I am not sure about a direct link between a Fear of Flying and a Fear of Heights.
Certainly in my case I have never been afraid of actually flying itself (although I have certainly been afraid in some scary situations).
In fact I have even laid face down on a perspex window at height watching the world go by below me with nothing solid at all in my peripheral vision.
But I do have a fear of heights, to the extent that I can get queezy standing on a chair! When I was younger I tried desperately to get rid of it by going mountain climbing and abseiling etc, but to no avail. I cope with it by simply focusing on overcoming it .......until something breaks the concentration!
So as has been said, it seems to be related to being in contact with the ground.
However during my time in Cyprus I did a lot of snorkling in the crystal clear blue sea (with panoramic views of the bottom a long way down) and this often gave me vertigo, which was tricky to deal with ......nothing to hold on to!
So not sure how this fits in?