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The news keeps talking about the black boxes on aircraft that end up in water, and one thing that keeps coming up is the underwater locator beacon that transmits a signal for 30 days if it gets wet.

What kind of signal does it transmit and what kind of range does it have? I'm guessing that the depth of water that it is in will affect how far away that it can be detected, but by how much?

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This excellent explanation is taken from Avionics News in 2006:

FDRs are equipped with an Underwater Locator Beacon (ULB). If you look at a picture of an FDR, you will almost always see a small, cylindrical object attached to one end of the device.

If a plane crashes into the water, this beacon sends out an ultrasonic pulse that cannot be heard by human ears but is readily detectable by sonar and acoustical locating equipment.

There is a submergence sensor on the side of the beacon that looks like a bull's-eye. When water touches this sensor, it activates the beacon.

The beacon sends out pulses at 37.5 kilohertz (kHz) and can transmit sound as deep as 14,000 feet (4,267 m). Once the beacon begins "pinging", it pings once per second for 30 days.

Newer FDRs can now transmit as deep as 20,000 ft.

The following table is taken from a fascinatingly detailed article on Hydro International about the retrieval of black boxes. It shows the radius you can pick up the signal of a pinger from:

Notice that if pingers were replaced with more powerful transponders, the range could be drastically improved!

The article also points out, that in very deep water:

under normal conditions, the existing pinger would not be detectable from the surface in depths exceeding 2km

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    $\begingroup$ I imagine that the performance characteristics of their underwater passive sonar is something that most Navies are not inclined to discuss openly. In deep water it will also be highly dependent on background noise, halo and thermo-clines the topography etc $\endgroup$
    – NobodySpecial
    Apr 6, 2014 at 16:09
  • $\begingroup$ I wonder if the pinger could send one pulse per second for 15 days, one every two seconds for another 15 days, then one every four for 15 days, one every eight for 15, etc., or if factors other than energy capacity are responsible for the 30-day limit? $\endgroup$
    – supercat
    Jan 19, 2015 at 16:35
  • $\begingroup$ You says it shows the "radius" in which we can pick up the signal, but then the figure states it's the max "depth"... So is it the radius at the surface of the water, or the depth distance below the surface ?? $\endgroup$
    – Matt
    Nov 23, 2016 at 16:52
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3km..just one mile away from the source. They must be right over it, if they heard the ping for 2 hours 20 minutes. But the Chinese guys with the more primitive gear...what were they hearing...maybe the other recorder.

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    $\begingroup$ The Haixun 01 and the Ocean Shield are 300 nautical miles apart so it's hard to see how the signals they each detected could be from CVR & FDR from the same aircraft. Also The Ocean Shield is reported to have detected two independent signals in the same vicinity. $\endgroup$
    – RedGrittyBrick
    Apr 7, 2014 at 8:04
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Sound waves travel through water for incredible distances. For example world war two mines were still to be heard many hundreds of miles from the original source of the explosion and more interesting, they were heard several years after the event that caused the sound wave, so it is probable that it was the sound waves of pings rather than the actual ping that has been detected.

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I think it does -- per the table in Danny Beckett's answer you have a maximum of 7km range. If the black box is 4km deep in the water, you would have to be no more than 3km from it to hear it.

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    $\begingroup$ If you're 4 km up and 3 km to the side, that's only 5 km straight-line distance. With your numbers the surface distance would be more like 5¾ km. $\endgroup$
    – hmakholm left over Monica
    Apr 6, 2014 at 15:33
  • $\begingroup$ Per the table, a pinger is only detectable within a max range of 5km in good conditions. $\endgroup$
    – Danny Beckett
    Apr 6, 2014 at 16:10

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