Once the crew "stuffed up" making the exit and stopped, did they intentionally use reverse thrust to back their Aircraft, yes or no?
The answer appears to be:-- yes they did as evidenced in the Video.
If their AFM says "not approved" then they shouldn't have attempted it and can probably expect Tea and Bikkies with their Chief Pilot, especially since it's on YouTube for the whole World to see.
In any case maybe they should plan their exit strategy a little better at this Airfield!
