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I was surprised that flight recorders only have enough battery to send out signals for 30 days. I'm guessing that it blindly sends out signals that help it to be located. But instead, why aren't they made to only send out a signal when they receive a ping? Surely, then, they could last for years?

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  • $\begingroup$ How do you propose to power a receiver that is sensitive enough to "detect a ping" under several thousand meters of water "for years". With a battery, maybe? ... ;) $\endgroup$
    – alephzero
    Sep 6 '16 at 14:48
  • $\begingroup$ @alephzero Receiving needs very little power. Surely it's more amazing how they can be located in the first place, given their limited battery to transmit $\endgroup$
    – CL22
    Sep 6 '16 at 14:51
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Think about it a bit: you'd need a monstrous piece of gear to be able to receive a ping from a distant source on a search platform. Transmitters are not the same as receivers.
The primary design goal of the flight recorder assembly is crash survival. Further, after 30 days, there's pretty much no chance of finding survivors (with the notable exception of the Andes 1972 crash . The world's a big place. Aircraft occasionally disappear and are found years later pretty much by blind luck. For that matter, cargo ships sink all the time and even given their bulk and the theoretical ability to put a very large, long-lived beacon on them, they're not found either.

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  • $\begingroup$ "For that matter, cargo ships sink all the time..." Actually very few ships are now "lost at sea" compared with the number of sinkings and other accidents. This is not really surprising, since all cargo ships of more than 300 tons gross tonnage (and all passenger ships of any size) must be fitted with the AIS automatic tracking system - which also tells the ship's own crew where they are and what other ships are close enough to be a safety hazard. en.wikipedia.org/wiki/Automatic_identification_system. Of course AIS can be deliberately switched off - but that's a different issue. $\endgroup$
    – alephzero
    Sep 6 '16 at 14:44
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    $\begingroup$ I disagree: you say it needs monstrous gear to receive a ping. The ping it receives could be sent from a search vessel at a high power. Receiving needs little power or size. The difficulty is surely getting the box to transmit at a high enough power. An aside: I read they are now planning to extend it to 90 days given Malaysia 370. While lives may not be saved immediately, it will help to improve future safety knowing what went wrong $\endgroup$
    – CL22
    Sep 6 '16 at 14:50
  • $\begingroup$ @Jodes making the receiver survive the crash is also a problem all on its own. $\endgroup$ Sep 6 '16 at 20:14
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An ELT or Emergency Locator Transmitter is built to be a reliable emergency device to locate a wrecked aircraft relatively quickly by satellite and ground triangulation. The goal of locating the aircraft is to save human lives; so 30 days is usually sufficient. Finding remains and satisfying people's curiosity is not part of the current design criteria.

Transmitting on an interval is the most reliable method, as it starts transmitting on the emergency frequency as soon as a plane goes down; not when someone decides to start looking for it. Satellites and ground towers are always listening on this frequency and will triangulate the signal as quickly as possible. Think of it like your house is on fire and you are trapped inside. You would yell "help, help, help" loudly and continuously instead of listening at the door for bystanders before you yell.

To extend the amount of time an ELT signal lasts could be done by:

  1. Adding a larger battery (obviously)
  2. After the 30 days go into a "Marco Polo" mode, waiting to receive a signal before transmitting like you mentioned. Remember it still takes some power to listen.
  3. After the 30 days decrease how often the signal is transmitted logarithmically. If the ELT synchronized with the aircraft clock it could emit signals at predictable times so that search personnel would be ready to listen. This would allow for very very low power use during the waiting period.
  4. Adding water pressure sensing capability to the unit so that it only sends sonar pulses after being submerged to a certain depth.
  5. Adding multiple directional antennas instead of an omnidirectional antenna. Sensors in the unit could fire the antennas that are not pointed at the ground increasing signal strength and reducing power consumption.
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  • $\begingroup$ I'm not sure I agree with the statement "satisfying people's curiosity is not part of the current design criteria." I think the purpose of flight data recorders and cockpit voice recorders is to satisfy our curiosity about why the plane crashed, so changes can be made to prevent it from happening again. $\endgroup$
    – Mark
    Sep 7 '16 at 13:33

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