I never have this problem as my trusty kazoo travels with me in my camera bag and has never been damaged (to the general chagrin of music lovers everywhere). However this summer I did see brave young people carefully handing guitars (in soft gig bags)at the boarding gate for hand loading. At the other end they came out on the carousel with everything else. As far as I could see they were undamaged (much nervous checking by worried owners) but I don't think I'd risk it if I were a musician, unless I had a fibreglass or Kevlar case. I think someone said on an earlier thread, jump up and down on the case and throw it down a flight of stairs to simulate the effect of baggage handling!