Between my green nomex “approved for flight” jacket and my classic “not approved for flight” leather jacket, I don’t think I’ve ever worn my green one inflight. It seems destined for driving to work or maybe, maybe, doing a preflight. The leather jacket is too comfortable, too warm (although this isn’t really a factor in the KC-10), and too awesome to not wear inflight on the regular. Thankfully, most everyone I work with agrees with the sentiment.
Both jackets are older than most new pilots and booms, and both are just about worn in