Check out pages 75-76
The law says, I can fly with my bass at no extra charge- In the cabin–Hurray.! I’ve always been a bit nervous about my basses. All you players, maybe print this out next time you fly. Unfortunately, my upright doesn’t fit the bill. He needs a whole seat to himself.
“An air carrier providing air transportation shall permit a passenger to carry a violin, guitar, or other musical instrument in the aircraft cabin, without charging the passenger a fee in addition to any standard fee that carrier may require for comparable carry-on baggage, if
(A) the instrument can be stowed safely in a suitable baggage compartment in the aircraft cabin or under a passenger seat, in accordance with the requirements for carriage of carry-on baggage or cargo established by the Administrator; and
(B) there is space for such stowage at the time the passenger boards the aircraft.”
Board early! Bon Voyage.