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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?

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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 [...]

NBAA

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]

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    $\begingroup$ 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
    Oct 24 at 15:33

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