By the current rules all airfields containing a structure such as a hangar, tower, clubhouse etc are brownfield sites and can be developed.
This means that an airfield in current use can be instantly confirmed as being brownfield for planning purposes. OTOH a field which was used in WW1, returned to agricultural use and later converted to a nature reserve can still be a brownfield site if the remains of a structure can be found.
The law is an ass.