, Marine Lt. Col. Jeff Pool, a Pentagon spokesman, said Friday. They work slowly and in tandem: The pinger locator detects the target; and the small, unmanned submarine maps the seafloor, Pool said.
The pinger locator and the Bluefin-21 are hindered by bad weather on the surface. The pinger locator helped find a Marine AV-8B Harrier that crashed in the Gulf of Oman in 2011. The Bluefin-21 helped recover an Air Force F-15 off the coast of Japan.
Diehl said it is important to find a signal from the Malaysia jet before its recorders stop sending their emergency transmissions in the next week or two.
'It's an extremely large ocean. The chances of finding anything once the pingers die is going to be unlikely,' Diehl said. 'This is the kind of thing where you can put away the stopwatch and get out the calendar because it could be months or years.'
Australia is coordinating the ocean search, and the investigation into the plane's disappearance is Malaysia's responsibility. Australia, the U.S., Britain and China have all agreed to be 'accredited representatives' of the investigation.
On Saturday, Southeast Asia's top budget carrier AirAsia withdrew its latest inflight magazine and apologized for an offending article boasting that its well-trained pilots would never lose a plane.
AirAsia Executive Chairman Kamarudin Meranun expressed 'deep regret and remorse,' saying the magazine was printed before the Malaysia Airlines plane disappeared.
Kamarudin says disciplinary action would be taken against the editorial team for the oversight.
The article sparked anger on social media after an AirAsia passenger posted a photograph of the text on Twitter late Friday. The last paragraph read: 'Pilot training in AirAsia is continuous and very thorough. Rest assured that your captain is well prepared to ensure your plane will never get lost.'
Meanwhile, the Air Line Pilots Association, a union that represents 30,000 pilots in North America, said in a statement that the Malaysia Airlines tragedy should lead to higher standards of plane tracking technology being adopted by the airline industry.
Contributing: Tom Vanden Brook; Associated Press
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