Direction of take-off in this plane! (8)

I believe the answer is:

vertical

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But I can't explain it at all! Could you help me understand?

(Other definitions for vertical that I've seen before include "Upright, perpendicular" , "Perpendicular to the horizontal" , "Straight up" , "At right angles to the base line" , "At right angles to the horizontal" .)

I've seen this clue in The Sun and The Times.

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