Mentioned through other channels in the past, I’m opening a thread here so this gets properly logged and addressed.
At this time, it is not possible to engage thrust reversers while in the air, on any aircraft that has jet engines. There is a hardcoded “weight on wheels” condition for thrust reversers to engage in MSFS.
Though this is generally uncommon among aircraft, there are a couple of notable examples where such a feature is important: DC-8 and most notably, on the C-17 to perform Tactical Descents, which is our case. It would be nice to have a variable on engines.cfg that we can simply flick to on to remove the WOW limitation.
Sure, we could use alternative methods such as an oversized spoiler axis to achieve a similar result, and faking the engine gauges to match the conditions, but we rather have a fully native solution as it seems like a far simpler and cleaner option.
Tactical descents is one of the most requested features on our 1st party C17 
@FlyingRaccoon @EPellissier