Echo the above. Nearly 20,000 flying hours with never a twinge of height fear, but tall hotel balconies and cliff tops set me off. In Alaska at Mt. Kosciusko, I had to walk on a coll- a high ridge that stretched between 2 peaks. It became just a path with a fallaway on both sides. I suddenly had to get on my knees and try and turn around and crawl back. Very hard to turn- felt terrible. Couldn't go on. I felt if I tried standing I would just fall over. Decided mountaineering was not for me, ever! Skiing seems somehow easier, but still scary.