1) Hot air Baloons - Probably exist in moderate and higher technology groups. Textiles and some gas burning technology required. Probably owned by richer eccentrics for pleasure, and used by organised governments (NCR, Vault city, etc) for scouting, cartorgraphy and aerospace research.
I'm with you on this one. Hot air baloons are most probable. After all, the first ones were made out of paper and didn't require a gas burner. IIRC it was held directly above a fire, flew up and landed when the air cooled.
2) Lighter than air baloons and Zepplins - Helium aircraft (like blimps are today) are probably non existant due to the difficulty in getting helium. This leaves Hydrogen, like the Hindenburg used, which is plentyful and relatively easy to produce (Any water source, even if undrinkable + Electric Current = Oxygen and Helium). Known to exist in the BOS (if Tactics is taken as Canon). Within technological grasp of the NCR, Shi, and Vault City, although the NCR is more likely than the others to build them it due to better resources. Can be used for transport, as well as scouting purposes. Probably out of reach of the regular citizen, although richer caravan masters may have an interest in this technology to get above the raiders.
Even better. Btw you made a typo, it's hydrogen, not helium Also, that hydrogen could be the gas for the gas burner of the hot air baloon.
A simple propeller aircraft powered by electric motors running on microcells/batteries would also be possible.
However, one thing surprises me though - lack of hovercraft. Take four robots apart for the propulsion, mount some sort of platform on top and use a fifth robot's propulsion system for forward drive. If you get the balance right, it'd be way better than aircraft - no need to worry about surviving the landing, and, also better than ground vehicles on the rough terrain that exists. Imagine the highwayman having a flat tire...
The jets in RC have a.) their electronics destroyed - EMP or not. b.) their parts rusted c.) no fuel d.) no pilots and e.) no perceived need to ever be flown. However, why haven't they been stripped for parts is a bit of a mystery. Besides, wouldn't their weight be causing some issues for RC's boyancy? One would presume that leaky rusty tub to be way better without that useless weight.