What the headline says - how to open/view .flt (OpenFlight) in Blender 2.79?
In Blender version 2.42a - 2.46 there is an option to load OpenFlight FLT files. In later versions this addon has been removed (most likely due to the lack of support by the community).
Best way currently is to download blender version 2.46 and import the FLT file (File → Import → OpenFlight(.flt)). Save the blend file and open this blend file with the blender version of your liking.
Second option is to find a developer who will migrate and maintain the OpenFlight plugin.
More information of the plugin can be found at https://wiki.blender.org/index.php/Extensions:2.4/Py/Scripts/Import/Openflight_flt