- Batteries powering the pingers on Flight 370 are expected to die in the coming days
- Pingers emit sounds that can direct searchers to flight data and cockpit voice recorders
- A pinger locator moves slowly -- at about 2 to 3 knots
- Searchers also have an autonomous vehicle that can search the ocean bed
They are inaudible to humans, but they would be sweet music to searchers hoping to find clues to the fate of Malaysia Airlines Flight 370.
They're the sounds believed to be emanating from the underwater locator beacons -- known as pingers -- that were attached to the jet's cockpit voice recorder and flight data recorder.
But the clock's ticking: Their batteries are not guaranteed to work for more than 30 days, and Friday marks day 28.
And they may not even make it that long: The recorders had been scheduled for battery replacements in 2012, but they were never returned for the overhaul, the manufacturer told CNN on Friday.
That revelation leaves three possibilities, said Anish Patel, president of beacon manufacturer Dukane Seacom of Sarasota, Florida:
• Malaysia Airlines could have replaced the old pingers with new ones;
• The airline could have hired another company to perform the necessary maintenance;
• It could have done nothing.
If it chose the third, and the original batteries were still driving the pingers when the Boeing 777-200ER disappeared from radar screens, their life probably would have dropped from 30 days to 25 or 20 days, Patel said.
The pingers would not die immediately but would continue to emit signals with "progressively lower output levels until the unit shuts down," he said.
Malaysia Airlines did not respond to a question from CNN about the devices. But the airline said in an e-mail last week, "We are unaware of any issue with the ULB (underwater locator beacon) or its batteries."
"This battery is not replaceable," the airline said. "The battery is built-in inside the (pinger) and installed by OEM -- Original Equipment Manufacturer."
On Saturday, the Malaysia Airlines CEO said the acoustic pinger batteries on the airlines' black boxes were due for replacement in June 2014.
"We can confirm there is a maintenance program. Batteries are replaced prior to expiration," Ahmad Jauhari Yahya said.
What happens if they sputter out? Is there any hope left of finding the jet that was carrying 239 people when it disappeared from view on March 8?
What are pingers, and how do you find them?
All commercial airplanes are required to carry pingers -- underwater locator beacons -- to help investigators find them should they crash into water. One is attached to the flight data recorder, another to the cockpit voice recorder.
The pings sound about once per second and can be detected from 2 nautical miles away by towed pinger locators, or TPLs.
The pinger locator is equipped with a sensor that looks like a 35-inch, 70-pound yellow stingray. It can recognize the flight recorder's chirps up to 20,000 feet below the water surface.
An Australian ship is dragging a TPL in the Indian Ocean that is on loan from the United States.
What are the challenges of hunting for the plane by pings?
Not only will the batteries powering the pinger die after about 30 to 45 days, but their sound can be obscured by weather, noise or silt.
And pinger locators must be towed slowly -- it could take days to cover the 150-mile (240-kilometer) track that officials have identified as the latest, best guess for where the plane might be.
"It is a very slow proceeding search, 2 to 3 knots depending on the depth that you want the hydrophone, that tow pinger locator trailed at," said Capt. Mark M. Matthews, the U.S. Navy's head of TPL operations. "It's going to take time. ... Again, we're searching on what information we do have, our best guess at where it would have been lost. It's the best we can do at this time."
So is all hope lost in finding a plane after the pinger dies?
No. Take, for example, Air France Flight 447, which disappeared in 2009 as it was flying from Rio de Janeiro to Paris. A towed pinger locator looked -- without success.
But two years later, searchers using an autonomous underwater vehicle found the flight data recorder and the bulk of the wreckage hundreds of miles off Brazil.
What other high-tech gadgets can searchers use to find Malaysia Airlines Flight 370?
An Australian search ship has an autonomous underwater vehicle called the Bluefin-21, which can scour the ocean bed looking for signs of wreckage.
But the AUV, on loan from the United States, would be deployed only if the searchers get a clear fix on the beacons sending out the pings, Matthews said.
AUVs are typically used in the oil and gas industry to conduct deep-water oilfield surveys.
"The smaller ones are only going to go down to about 5,000 feet," analyst David Soucie said. "The next class is a much more expensive, much larger device. It's 15 by 25 feet because it adds a lot of battery capability and a lot of hydraulic capability."
One of the most sophisticated AUVs owned by Phoenix International was activated and flown to Perth, Australia, to help with the search for Flight 370. The device is yellow, 17.2 feet long and has an in-air weight of 1,600 pounds.
It can search 20,000 feet below the water surface and travel 2 to 4.5 knots, using side-scan sonar to map the seafloor. Its probe, equipped with a still camera, moves rapidly.
"A picture will scroll, and you will see the seafloor be painted in front of you," said Jami Cheramie, vice president of systems development and IT support for C&C Technologies Inc., whose AUV has been used to search for plane debris in the past.
Have these underwater vehicles found plane wrecks in the past?
Yes. AUVs helped find the downed Air France flight, the wreckage of the plane that was carrying Italian fashion designer Vittorio Missoni when it disappeared last year off Venezuela, and the HMS Ark Royal, a ship sunk by a German U-81 submarine in World War II. The AUV provided black-and-white images of the wreckage site.
Will the mystery of Flight 370 be solved once the data recorders are found?
Not necessarily. The voice recorders retain only the last two hours of recordings. And, since officials believe Flight 370 flew almost seven hours beyond the point where something went terribly wrong, crucial data have almost certainly been erased.
On the positive side, the depletion of the battery will not wipe out data. Data has been known to survive years in harsh sea water conditions on modern recorders.