All the airlines surveyed as of 2010 have set a maximum weight limit for checked baggage at 100 lbs. Anything over this is considered cargo and must be shipped as such. Rarely can cargo be shipped on the same plane you're traveling on.
Most airlines in 2010 consider 50 lbs. to be the norm for checked baggage. Some don't charge any fees for one piece, most however, do. Baggage heavier than 50 lbs. but less than 100 lbs. are divided into two tiers: 51 lbs. to 70 lbs. and 71 lbs. to 100 lbs. All airlines surveyed charge a fee for bags in these ranges. The average is $50 for the first tier and up to $175 for the second tier.
Title 14 of the US Code of Regulations covers the airline industry, and while section 21 covers operation procedures and weight limits of the planes themselves, it does not specifically cover checked baggage. It leaves the airlines to set those rules and regulate themselves.
