Flight data recorders store massive amounts of data per flight and despite improvements year on year with data storage as with Moore's law, carrying larger and larger amounts of data can become expensive, through larger hard drives and the extra weight.

So, is a flight data recorder erased after every flight? If so, after how long and by whom, is it automatic on landing, engine shutdown or it's one of the pilots procedures on landing?

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    $\begingroup$ "larger and larger amounts of data can become expensive, through larger hard drives and the extra weight." Patently false. Standard 128GB microSD flash cards are cheap, have massive storage, and weigh nothing. $\endgroup$ – abelenky Apr 9 at 11:54
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    $\begingroup$ I don’t feel Moore‘s law is applicable here, as cost would be prohibitive when attempting to certify aviation hardware (which has completely different reliability, environmental and robustness requirements) at the same rate as consumer hardware is developed. $\endgroup$ – Cpt Reynolds Apr 9 at 18:01
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    $\begingroup$ @Aron I think the implication is that if we can fit 128GB on a thumbnail sized chip, we don't need to worry about "size and weight" of additional storage. Just harden your solid state storage and you're done. You're not adding additional drives or making a bigger tape these days. $\endgroup$ – JPhi1618 Apr 9 at 19:52
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    $\begingroup$ @CptReynolds exponential divided by constant factor is still exponential. $\endgroup$ – Acccumulation Apr 9 at 20:53
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    $\begingroup$ @Aron Nor does any hard drive, so that's a silly thing to say. The reason these things survive such extreme conditions is entirely down to the casing. And in point of fact, SSDs are way more robust to impact and temperatures than any hard drive. $\endgroup$ – Graham Apr 9 at 23:19

Flight data recorders have changed over the years and evolved to what they are today. Modern recorders are recording up to 88 parameters. The storage of the recorder is dependent on the range of the aircraft. Early recorders were using a circular tape that ran through the recorder. E.g. loop recording all the data and automatically overwriting the oldest data on the tape.

The recorder of a Boeing 777 is based around modern solid state storage, capable of recording up to 25 hours continuously. Modern flight recorders are still recording continuously, so after the initial 25 hours, the storage is full. It will then start overwriting the oldest datablock stored. This ensures that you will always have the last 25 hours of data before a crash available for research.

The following quote comes from AERO Magazine Issue 02 - Spring 1998:

Flight data recorders were first introduced in the 1950s. Many first-generation FDRs used metal foil as the recording medium, with each single strip of foil capable of recording 200 to 400 hr of data. This metal foil was housed in a crash- survivable box installed in the aft end of an airplane. Beginning in 1965, FDRs (commonly known as "black boxes") were required to be painted bright orange or bright yellow, making them easier to locate at a crash site.

Second-generation FDRs were introduced in the 1970s as the requirement to record more data increased, but they were unable to process the larger amounts of incoming sensor data. The solution was development of the flight data acquisition unit (FDAU).

As shown in figure 2, the FDAU processes sensor data, then digitizes and formats it for transmission to the FDR. The second-generation digital FDR (DFDR) uses tape similar to audio recording tape. The tape is 300 to 500 ft long and can record up to 25 hr of data. It is stored in a cassette device mounted in a crash-protected enclosure.

FAA rule changes in the late 1980s required the first-generation FDRs to be replaced with digital recorders. Many of the older FDRs were replaced with second-generation magnetic tape recorders that can process incoming data without an FDAU. Most of these DFDRs can process up to 18 input parameters (signals). This requirement was based upon an airplane with four engines and a requirement to record 11 operational parameters for up to 25 hours (see "Parameters Explained" below).

Another FAA rule change that took effect October 11, 1991, led to the installation of digital FDAUs (DFDAUs) and DFDRs with solid-state memory on all Boeing airplanes before delivery. This FDR system was required to record a minimum of 34 parameter groups. The DFDAU processes approximately 100 different sensor signals per second for transmission to the DFDR, which uses electronics to accommodate data for a 25-hr period.

Today all Boeing current-production models use DFDR systems, which will store 64 12-bit words per second (wps) over a 25-hr period in electronic memory. At the end of the 25 hours, the DFDR will begin recording the most recent data over the oldest data. No tape removal is required with these systems. Each of these systems on every Boeing model (except the 777) have at least two data frames that are transmitted from the DFDAU to the DFDR (see "What Is a Data Frame?" below).

These separate data frames accommodate the different regulatory agency requirements. A 128-wps DFDR was available for the Boeing 777 and MD-90, allowing the development of one data frame that incorporated all regulatory agency requirements and that required operators to develop only one data frame decode algorithm. "How a FAA Rule Is Changed", below, explains the basis on which the FAA may propose rule changes.


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    $\begingroup$ Makes me wonder what kind of ancient memory technology they are using. 64 x 12 bit words per second for 25 hours is not even 10MB. $\endgroup$ – rghome Apr 9 at 13:14
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    $\begingroup$ @rghome Remember that reliability is of extreme concern, and that includes reliability after being lost in the ocean for a few years. There aren't many situations in which you'd need more than 25hrs worth of data, and 64 parameters once a second is enough to store most of what's needed. $\endgroup$ – David Richerby Apr 9 at 13:27
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    $\begingroup$ @T.J.Crowder The first paragraph says, "After these 25 hours it will start overwriting the oldest datapoint." That tells us that it's not totally erased after every flight, but rather partially erased on every time interval it records a new datapoint, to always have 25 hours of recorded data. $\endgroup$ – JoL Apr 9 at 15:49
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    $\begingroup$ @JoL - No, it doesn't tell me that. It tells me that if it's not erased, it will start overwriting after 25 hours. It doesn't tell me that it's not erased. (This isn't pedantry. I genuinely couldn't tell from the answer whether the common practice was to erase them or not. David Richerby's answer is quite clear on that point.) $\endgroup$ – T.J. Crowder Apr 9 at 15:56
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    $\begingroup$ @T.J.Crowder I see. I was looking at this question from the angle that the data needed to be erased to free up storage space, and so how is it erased? after every flight? Since "after these 25 hours it will start overwriting the oldest datapoint" solves that, I didn't see any reason why there would be any other type of erasure, and took that sentence as saying there wouldn't be. If there is any other motivation for erasing the data, I suppose you're right that this answer isn't explicit on that point. $\endgroup$ – JoL Apr 9 at 16:51

So, is a flight data recorder erased after every flight? If so, after how long and by whom, is it automatic on landing, engine shutdown or it's one of the pilots procedures on landing?

Absolutely not! Recorded data isn't just needed for flights that crash: there are plenty of cases where something bad happens in mid-air but the plane can still make a controlled landing, in one piece. Automatic erasure would mean you'd never have data from those incidents.

Rather, recordings are made on a loop. Originally, this would have been a looped tape; now it's digital memory. Once the memory is full, each new chunk of data overwrites the oldest stored data, so there is always a record of the last X amount of time.

There isn't really ever a reason to erase data from the recorder, except by overwriting it with new data. Having data from previous flights on the recorder is only a very minor inconvenience for investigators: the data is timestamped, so it's easy to extract the data they need. Not erasing the old data means that it's still there even if you only realise you want to see it after the plane has set off on its next flight. It also allows investigators to look a little farther into a plane's history and see if problems on an incident flight had also occurred on previous flights, without major consequences.

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    $\begingroup$ "plenty of cases": a count which probably exceeds actual crashes by at least an order of magnitude. Given that a crash usually requires a whole series of things to go wrong (and each of those "things" usually doesn't go wrong), "we had a problem but were able to recover" is going to be much more common. $\endgroup$ – Martin Bonner Apr 10 at 10:19
  • $\begingroup$ This doesn't say anything about the possibility of erasure between flights more generally, say at the beginning of a flight (and it need not be automatic). I'm a little surprised that this isn't indicated, for I would have thought that if a crash does occur you don't really want the dataset peppered with old, irrelevant information when you're trying to do forensics on a potentially damaged recorder. Then again, as I've commented elsewhere, full erasure is likely to have a lifetime cost for the device. $\endgroup$ – Lightness Races in Orbit Apr 11 at 2:24
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    $\begingroup$ @LightnessRacesinOrbit Lifetime cost is far from the biggest issue. Having data from previous flights on the recorder is at worst a very minor inconvenience for investigators: the data is timestamped. And it has the (huge) advantage of being able to obtain data from a recent flight if you decide you want it after the plane has already begun its next flight. Storing data for the last 24hrs means that you might even have several days to decide "Hey, we need the data from that flight on Tuesday now that three more pilots have reported that slightly weird thing." $\endgroup$ – David Richerby Apr 11 at 8:45
  • $\begingroup$ @DavidRicherby Okay, that seems reasonable. :) (Would you consider merging some of this to the answer? You have some great points with value here.) $\endgroup$ – Lightness Races in Orbit Apr 11 at 10:47
  • $\begingroup$ @LightnessRacesinOrbit Done! $\endgroup$ – David Richerby Apr 11 at 12:51

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