§ 251.4 Large musical instruments as carry-on baggage.
Primary source
Verbatim text below is from the Electronic Code of Federal Regulations (eCFR), a public-domain U.S. government work. Always verify the current version with the eCFR before relying on it for any legal matter.
Full Text
Each covered carrier shall permit a passenger to carry a musical instrument that is too large to meet the requirements of § 251.3 in the aircraft cabin, without charging the passenger a fee in addition to the cost of an additional ticket described in paragraph (e) of this section, if:
(a) The instrument is contained in a case or covered so as to avoid injury to other passengers;
(b) The weight of the instrument, including the case or covering, does not exceed 165 pounds or the applicable weight restrictions for the aircraft;
(c) The instrument can be stowed in accordance with the requirements for carriage of carry-on baggage or cargo established by the FAA;
(d) Neither the instrument nor the case contains any object not otherwise permitted to be carried in an aircraft cabin because of a law or regulation of the United States; and
(e) The passenger wishing to carry the instrument in the aircraft cabin has purchased an additional seat to accommodate the instrument.
This is legal information, not legal advice. Laws vary by jurisdiction and change frequently. Always verify current law with official sources and consult a licensed attorney in your jurisdiction for advice on your specific situation.