The passages below come from 'Cambridge English: Compact Advanced.'
a. The thunderstorm will have ended by the time our plane takes off.
b. The thunderstorm will be ending by the time our plane takes off.
It asks the difference in meaning between a and b.
Let me tell you my thought.
'a' more strongly suggests than 'b' the possibility of the end of the thunderstorm. 'b' suggests not the end of the thunderstorm but just a hopeful possibility (maybe since it has been in its force long enough).
What do you think of my thought?