Brompton owners

I have taken Brompton's on flights using soft bags with the bike wrapped in various things (including my clothes) to cushion them, also both versions of B&W's folding bike cases.

1. Soft bags - don't. I was lucky that it was just the EZ wheels that were broken. None of the commercial soft-but-padded bike bags are good enough for the tough treatment inflicted by airline baggage handling.
   
2. B&W Foldon case - awesome. All you have to do is remove the folding pedal, fits snugly around the bike but has some space around it where you could fit some clothes or other things. Looks just like regular luggage, so we've never had awkward questions about it from the check-in staff. Small enough that it doesn't get diverted to Oversized Luggage. Only drawback for us is that our flat isn't huge, so at home it sits on top of the wardrobe holding other things and we had to think twice about how much luggage we would own and how flexible it needed to be. Helps that it's reasonable to use it as a regular luggage case if you aren't travelling with the bike.
   
3. B&W foldon Box S - tried this because I didn't want another big case taking up space. The sides of this fold so the whole thing collapses to take up less space. Works well enough but I don't know how many journeys the sides will take. Also, it's big enough that it becomes Oversized Luggage and I have to persuade the airline staff that it's not a bike box (it isn't just one, my clothes go in there, but extra charges can become complicated).
   
   

So when we travel by air and take the Bromptons, one is in the Foldon case and the other is in the Box S.

Whatever you use, I recommend getting one of those handy hanging scales you can use to weigh luggage. When we travel with the bikes, I find myself having to do careful moving of clothes between the bags so that they all fit the individual limits as well as not going over the total weight allowance.

I've used one of these a few times https://www.bicyclehero.com/gb/vincita-b132b-soft-transport-bag-for-brompton-bike.html padded out with clothes and a bit of bubble wrap and had no issues. A few people have borrowed it and didn't have any issues with bike damage (although one airline managed to break a wheel off the bag).

Goes through as a normal size bag so you don't need to bother with bike allowance, oversized, etc

I did pay much less than that though, if you can't find it cheaper I'd probably get the hard case unless you're short of storage space. Although it is worth thinking about who you'll be flying with and whether you pack light or not. The Brompton and the hard case will actually be heavier than some luggage allowances on some airlines.

I've flown lots with Bromptons. I got the EVA Brompton Folding Bike Case, which was being discontinued at Evans and cheap. It's been really good, the bike just fits in. With nothing removed and no tinkering, just folded normally. The case is also light enough (4kgs) + Brompton (11kgs) to stuff loads of stuff around it. Even with a 20kg luggage limit I can stuff 5kgs of other shit inside. .

The only issue I've had so far, is the last trip just before lockdown there was a little damage to the pull out trolley handle. It looks like a few things were stacked on top of it and the mechanism has dented a little and doesn't move smoothly. But the case have flown enough trips now without any excess baggage fees, so they've sort of paid for themselves a few times over. At least that's how it feels in my head.