While I don't quite agree that the severity of the turbulence and unstable hovering flight would be tremendously severe (I believe the size and shape of the tunnel would be a significant factor), just much higher power required to hover. Additionally, I believe the other factors would play a role in the forward flight portion. In the MI5 senerio, the aircraft was chasing a speeding train at say, 50-70 mph. What do you think the wake turbulence coming off the train effect would be on the aircraft? In the confined space of that tunnel, the margin for error for managing precision forward flight would be slim. Most likely, most pilots would only manage a few hundred yards before a blade stike or some other type of contact with the interior of the tunnel. Just my .02 worth.