The law says of you are qualified and under the hood for practice you have to have a competent observer to warn of any dangers, most people took this to be another pilot, I would take a mate of mine who was an RAF flight engineer on the basis that if he s OK to sit on the center seat of a C130 keeping four engines running and helping with the lookout when the aircraft was traveling down a Welsh valley at 230 Kt and 200 ft he was good enought to keep me out of trouble in a DR400 !