I know a trainer whose barn is in the flight path of a private airport. Within a couple of days the horses don't bat an eye! There is lots of other activity at the farm incl'g camp and small farm animals of all kinds. Great exposure...lots of extra benefits of training at that farm!
When I took Jet to his first show as a 3 almost 4 yr old, a helicopter landed right beside the warm up ring. Lots of people jumped off their horses. Jet just stood there looking at it and all of the dust the rotor wash was kicking up, like " Well I'll be...would you look at that! That's really interesting...hmmm."
Then a week later as I walked out of the ring at home past an irrigation ditch, a frog that was sitting nearby went "Riibbiitt" and hopped about 2 feet. All 4 of Jets feet went 4 different directions, and he got about 3 feet lower to the ground suddenly. I guess frogs eat horses and I didn't realize it! So much for a spook-proof horse!
I went to a carriage driving show that was about 300 feet from the runway of a small airport. I didn't realize how much I focus until DH asked me if "that jet" bothered me. I answered "what jet?" Had a vague recollection of a small jet powering up for take off but nothing much after that.
At the Region 15 Arabian championship over July 4th a few years ago, there were hot air balloons going by-very pretty, but it did spook a few horses.
Where I board, next to Crowder's Mountain, they will have to use the neighboring hay field to set down helicopters when they do rescues from the mountain. Two years ago they did a whole mock disaster scene drill with several Black Hawk helicopters coming and going.