For analysing the FDR, data is downloaded to the local computer from the FDR and feed into the Analysing Software. I would like to know what is the file extension of the data stored on the FDR. I came to the conclusion that it must be csv for storing all the data about Height, longitude, latitude and so on. Then this .csv file is feed into the Analysing Software for analysing. But I am not sure if I am correct.
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The builders of an FDR must comply with DO-178C (the older DO-178B may also be of interest here) as per AC 20-141B issued by the FAA. These documents do not specifically define a data format but provide requirements for what it must be resilient against. Here is an interesting research paper that covers some of the topics you are asking about. Since most avionics use ARNIC 429 as their standard of communication some FDRs provide ARNIC 429 inputs to record the avionics instructions at the time. I would assume these inputs dump the bus data right to the storage medium (most likely solid state memory these days). Since the formats the device uses may be proprietary as well as very low level (and memory efficient) I am sure they make software to dump it to other formats. For example L3 makes a full unit for their FDRs.
After some more searching I found what may be the most comprehensive answer from this Boeing summary document
What Is a Data Frame?
A flight data recorder (FDR) data frame is the order of the words that are transmitted from the digital flight data acquisition unit (DFDAU) to the digital FDR (DFDR) each second over many seconds (see ARINC 717 for additional information). Most FDR system data frames are made up of four subframes within one superframe. For a 64-words-per-second (wps) FDR system, a DFDAU will output 64 12-bit words to the FDR each second, where each word typically contains the value of an analog parameter. The order of the words (for example, word number 12 of the 64 words) within a subframe, as well as the order of the subframes, define an FDR data frame. This order is important to understand in order to decode the data recorded in the DFDR.
It should also be noted although they may have all been phased out by now, some older cockpit voice recorders and FDRs recorded on analog mediums and may still be inservice doing so.
We can use inductive reasoning to make a guess at a probable answer to your question.
I came to the conclusion that it must be csv
is likely wrong.
The only purpose of a CSV file is to transfer data between a source and a sink which do not understand the way in which the other stores data.
An FDR does not need to be understood by any other sink other than the manufacturer and/or incident investigators.
Adding the ability to store or retrieve data in a CSV format would add some complexity, through additional problems to solve, which is simply not needed.
All that is required is a method to extract the raw data and display it in a format that humans can understand. This is trivially done with a piece of custom software.
The designers will have no requirement to provide data in any format other than that required by the extraction software so they simply won't implement it.
CSV, and other data interchange format files, all have limitations. They lose resolution, they are inefficient in their use of resources and only exist to work around problems introduced by a lack of standards, differing interpretations of standards and attempts by commercial, general purpose software vendors to differentiate themselves or make it more difficult for systems to interoperate.
We can therefore reasonably conclude that an FDR does not store data in a CSV file. Of course, the custom software which does extract the data might contain such a feature but then again, why would it?