There's heights and there heights. The common fear is of heights that are large enough for a fall to cause serious injury, and not too large to be out of range for biological fall protection

I used to be afraid of heights, but trained myself out of it as an adult. I had trouble abseiling, walking on elevated walkways, standing near windows in tall buildings

Three things I never had a height problem with:

• Front seat of a plane (or any other seat, for that matter)
• Basket of a balloon
• On top of a tall hill

I have some fear of heights and for some reason it doesn't get triggered when flying on a plane, even on a prop-plane a km or two above the ground.

High up in tall buildings or mountain or coastal cliffs, sure, planes, not at all.

It's not exactly rational.

If you're not actually afraid of flying when on a commercial flight, I bet you won't either when at the commands of a poky prop-plane.