Official aircraft can be downloaded at FlightGear.org Aircraft, which then usually require unzipping software to extract from a compressed file format. Then these must be manually installed in a FlightGear software installation to use. The details depend on the method of download, operating system, and user setup.
In your FG_ROOT directory you would find the following directores
./Translations ./Protocol ./Textures ./Timezone ./Airports ./gui ./Aircraft-uiuc ./Scenery ./Traffic ...s