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If CVR/FDR cannot be seen visually on the seafloor (because covered by a thick layer of mud/sediments) and no pinger signals are detected, how can they be found? Metal detectors? Any other methods?

case in point: The FDR of crashed Lion Air flight 610 was recovered on 1 Nov 2018, but 2 months later now the CVR still has not been found (while the pinger batteries are rated for 30 days only):

« The search for the CVR is hampered by thick mud. The signal source is difficult to ascertain its position considering the sea floor is mud with a depth of more than 1 meter. The ping signal from the CVR has not been received for 2 days now (5 Nov). There are other means to find the CVR however. » (source)

Which "other means" are this ?



update 18 Dec 2018: Lion Air contracts MCS to locate and recover the CVR of the 737MAX.

update 21 Dec 2018: an expert answered the question for me. I'll paraphrase his explanation as an answer below.

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    $\begingroup$ Or it may be attached to a larger piece of wreckage that they bring up. Once they start bringing up large sections, they can figure out the approximate location of the CVR in relation to the parts they recover. This will narrow the search area and then they can dredge to find the box if it is not part of the larger wreckage. $\endgroup$ – Ron Beyer Nov 29 '18 at 14:48
  • $\begingroup$ @RodrigodeAzevedo: My question was not how to find the haystack (huge debris field) but the needle (one specific piece) in the haystack. I hope you see the difference. Also the Titanic wreckage was easily visible on sonar images. You can find an entire debris field by ensonification, yes, but not a single piece as small as a CVR (let alone if covered by mud). $\endgroup$ – summerrain Dec 9 '18 at 19:08
  • $\begingroup$ How do you find a flight recorder underwater if it isn't transmitting? With great difficulty. $\endgroup$ – Sean Jan 22 at 4:24
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The CVR on a Boeing 737 is located in the aft cargo compartment. The CVR itself is not built of the same materials as the aircraft, which is mostly aluminium and composites. By mapping the location of other components of the aircraft found in the sea bottom it should be possible to make an educated guess of the general location of the CVR (assuming the aircraft did not disintegrate, sending everything in different directions). One common way of tracking down the CVR is by using an underwater magnetometer to detect ferrous metal objects. The problem is that something as small as a CVR will only be detectable up to about 20' under the ocean floor. If it is buried deeper than that it will be difficult to find. That's why they have pingers, but those stop working when the battery runs out.

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  • $\begingroup$ re: "it should be possible to make an educated guess of the general location of the CVR": If you look at the quote above and click on its link, you will see that the FDR of crashed Lion Air flight 610 was recovered on 1 Nov 2018, but the CVR still has not been found yet. / re: "If it is buried deeper than that it will be difficult to find." --> hence my question $\endgroup$ – summerrain Dec 9 '18 at 18:47
  • $\begingroup$ re: "That's why they have pingers" : please note that the question was about cases WITHOUT pinger signal (malfunction, dead batteries, signal not detectable because CVR is covered by aircraft debris, etc.). From the same source: "The locator ping signal, believed to originate from the CVR, had died down, possibly the CVR is covered by mud". Although counter-intuitive, the pinger location is difficult to detect in shallow water. $\endgroup$ – summerrain Dec 9 '18 at 18:50
  • $\begingroup$ In re: the educated guess, my answer explains itself. As to the lack of pinger signal, I read the question, I am a pilot and have worked for major airlines, I know what i am talking about and I know what you meant. The pinger is not difficult to detect in water. The sound of the pinger carries for a great distance. But sound does not carry well in mud. $\endgroup$ – Juan Jimenez Dec 10 '18 at 10:53
  • $\begingroup$ To clarify, what happens when the CVR is buried is that the ping can only be heard if you are very close to the source. That's why traditional listening methods when searching for the CVR don't work if the CVR is buried. $\endgroup$ – Juan Jimenez Dec 11 '18 at 15:32
  • $\begingroup$ I repeat, this question is about the case where NO pinger signal is emitted AND/OR detected! This always happens after a couple of weeks when the pinger batteries have died and can also happen already on day 1 for a multitude of reasons. $\endgroup$ – summerrain Dec 11 '18 at 17:46
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I don't know the details, but since there is still no good answer, I'll wager an educated guess.

Metal detectors do work under water, but I believe sonar can also scan through a layer of mud and can work from larger distance, so it can scan the debris field more effectively.

Then the mud will be sucked or blown away and the items identified visually. Usually this is done using an remotely operated submersible vehicle.

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  • $\begingroup$ This isn't an answer and would be best posted as a comment... $\endgroup$ – Juan Jimenez Dec 3 '18 at 14:24
  • $\begingroup$ And it would more likely be done using a remote-control submersible vehicle rather than an autonomous one, wouldn't it? $\endgroup$ – Transistor Dec 8 '18 at 22:50
  • $\begingroup$ @Transistor, it would only be “autonomous” in the sense it does not need to be attached to the ship by a cable; of course there has to be operator watching the camera and sonar images. And in shallow waters like this it does not really need to be autonomous at all; it may well be on a cable. $\endgroup$ – Jan Hudec Dec 9 '18 at 13:17
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    $\begingroup$ @JanHudec: these are technical terms. There are ROVs (remotely operated (underwater) vehicles) and AUVs (autonomous underwater vehicles) $\endgroup$ – summerrain Dec 9 '18 at 18:32
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When the underwater wreckage site has already been found, but CVR/FDR are still missing, at first hydrophones are use to listen for the black box acoustic pinger signals. Absent these signals (or when they have stopped emitting after the batteries have died down), in the second stage another method is used: Sub-bottom Profiling

"High Penetration Sub-bottom Profiler":
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"The systems generate high-resolution images of the sub-bottom stratigraphy in oceans, lakes, and rivers." (source)

"Sub-bottom profilers work by transmitting sound energy in the form of a short pulse towards the seabed. This sound energy is reflected from the seabed and the sub-surface sediment layers. The reflected energy intensity depends on the different densities of the sediments, the denser (harder) the sediments, the stronger the reflected signal. The reflected signal then travels back through the water to the receiver (either a towed hydrophone or transducer). The received signals are then amplified, processed and displayed in the acquisition system." (source)

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Deployment of Various Shallow-Water Sub-bottom Profiling Systems

In the case of JT610 a ROV is also used:

« The recovery team is set to utilize an ROV equipped with the needed features to search for the missing CVR that is believed to be buried under the seabed. “The device that we prepared is able to detect objects buried 4-meters under the bottom of the ocean.” » (source)

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    $\begingroup$ FYI, setting up questions that you already know the answers to, then providing your own answer while arguing with sincere responses, makes you come across as full of yourself and a know-it all. This isn't the first time you have done this either. Just my humble opinion... $\endgroup$ – Michael Hall Jan 8 at 18:57

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