First Flight yesterday. Thought I followed all instructions. 1st noticed drift in GPS mode. When I took it to high elevation, it stoped responding to inputs. Figured it would engage RTF. Nope! Shut off transmitter to engage RTF. Still no. It maintained it's position until battery low, descended slowly, but battery exhausted before landing. Dropped last 100'. Interestingly, at about 20' I heard the engines rev strongly. Upon inspection, it looks like the props were turning backwards at impact! (Damage to all four trailing edges) Amazingly, the props are the only apparent damage. Any ideas why it would not return from altitude?