I've been seeing a lot of instruments on planes lately and there's a reason. Late last year the U.S. Department of Transportation ruled that musicians who travel on planes must be allowed to carry their instruments on board as long as they fit in the overhead bin.
What if the space isn't big enough for the instrument? The musician is permitted to purchase a second seat where the instrument could be stowed.
Sounds simple, right? You'd think that airlines would be allowing this all along, but the fact of the matter is that carrying an instrument on board had been at the discretion of the airlines before, which could lead to having to check it as baggage and some disastrous results. Now, if you can carry it on, it should be safe.
There is a caveat however. The airlines won't prioritize an instrument over other types of baggage, so you have to be able to get on the plane early to make sure there's space available.
The best way to do this is to either enroll in a loyalty program or buy priority boarding so you can be one of the first to board the plane to make sure there's room in the overhead bins. It also has to be one of the your two carry-on items, so the other must fit under the seat in front of you.
That said, this is good news in that we no longer have to worry when carrying instruments on a flight (at least in a gig bag).
If you like legalese, you can read the full ruling here. Here's a list of the individual airlines instrument policies.