NOTE: It is vital to use the same size and depth of nut as originally fitted to your hang glider.
Using the wrong nut courts disaster. So check the nut you remove carefully and make certain that you order the same size and thickness replacement.
The 5/16" inch UNF is not used in many locations on hang gliders. Generally the nut is only used at the top of upright bolt and cross tube leading edge junction. (However this is not a thin nut which could be required on Icaro Hang Gliders.)
Note 5/16" is very nearly identical in diameter to 8mm so you do need to check the colour of the nut insert.