Honeywell, who made MH370's pinger, say the signal can typically only be picked up a mile away. David Mearns, director of Blue Water Recoveries, says this means aircraft really needs to be almost directly on top of the black boxes to hear the ping. There are also factors that may diminish the signal. Wreckage on the surface, or if the black boxes are submerged in mud or sediment on the seabed, he adds. But if the pinger is deep on the ocean floor navies have hydrophone technology that has a better chance of locating it than conventional detectors. The Air France black box was not found until after its ping had expired. It was eventually located by slow moving unmanned underwater vehicles. A modern submarine - such as one of the Royal Navy's hunter-killer models - could potentially at least hear a ping from many miles away, Gratton says. The US, China and Australia all have similar submarines, he says. "By now there will be a submarine down there. I'm certain the Chinese will have put something out there." The US has deployed a ship that will tow a special black box locator through the water. According to the Associated Press, "the Towed Pinger Locator, which is pulled behind a vessel at slow speeds, has highly sensitive listening capability so that if the wreck site is located, it can hear the black box pinger down to a depth of about 20,000ft (6,100m)". However, there is a further complication, says Barry. The black box may be giving off pings from the ocean floor. But if those pings hit a layer of warmer or colder water above, the signal might be refracted or reflected.
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