never had the benefit of a F/E in the civil world but many hours spent in the RAF with them and when many hours into a very long patrol with more hours to go if something wasn't quite right it was always the flight engineer who advised us whether it was ok or whether to high tail it back to base, and what to do en route.
He was I thought the only man on board who knew how the thing worked,
all of it. The Flight engineer to us was priceless. We called it "crew cooperation" . CRM hadn't been thought of.