I wouldn't call it an irrational fear (like, being afraid of bunnies or the dark), but a healthy respect for something that will easily kill you if you dont pay attention. I have no problem in an airplane including aerobatics, and no real problem climbing things like ladders or trees, but when I'm on something without a good grip I get very tense. For example when I built my house, standing on the edge of the roof to work on a piece of trim makes me tense. Standing on the edge of a cliff also gets me a little tense. It's not the height so much as the lack of a secure grip that bothers me.
Starting about 1:30 makes me pee myself just watching this from my office chair.
3:50 gives me a mild heart attack.
At 6:30 I pass out.
I probably wouldn't sign up for this either: