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On Jan 14th 2019, at 9.10 Indonesia time, the CVR of the crashed Lion Air JT-610 has been found. Another vital device was found earlier.

There are three modern airplanes crashed in to water in Indonesia:

  1. Adam Air, registration PK-KKW, Boeing 737-400, flight KI514, on 1-January-2007, crashed into Majene Sea due to the failure with the Internal Reference System (IRS), rested in 2,000m below the sea surface.

  2. Air Asia, registration PK-AXC, Airbus A320-216, flight QZ8501, crashed on 28-December-2015, into Karimunjawa Sea, due to the failure with the rudder-travel-limiter.

  3. Lion Air, registration PK-LQP, Boeing 737 Max-8, flight JT-610, crashed on 28-November-2018, into Java Sea. The crash cause is still under investigation.

Those three crash locations were found not by the electronic instrument system, but by fishermen.

Then my question is: why did the modern airplane has not been located via its ELT?

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    $\begingroup$ Ordinary electromagnetic waves don't work under water (extra low frequencies work to some extend, and not very deep). That 's why submarines use (ultra)sonic transmissions to communicate between themselves (like whales do). Ultrasonic "pingers" are added to CVR and FDR, to locate them underwater. $\endgroup$ – mins Jan 15 '19 at 17:47
  • $\begingroup$ How does an ELT work? What does trigger it? $\endgroup$ – AirCraft Lover Jan 16 '19 at 1:42
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    $\begingroup$ @mins: And the frequencies that do go through water need antennae many kilometres in size in order to produce useful amounts of them. Not exactly something you can fit into an aircraft's ELT... $\endgroup$ – Sean Jun 6 '19 at 2:50
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    $\begingroup$ @Sean, agreed (more on this). However this is the electric length taking into account the velocity factor which counts. By selecting the proper material and using clever current-to-wave conversion techniques we can sometimes make miracles. An alternative to the plain old Gertrude. $\endgroup$ – mins Jun 6 '19 at 6:37
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ELT's are not required to be built to the same standards as the CVR and FDR. That means they are much more likely to be destroyed in a severe crash. Also, ELT's (the old style 121.5 and 243 mhz and the new 406 mhz) are not designed to function under water or mud.

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    $\begingroup$ And they really cannot be built to the same standard as the recorders, because they need antennas and there is no practical way to make an antenna that will not tear off at hundred G deceleration. It usually tears off beyond some 20–30 G. $\endgroup$ – Jan Hudec Jun 18 '19 at 5:46
  • $\begingroup$ Good point, @JanHudec. My first thought was "external antenna" and then the "D-Oh!" hit me with a great big "THUD!" LOL! $\endgroup$ – Juan Jimenez Jun 18 '19 at 9:11
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Salt water is conductive. The e-field part of the radio wave is effectively short-circuited in salt water. Radio waves attenuate very quickly when travelling through it.

This is out of date, but may lead you to more current information.
My notes from when I was involved with ELTs on commercial jet transports. https://eltrequirements.homestead.com/ https://eltrequirements.homestead.com/files/fars.doc

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