Often the airline won't be able to tell you in advance what equipment (type of plane) your flight will be using, or they may say it's ok but it will end up being otherwise. What led me to stop carrying a bass onboard in a gig bag was a 3-week tour I did several ago, all fly dates, where about half of the flights were on planes with smaller overheads - big enough to fit a guitar in a gig bag but not a bass. On those flights I ended up having to gate check my bass, which made me uncomfortable based on the incident I referred to in a previous post in this thread. When I got home from that tour I did some research, and thanks to information I got in this forum I ordered an SKB Bass Safe and have used it happily ever since.