Hmm, I wondered about that, but unlike other FC3 aircraft, the A-10A doesn't show a Jettison Fuel Tanks key mapping in the options>controls screen (at least not for me).
Strangely enough, in the default.lua script under ..\Mods\aircraft\Flaming Cliffs\Input\a-10a\keyboard I see the following in the "-- Systems" section:
{combos = {{key = 'R', reformers = {'LAlt'}}}, down = iCommandPlaneJettisonFuelTanks, name = _('Jettison Fuel Tanks'), category = _('Systems')},
but apparently it doesn't work with the A-10A (for me) - does it show up in anyone else's options>controls screen?
Thanks