They have specific rules about cellos and basses (or "bass violas" as they call them). This is quoted from their website:
http://www.usairways.com/awa/content...cialitems.aspx
Musical instruments
Musical instruments are considered a fragile item. Fragile items will be accepted as checked baggage if they are appropriately packaged in a container/case designed for shipping such items. Fragile items without appropriate packaging will be accepted upon the execution of a release furnished by US Airways relieving US Airways of liability for loss/damage of checked baggage.
Musical instruments are permitted as carry-on baggage as long as they can be safely stowed and have dimensions no greater than 75 in/190 cm (length + width + height).
US Airways allows customers to carry their musical instrument in the cabin with them if the instrument exceeds the size restrictions for carry-on baggage and/or the customer believes that the instrument is too fragile to be handled as checked baggage. To do this:
The customer must purchase an extra seat for the instrument. Cost of the seat is the applicable adult fare for the portions of the flight that the extra seat is requested plus sales tax.
The musical instrument must be secured in or against a window seat (not in an Emergency Exit Row).
The customer must be seated in the seat directly next to the instrument.
Seat baggage must not weigh more than 170 lbs/77 kg and must meet the size restrictions based on aircraft type.
Cellos and bass violas will only be accepted as seat baggage.