I had to fly with a walking boot this past June. Fortunately, it is visible so they know it is coming
I wore the boot through the metal detector. It and my MedicAlert bracelet triggered the alarm. I stood for a pat-down. I was asked if I was able to take the boot off and I explained I could, but only if I was sitting down. They led me to a chair and I took the boot off (they watched as I did and it was clear there was nothing on underneath the boot except the smelly tube sock!). I remained seated while they sent the boot through the xray machine your carryons go through. Then I got the boot back, put it on, and was good to go to where they were inspecting my medical liquids
A few things to think about:
- consider if you will need or could use wheelchair assistance. Airports can be huge and walking those long distances with a boot is a pain in the butt (and knees, and...). At one airport I regretted not having arranged for it -- the walk was quite tiring and uncomfotable.
- if your walking boot has air bladders -- you will need to deflate them as or before the plane gains altitude. As with other things, they expand as the plane goes up. This can burst them if it is enough pressure. But, more importantly, IT HURTS. I forgot this on the first flight but was rudely reminded as they expanded and the boot suddenly became painful; then there was my mad scramble while still seatbelted in to hit the pressure release valves. For the other flights, once I was seated and seatbelted and not going anywhere I let the air out pr-emptively. Even then, the residual air expanded and had to be let out at altitude. Then, once we were on the ground I was able to reinflate.
- I pre-boarded each flight and waited to be the last or nearly last off. Much easier navigating the plane and jetway with no or few other humans instead of the streaming masses.
- think carefully about what shoe you will be wearing and how comfy it is. I had no choice as only one pair of shoes was tall enough for the boot, but those shoes were not as comfy as my normal shoes and that took some adjusting.
- planes don't give you much room between you and the seat in front of you. this lack of space is magnified when you are wearing the boot (plus, finding a comfy position for it is challenging -- tho it does make a good foot rest for your other foot

). My flights did not offer it, but if they did I would definitely pay to upgrade at checkin to a "plus" (more legroom) type seat if available.
SW