The pinger locator is designed to detect signals at a range of 1.8 kilometres, indicating it would need to be almost on top of the black boxes - the flight data and voice recorders - to detect them if they were on the ocean floor, which is 4.5 kilometres under the surface. However, the latest detections indicate the pinger locator may be effective at a longer range than designed. The first signal from the black boxes was picked up on Saturday and lasted two hours and 20 minutes before it was lost as the ship moved forward. The ship then turned around and a few hours later picked up a second signal that lasted for 13 minutes. It picked up signals twice again on Tuesday. The signals The Tuesday signals lasted 5½ minutes and 7 minutes but they were weaker, indicating that the black boxes may be running out of battery. They have a stated shelf-life of 30 days, but sometimes they last longer. The plane disappeared just over a month ago, on March 8. The signals have given searchers a better idea of the location of the devices, which are now believed to be within a roughly 20-kilometre radius. Still, that is a 1300-square-kilometre plot of the ocean floor, an area as wide as a large city. Why not send an underwater craft now?

When crews determine the best possible location, the next step will be to send down the US Navy's autonomous underwater vehicle Bluefin 21, an unmanned submersible that can create a sonar map of the sea floor and any wreckage, as well as take photos. But the sonar can scan only about 100 metres. As for its ability to take photos, it can see with lights and cameras only a few metres away in a landscape that is completely dark. So, even after the search area has been narrowed down, deploying the underwater vehicle now to find the black boxes would be the equivalent of looking for an object the size of a desktop computer in a city the size of Los Angeles. Indeed, the Bluefin 21 would take six times longer to cover the same area than the towed pinger locator. So it makes more sense to use the towed pinger locator to zero in on the location of the pinger, or at least try until searchers become certain the batteries of the black boxes are dead and are no longer emitting any signals. Then searchers would have no option but to use the Bluefin 21.

Can a conventional submarine not help? No. Militaries will not disclose the depth to which their submarines will go down because it is classified information. But it is believed some of the most sophisticated nuclear powered submarines can dive only a few hundred metres down. AP