@dgar This is similar to my approach.
One of my alts posts photos of aeroplanes – many people would be happy to have alt text with something like “Boeing 737 operated by Ryanair about to land at Manchester Airport”, but that is what the main post text field is for.
Instead, go for more detail but less precision.
What type aircraft is it (airliner, helicopter, bizjet, etc), how many engines does it have, and which kind (jet, propellor, etc), and where are they (under/over/through the wings, sides of the rear fuselage, in the tail, etc), describe the livery, including colours, stripes, and any text, and what it is doing and which directions (flying from left to right at a low altitude, taxiing towards the camera, etc).
Precision means you expect the reader to know what something looks like – if you just say “Boeing 737”, you expect them to know what a Boeing 737 looks like, but saying “Mid-sized twin engined jet airliner with an engine mounted below each wing” explains the basics, and it’s the same with colour schemes.
And do something similar with foreground and background.