I know there have been more recent posts on this subject but this came up first when I did a search, so here you are.
Item in my daily paper today about violins, but must apply to other instruments. BA have said they have reduced the length of instrument cases allowed in overhead lockers from 50 inches to 22. That would seem to rule out most things apart from a curved sop. Forum members will no doubt use their usual resourcefullness to get round this (don't fly BA?), but you might need to be more ready with the strategy than previously!
YC
This item is described at
http://www.musiceducationuk.com/news/2012/5/29/ism-condemns-bas-change-to-carriage-of-musical-instruments-p.html. There's a lot of posts about this on BA's Facebook page:
British Airways. You can't start new threads, but musicians are adding their comments to existing threads faster than BA can delete them.
...A lot will depend on the type of plane - if you're on something small like a Bombardier or Avro, then you've no chance...
I've taken my tenor (Hiscox case) on Bombardiers on six journeys. On one, the cabin crew put it in their cupboard with their own bags. On the second, I took it to the bottom of the steps and handed it to the crew, who put it in the hold with the pushchairs. (These two journeys were from Southampton to France and back on FlyBE). For the other four journeys (Air New Zealand at Christchurch, Nelson and Auckland), I put it in the overhead storage. I was also able to take it from Gatwick to New Zealand via Dubai on Emirates 747s, again in the overhead storage lockers.
So in conclusion, large or small airports, large or small planes, (except BA's) it is possible to fly with a tenor without the baggage handlers handing it. I'm not sure about the super budget, "charge extra for everything" airlines like easyJet or Ryanair though.
And remember, United breaks guitars.