Getting a scope through as carry on isn't about the amount of space in the cabin (although with an empty flight and nice cabin crew just about anything is possible) ... the problem is getting past the carry on size requirements at the gate. Those chrome tubed sizing tools so prevalent with the airlines (and the sign over it something like "Your carry on luggage must fit here") and extra vigilant gate staff are your enemy when pushing the limits.
Within North America the smallest dimension was (and generally still is) 9"/23cm so a 6" scope (with it's 7" or so diameter OTA) and some soft padding is usually the largest aperture you can "legally" get on a regular mainline flight. A 7 or 8" scope with some padding is just about always over this smallest dimension - although a custom minimal truss tube 8" newtonian is possible. I also mention "mainline" because there is also the added problem of smaller regional airlines and their associated smaller aircraft that have smaller overhead bins and therefore even the 9" dimension may not be small enough (although most of the 50+ seat regional airliners have had upgraded interiors over the past 10 years and this has helped with carry on sizes). Still,it can a bit of crap shoot determining what aircraft type you will be on when there are multiple hops with multiple carriers.
I have at times (such as when being rerouted onto a smaller aircraft at the last minute) even resorted to removing my C6 from it's nylon soft case and bundling it with a sweater or small jacket to get it to fit ... and yeah I had to be extra vigilant when others were trying to get some last minute items stowed close by. In these full cabin circumstances a kind and understanding cabin crew is your savior here.
In general my guidelines for scope travel follow what many have already mentioned ...
- A simplified mount (shortest practical tripod) and accessories well packed in the largest checked suitcase available (and keep a scale handy when packing as it's very easy to bust that 50lb/22kg weight limit)
- A scope that is under that max 9" dimension and has flexible packaging to accommodate as many of those unforeseen cabin/personnel issues as practical. I also tend to pack my eyepieces and diagonal separately from the scope.
- Watching your gear like a hawk through the entire boarding process. Turning your back for even just a second will ensure that the big guy just now coming down the aisle will try and jam his hard shell carry on into a space much too small and currently occupied by your scope.
- Being as nice as you can be to the cabin crew no matter what is going on around you.
