Well, I just went through two TSA checkpoints (not in PIT, in BUR and OKC) with a back brace. None of the TSOs at either checkpoint had ever seen one before, and seemed completely befuddled. Expect the same over your air cast.

They both initially demanded that I remove it and put it through the x-ray. When I told them it was a medical device, and as such I would NOT be removing it, they subjected me to intense pat-downs (worse at OKC than BUR). They also thoroughly swabbed it and tested the swabs for explosive residue. In OKC, they also pressed on it hard enough to cause me excruciating pain (perhaps to see if I was telling the truth that I had two fresh, very sensitive surgical incisions on my back?).

According to the TSA website, you are not required to remove it (I'm not sure if an air cast CAN be removed). But don't expect the TSOs to know this - mine didn't, until I told them. Just stick to your guns. Expect a thorough prison-style frisking.

They also may try to stick their fingers underneath it. Don't let them (I didn't). They can swab the outside of it, and that should be sufficient to convince them that you are not a suicidal terrorist carrying explosives or other "prohibited items" in your cast.