We recently had the same issue with the aircraft seat belt buckle with our daughter's Britax Frontier. What we found works best is to twist/rotate the non-adjustable part of the seat belt. This makes it shorter and therefore moves the buckle over to one side of the seat, often enough that it sits far enough off to the side that it won't bother her at all.. It will of course depend on the aircraft seat and seat belt, but we've never not been able to do this. It was actually our daughter who figured it out! Hopefully that's of some help.