It believe the primary question would be, why were they grounded? I mean, the military will suspend use of it's equipment for some very simple reasons. How long were they downed?
Not meaning to sound "old", but when many of us began skydiving, we were jumping piston prop Beech-18's and Twin Bonanzas. Now, these planes leaked oil like pouring soda from a can. They spat 4ft flames on startup and lost engine power in flight sometimes. I (and my gear) have been pelted by oil drops thrown from high RPM engines. Still, we survived. Was it safe...probably, but not as safe as the planes today.
Sorry to ramble, guess I'm just feeling nostalgic. Geez, maybe I AM just getting old?!?