I just used a helicopter to fly over rough terrain to a desert to get me some cactus. Great version of flight, thanks!

I fell out several times. Lucky I have bouncy boots to protect me, but as I watched my unmanned chopper sail off into the distance I really wished I hadn't accidentally pressed the dismount button.

Would a seat belt be a fun thing to add/make?

What do you mean by seat belts? You exit the aircraft after trying to crouch, just like in boats, so... just don't crouch and then you won't fall out, I guess?

Yes, that's both correct and also didn't happen that way.

What I mean by seat belt would be a new invention for Minecraft, something that hasn't been done before: A method of preventing accidental dismount from pressing a button that also serves as the down button when swimming. Muscle memory is funny like that.

Maybe it's an item that when placed on the hotbar prevents dismounting/crouching. Such a device might even be useful across other mods.

If this doesn't interest you then feel free to close the issue, and I'll investigate making it as a separate mod. Might be possible to implement as a setSneakingAllowed(TRUE|FALSE) game rule and listener, if that system allows for player-specific game rules. Might trigger some sort of highlight animation/sound when it blocks a crouch attempt, to remind the player why they didn't crouch.

Either way still loving this mod

This would be a separate mod as it wouldn't be plane-specific. If there would be something to change in simple planes in order for it to work, I'll be happy to do that.

This may not be exactly what you're looking for but I do think it's worth mentioning - there are a few mods that let you separate the crouch and dismount keybindings. Quark is one, if memory serves, but there are other standalone mods that should work just as well.