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In a recent BBC article regarding the disappearance of Malaysian Airlines Flight 370 (MH-370), the writer refers to a US Naval officer's statement:

Commander William Marks from the US Seventh Fleet, which is taking part in the search, says he expects the plane's flight recorders to be floating in the water.

"In calm seas, if there were a soccer ball [football] or a basketball floating in the water, the radar could pick it up. They [flight recorders] typically have a radio beacon and so for example our P3 [radar] - if they are flying within a certain range of that - will pick up that radio beacon. We have not yet picked up anything, but that's typically what those black boxes contain."

I was under the (potentially incorrect) impression that flight recorders, by nature of the materials needed to protect their contents in the event of a crash, are quite dense and unable to float in water. I base this impression on news reporting of other airliner crashes (such as Air France 447, which crashed in the Atlantic), where the recorders were found some time later on the ocean floor.

Additionally, it seems reasonable to me that one would want to have the recorders not float so that they would come to rest near the other debris on the ocean floor since the recorders are equipped with underwater locator beacons which would aid in locating not only the recorders themselves but also the debris of the plane. If the recorders floated then they could move significant distances from the crash site and other debris, complicating search efforts.

It seems that Commander Marks, the writer of the article, or I (or some combination thereof) misunderstand something about flight recorders and their behavior in water.

My questions are these:

  1. Do flight recorders float on water? If so, why?
  2. I know that flight recorders are equipped with underwater locator beacons. Are they also equipped with radio beacons (either an active transmitter or a some passive device like a corner reflector to make them easier to detect with radar)?

Edit: The article has been updated, with the section regarding floating data recorders now reading:

Commander William Marks from the US Seventh Fleet, which is taking part in the search, said he expected the plane's flight recorders to be floating in the water.

He said the recorders, also known as "black boxes", are fitted with radio beacons that can be picked up by radar.

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    $\begingroup$ they are painted bright orange with white stripes for easy visual detection $\endgroup$
    – ratchet freak
    Commented Mar 10, 2014 at 14:13
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    $\begingroup$ @ratchetfreak Yes, but do they float in water? $\endgroup$
    – heypete
    Commented Mar 10, 2014 at 14:26
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    $\begingroup$ It would seem to me that they should float, it is trivial to have two gps/beaconing devices and put one on the floating black box and one on the plane somewhere. That way you can easily recover the black box and still have a beaconing sunken wreckage. $\endgroup$
    – TheZ
    Commented Mar 25, 2014 at 16:31
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    $\begingroup$ @TheZ Very little in aviation can be classed as trivial - especially anything requiring wholesale changes! $\endgroup$
    – Dan
    Commented Jan 8, 2015 at 11:04

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You are correct in the assumption that the typical data recorders being used today are too heavy to float. They are designed to withstand very high forces, extreme temperatures, and to withstand corrosive environments (like sea water) for long periods of time.

For example:

A DFDR must obviously have a high degree of ‘crashworthiness.’ The first units were required to withstand a momentary shock force of 1,000gs, the latest test standards now call for a test to 3,400gs for a duration of 6.5 milliseconds. The units also have to withstand a static crushing force at all of its six axis points of an applied load force of 5,000lbs for 5 minutes on each axis. Third, it must withstand a 500lbs piercing force test conducted by dropping it onto a ¼ inch steel pin from 10 feet. Lastly it must withstand a 1,100°C fire test for 60 minutes, and a 260°C oven test for 10 hours. 

It is also required that these units are mounted within the tail area of an aircraft, away from the potential crushing force of any engines mounted nearby. The DFDR must be watertight to a depth of 20,000 feet in sea water, and survive at this depth for 30 days - and it must be fitted with an underwater locator beacon which will act like a sonar transmitter, by ‘pinging’ a signal through the medium of water that it might be laying in.

Time has an article showing the Air France 447 flight data recorders resting on the bottom of the ocean (there are others there):

FDR image 1 FDR image 2

This particular FDR is pretty common, and weighs about 13.5 pounds1.

Without seeing the original interview, I suspect that the author of the article got confused during the interview and was told that radar could detect debris the size of a soccer ball that is floating in the water and at a different time mentioned that they would be looking for the radio signals emitted by the black box (which wouldn't even be detected by radar).

UPDATE:

That being said, there are black boxes that can float and even eject from the airplane when a crash is detected. They seem to be more common in military aircraft, and there have been a few attempts to require them in the US (and even have the government pay the $60,000/airplane price) but the bills stalled in congress. The lawmakers here have recently started discussing this again though.

Deployable Black Boxes have been around since the 60's and are a second set of black boxes meant to be a backup to the main ones. They can be particularly useful when crashing into water or in the event of a major fire because they would not be within the main crash site.

Deployable Black Box Photo

Here are a couple of articles about them:

1 See specifications on Honeywell's webpage and page 7 of their product description.

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    $\begingroup$ Excellent. Thank you. I was trying to understand what reasons there might be for wanting a flight recorder to float -- it definitely seems like there was some misunderstanding with the Commander or reporter. Do you happen to have any information in regards to my second question (i.e. do flight recorders have radio beacons)? I've not found any information that says they do have radio beacons or radar retroreflectors, so I'm assuming they typically don't. $\endgroup$
    – heypete
    Commented Mar 10, 2014 at 15:57
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    $\begingroup$ Well, they are normally inside the aircraft so a passive device wouldn't really help any. As far as a radio beacon, aircraft are required to have ELT's which are radio beacons but the actual FDR's aren't. They should be relatively close together though so that's probably why! $\endgroup$
    – Lnafziger
    Commented Mar 10, 2014 at 16:03
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    $\begingroup$ @ratchetfreak - A flotation device yes, one that could survive a 500kt impact and, possibly, ensuing fire...not terribly easy. Granted, perhaps it could be worth it... $\endgroup$
    – Jae Carr
    Commented Mar 10, 2014 at 16:54
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    $\begingroup$ @Lnafziger Why not have 2 flight recorders, one floatable and another not? Also isn't 1000g enough? can a plane accelerate > 1000g? $\endgroup$
    – user13107
    Commented Mar 11, 2014 at 1:42
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    $\begingroup$ @user13107 Actually, the standard used to be 1000g and they raised it to 3400g because it wasn't enough. Like I said in my first comment, I know that water landings have been considered but apparently the cost -vs- benefit ratio is not good enough for them to require it. Even Air France 447's FDR was recovered, and that's pretty close to a "worst case scenario". $\endgroup$
    – Lnafziger
    Commented Mar 11, 2014 at 1:49

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