You may carry small knives on airplanes except that you may not.
The US government has done a 180 on a recent decision that would allow passengers to carry small knives (those under 2.36 inches long), bats and golf clubs onto planes. Several airline CEOs, lawmakers, and relatives, friends and victims of terrorist attacks balked at the idea, and now the TSA has changed its mind. Again.
So keep your commemorative Yankee Stadium bat and Tom Mix pocket knife in your checked luggage. Unless you want a friendly TSA agent to take it home.