I`ve always wanted to know this..out of interest.
All you 747 captains,ready??!!
One day your happlily flying along at...31,000 ft and at cruise speed..say 2 hours into a loooong flight somewhere..you`ve taken off with full payload and all engines are operating in normal limits.
Suddenly a large flock of high flying budgies fly into all 4 of your engines rendering them absolutely useless,so you have to shut them down
How far can your plane glide,and how long have you got before your guardian angel does a runner???