When a plane crashes in the water and the investigator team looks for the FDR and CVR, why do they put it in a container of water when they find it ?
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2 Answers
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This is actually a fairly standard thing to do with electronics that have been submerged: They are placed in water (ideally fresh, clean water) to both delay the onset of corrosion and dilute any salts or other chemicals that they came in contact with while submerged.
When you remove electronics from water and let them dry out they begin to form corrosion on all the little exposed bits of metal - chip legs, solder joints, capacitors, etc. - which can badly damage the device when it is powered back on.
Keeping the device submerged in water delays the onset of this corrosion because the dissolved oxygen in the water is lower than the oxygen in the air that would be hitting those parts as they dry out.
The Cockpit Voice Recorder and Flight Data Recorder are water resistant, but they are not necessarily waterproof (particularly after a crash, which may puncture their casings or damage seals), so they get placed in a container of fresh water for their trip to the NTSB's lab.
When the still-immersed equipment arrives at the NTSB laboratory it can be examined and dried in a controlled environment, maximizing the chance to recover data.
For more gory details check out The NTSB Flight Data Recorder Handbook for Aviation Accident Investigations.
Bonus Factoid: Archaeological recoveries from bodies of water use similar procedures, for similar reasons: If you bring up a clay pot that's been sitting at the bottom of a lake for a thousand years and let it air-dry naturally there's a good chance it will crack and be destroyed.
In archaeological recoveries the usual practice is to keep the item "waterlogged, ideally in waters associated with the original context" (i.e. "in a bucket of water from where you got the thing").
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After being sunk in water, especially salt water, the devices are more susceptible to corrosion when removed from the water. In order to better ensure a successful recovery of the data contained within, the investigators keep them immersed in water until they can be properly cleaned and dried in a laboratory. If they were to be simply removed into the air, corrosion would begin immediately, increasing the odds of data loss.
answered Jun 21, 2014 at 1:18
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1$\begingroup$ Is that just because of the damp surfaces now being exposed to air? Otherwise, you seem to be saying that the lump of metal "knows" that it used to be in water so it somehow corrodes more quickly. $\endgroup$ Commented Feb 26, 2015 at 15:03
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4$\begingroup$ @DavidRicherby: Yes. Corrosion (at least, the kind we're worried about here) requires both oxygen (usually, as in this case, coming from air) and water (with saltwater, such as from the sea, promoting corrosion more aggressively than fresh water; pure, distilled water is virtually noncorrosive). Before a crash, the recorders are immersed in oxygen (from the air), but are exposed to either no or very little water (depending on the ambient humidity); hence, they corrode slowly, if at all. After a crash into the ocean, they are immersed in saltwater, but exposed to only a little oxygen... $\endgroup$– VikkiCommented Nov 22, 2018 at 1:36
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3$\begingroup$ ...(the exact amount depending on how much air is dissolved in the water), so, although they do corrode, they only do so slowly. However, if the recorders are taken out of the water and left out of it, they will now be simultaneously wet with a still-considerable amount of saltwater (which will become much more concentrated, and, hence, more corrosive, as it evaporates) and bathed in oxygen-containing air. Under these conditions, with ample quantities of both saltwater and oxygen, corrosion rapidly ensues. $\endgroup$– VikkiCommented Nov 22, 2018 at 1:41