IIRC,
An a/c shall not fly lower than 500' over a "built up area, or habitation". A single engined a/c is [I think] limited to a
minimum of 7-8000 ft over Londinium, so that it can "glide clear" in the event of an engine failure. However, a chopper can "autorotate" into some fairly tight spots. That may be the reason, or I am talking bolleaux [as usual]
