Daft question why? If remember correctly futaba made no distinct ion between air and surface 2.4 sets originally it was just FASST . I will of course make the usual proviso that I, m completely happy to be proved wrong and im so old I csnt remember anything for longer than 6 month's
I thought the idea of 2.4 was to eliminate the idea that the pilot/driver would have remember to shout/ask what anyone else was using
kind regards Fid