Most airshows I’ve been to are too crowded to usefully use a tripod, if you’re in the “photographer’s box”, press stand, etc. If you’re anywhere near the front-and-center location, you need to be able to shoot far left and far right. With a tripod, that means you are moving your body around the tripod. But everybody else is hand-holding, so their feet stay mostly planted. You just wind up taking up too much space, bumping into people, tripping over the tripod legs, etc. It just doesn’t work.
The solution I came up with was to use a monopod kept less than half-extended, with the foot tucked into a pouch or pocket added to a belt. That definitely helped take the weight of a heavy lens+camera combo off my arms, and I could shoot mostly all day. Note: this absolutely requires use of a monopod tilt head (basically just a vertical-tilt-only head). A ballhead is suboptimal, but if you have one with a 90º side dropout, that can be used instead of a dedicated monopod tilt head.
Regarding tracking at zenith: I agree with xenoid, it probably isn’t worth it. Except for the very low/close overhead flybys, the photos aren’t good. And for the low/close flybys, you want to be able to move fast, so tripod tracking won’t help. With the monopod-in-a-pocket method, you can quickly lift the camera so that the monopod isn’t supporting it, and still track vertically if necessary (assuming you are using a monopod tilt head).