I’ve flown with my fiddle twice (or four times depending on how you count it) and always carried it on, once on LAM (Mozambique airlines) and once on air France. As long as there’s room in the overhead bin (lots of people put suitcases up there) you should be fine, as long as your fiddle (and bow) doesn’t have any banned woods or ivory or anysuch
Bruce, I have flown a number of times both domestically as well as internationally with various airlines and most airlines (BUT NOT ALL) as I discovered, will allow you to take your violin on board as one piece of your carry-on baggage. Note that some airlines will only allow ONE item as carry-on.
I (and many others) would suggest that you try to get onto the aircraft as quick as possible in order to find a vacant overhead locker near to your seat. I have found that it worked best for me to put the violin in the overhead locker opposite my seat so I could keep a constant eye on it when other passengers were either cramming their bag in or taking it out. A few passengers may think you crazy to be so concerned about a violin (as I experienced) but they are just the ignorant ones (whom we must bless) who know nothing about the fragility of a violin.
Also, if you smile a lot when boarding, you will often find a very helpful flight attendant who will help you quickly find a vacant locker. BY NO MEANS EVER LET YOUR FIDDLE GO INTO THE CARGO HOLD.