DevX - again, speaking Boeing here, but aside from some of the very early jet aircraft (which had flight operable T/R - I'm thinking maybe the DC-8 had those), there was always 'air/ground' to enable the T/Rs on the ground. Unfortunately that could also cause problems: A 737-200 crashed in Canada in the mid 1970s - they landed then discovered a snowplow on the runway after they deployed the reversers. The crew closed the reverse levers attempted to takeoff again, but one of the reversers hadn't completed the stow cycle when air/ground went air which closed the T/R hydraulic isolation valve. Without the reverser locked in forward, the aero forces opened the reverser again (those big clamshell type) and they crashed.