I was more than sure that each subsequent flights causes to overwrite earlier FDR and CVR recordings. How then should I understand the following part (emphasis mine) from this Wikipedia article:

The National Transportation Safety Board downloaded data from the aircraft's recorder and found it was a habit: 98% of the previous 175 takeoffs were made with incomplete flight-control checks. The National Business Aviation Association analyzed 143,756 flights in 2013-2015 by 379 business aircraft and only partial flight-control checks were done before 15.6% of the takeoffs and no checks at all on 2.03% of the flights.

Is it truly possible that single FDR unit kept 175 past flight's data? Is it possible that NBAA was able to retrieve approx. 379 earlier flights' data from 379 planes (143,756 / 379 ~= 379), spanning even two years into the past?

What am I missing here? How long data is kept and not overwritten in single FDR unit?


2 Answers 2


From the official report (AAR-15/03) of the accident being referred to on Wikipedia, page vii:

A review of data from the airplane’s quick access recorder revealed that the pilots had neglected to perform complete flight control checks before 98% of their previous 175 takeoffs in the airplane, indicating that this oversight was habitual and not an anomaly. [emphasis mine]

What the investigators used for this data was the QAR, not the FDR.

The QAR recording contained 303 hours of data and 176 takeoff events, including the accident takeoff. [ibid. p. 16]

For the FDR, it varies, see: Is flight data recorder erased after every flight? For example:

The recorder of a Boeing 777 is based around modern solid state storage, capable of recording up to 25 hours continuously [...]


The NBAA analysis did the same, essentially using regularly downloaded and anonymized QAR data over the time period stated on Wikipedia:

The data group was tasked with obtaining and aggregating de-identified data from business aviation FOQA [Flight Operations Quality Assurance] programs to determine the compliance with manufacturer-required routine flight-control checks before takeoff.

— NBAA Report: Business Aviation Compliance With Manufacturer-Required Flight-Control Checks Before Takeoff. p. 3. (PDF; nbaa.org) [emphasis and link mine]

  • 3
    $\begingroup$ And today a non-aviator learned about the QAR. Obviously capable of recording much more data, and faster, because it is not subject to the extreme accident-survival requirements of the FDR. $\endgroup$
    – PcMan
    Commented Oct 24, 2021 at 15:33

The amount of data that an individual FDR contains, while dependent on the particular unit, is fixed, and would correspond roughly to some amount of flight time. How far into the past that extends, and how many flights it captures, depends on how often the aircraft flies and how long the flights are. If aircraft 1 is operated for a single long flight every few days, and aircraft 2 makes 10 short flights daily, the memory in the FDR in the second aircraft would contain more flights, but going back fewer days on the calendar, than a comparable FDR in the first aircraft (before the data is getting over-written).


At major airlines, and probably many smaller operators, the FDR is downloaded regularly by maintenance as part of a FOQA (Flight Operations Quality Assurance) program, and the data is stored so it can be analyzed for trends over time. This way, if some worrisome event comes to light, it's possible to have a server go back through data across a fleet of perhaps hundreds of aircraft and years of operations to see how often something similar happened, or almost happened but wasn't noticed.

Thus, to take the example from the OP, if somebody noticed that flight control checks weren't being performed correctly, a FOQA analyst could write a query to determine if a particular flight control check was "good" or "bad", and then have the query run against the data from tens or even hundreds of thousands of flights. If that found that most flight control checks were done correctly, they might go further to see if the bad flight control checks correlated with something... short turn arounds, or late night flights (i.e. tired crews getting sloppy), or something else.

Then, instead of a message to the crews "hey, make sure you do your flight control checks", the message could be more specific: "Even on quick turns, it's important to make sure you get full travel on the controls, because we've seen XXX occur even when the aircraft was only parked for a few minutes..." That sort of a specific, focused message tends to affect behavior more than "Hey-- do your job right, darn it!" messages do.

Thus, data like what the NBAA referred to in the OP doesn't necessarily have to all be contained in the FDR; it may have been downloaded & stored over time instead. Data for 175 flights from one aircraft seems like it could have been all in memory; if that were a couple hundred hours of recording, that doesn't seem like an unreasonable amount of memory to put in a FDR. On the other hand, the analysis of data from over 100,000 flights over two years is almost certainly from data that had been downloaded regularly over that span of time.


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