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Cockpit Voice Recorder and Flight Data Recorder's "Underwater locator beacons" only send their position when underwater.

Why don't they do so when not underwater?

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    $\begingroup$ Yet another change-to-come. $\endgroup$ – CGCampbell Mar 31 '15 at 18:24
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    $\begingroup$ If it is a change to come, I wonder why they made it is this way in the first place. It must have some reason. I mean, detecting whether it is underwater, perhaps by measuring the pressure, isn't for free, and it seems to me they could easily have omitted it and got an even better findable blackbox. I'm confused. $\endgroup$ – Johannes Schaub - litb Mar 31 '15 at 18:40
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    $\begingroup$ The black boxes themselves don't send their position, the ELT does and it is a separate box which is used to find the aircraft, not the black box. $\endgroup$ – Lnafziger Mar 31 '15 at 18:53
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    $\begingroup$ The underwater pinger is acoustic on ultra sonic frequencies. That doesn't travel very far in air. They are activated by a water contact switch, not on pressure. $\endgroup$ – DeltaLima Mar 31 '15 at 18:59
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    $\begingroup$ Related, but not quite a duplicate: Why don't black boxes have their own GPS? $\endgroup$ – voretaq7 Mar 31 '15 at 19:08
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The short answer is that when a plane crashes on land we (generally) have a pretty good idea where to look for the black boxes - there will usually be a relatively clearly defined debris field to search.

When a plane goes down in the water there's a lot more ocean to search, and currents can deposit parts of the airframe over a much wider area, so the underwater locator beacon ("pinger") helps guide people to the debris field.


The longer answer is that sending a signal when you are underwater is relatively easy. The pinger has a water switch: when it gets wet the water closes an electric circuit and the pinger makes a noise that can be heard on standard passive SONAR gear.
Water is pretty good at conducting sound waves, so the pings can be heard over a large distance and can guide searchers to the black box (or at least the pinger: sometimes they get ripped off, but they're usually still near the debris field).
We get all of that functionality in a fairly small self-contained device that requires effectively no maintenance (you replace the unit when its battery expires).

Sending a signal when you're NOT under water is harder: you can activate the signaling device with a "G Switch" the same way the ELT is activated, but after that the problems get harder:

  • You can't use sound (effectively)
    A "pinger" beeping in the air is much less effective than under water - the range would be extremely limited even at very high volumes. Add to that the fact that crash scenes are pretty noisy with firefighting and rescue crews working and the chances of anyone hearing an acoustic beacon on the surface are pretty slim.

  • You can't use light (effectively)
    A strobe on the black box may work OK in the dark, but it's easily damaged and not terribly effective during the day, or if the recorder is upside-down, in dense foliage, under snow/mud, etc.

  • Radio locator beacons are problematic
    Radio locator beacons require an antenna - if the antenna is damaged or torn off in the crash the effective range of a radio beacon would be drastically reduced, possibly to the point of being completely ineffective.
    If the antenna remains attached the signal can still be reduced drastically if the box is buried under natural features (dirt) or aircraft debris (aluminum skin).
    (Both of these issues are frequently raised in criticism of ELT technology.)


It's possible a locator system could be devised that works on the surface, but given the challenges and the fact that the location of the black box is generally known (to within the size of the debris field at least) when an aircraft crashes on the surface there hasn't been much effort invested in this.

Finally, remember that while the recorders are a very valuable part of the investigation process remember that from an investigation standpoint it doesn't matter if you find the black boxes on the first day or the last day of recovery operations: Finding the recorders will not undo the crash or bring the dead back to life, so if it takes a little longer to find them while sorting through a debris field on the surface it's really not a major problem.

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  • $\begingroup$ It feels like you're ignoring accidents like Varig 254 in which we had almost no idea where the plane might be, as well as the fact that most ELTs also transmit on 406 MHz in order to tell investigators roughly where the crash site is. $\endgroup$ – raptortech97 Mar 31 '15 at 20:40
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    $\begingroup$ @raptortech97 There are several examples like that (Flight 404, searching for Fosset's plane etc.), but voretaq did specify that generally speaking, debris of land crashes are relatively on a much smaller area than crashes in water, where pieces of airplane can travel 100s of miles from the original point of contact. $\endgroup$ – Farhan Mar 31 '15 at 20:50
  • $\begingroup$ @raptortech97 It probably feels that way because I am ignoring those cases where investigators had trouble finding the debris field. That's mainly because usually when a plane hits the ground we can find it pretty easily, and because there's already a technology specifically designed to help you find the debris field on land (the ELT ). You could build an ELT into each flight recorder for redundancy (or put a dozen ELTs on the plane in the hopes that at least one will remain operable after a crash) but you'd still have all the problems listed above - just spread over more devices. $\endgroup$ – voretaq7 Apr 1 '15 at 20:51
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The premise is inaccurate. Modern aircraft do output a radio signal when they crash in order to guide search and rescue teams when the crash is over land.

Specifically, (some? most?) modern aircraft carry ELTs, which activate after a crash and use a GPS unit (accurate to about 100m) to determine their location. They then transmit this information on the 406 MHz band, which is monitored by satellites.

When a signal is picked up, search and rescue teams will be notified and provided with a location accurate to about 100 meters. This allows rescue teams to find survivors quite quickly after a crash, which is vital for survival.

Note that, as voretaq said, it's not important to find the black boxes very quickly after a crash. Whether it takes 1 day or 10, the investigation will take far longer anyways. The reason we have ELTs is to find survivors. If they happen to be useful for locating the black boxes afterwards, that's a plus.

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