I have a fear of heights. Especially when I'm physically connected to whatever I'm standing on. Famously, this is why I never got higher than the second floor of the Eiffel Tower or why I feel really uncomfortable looking out over edges of cliffs, buildings, whatever.
When I fly less so, as I'm disconnected from earth. But, I have to admit having flown both Helicopter and FW, that the helicopter gave me that uncomnfortbale feeling when we flew high. Anything say above a 1000ft and I felt very vulnerable. Don't have the same problem with FW, but if I start thinking about structural or wing spar failure, it comes! So I try not to.
It's my greatest fear in flying - catastrophic structural failure and not having control. Thankfully, it's rare.