The U.S. Department of Transportation issued a final rule to implement section 403 of the FAA Modernization and Reform Act of 2012. The rule requires that each U.S. airline allow passengers to carry into the cabin and stow a small musical instrument, such as a violin or a guitar, in a suitable baggage compartment.
This rule also requires that US airlines are prohibited from charging passengers carrying a musical instrument an additional fee other than the airlines standard fee for carry-ons.
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Great! I always wanted to bring my tuba on the plane with me!