About 5% of U.S. flights have undercover air marshals.
Since 9/11 there have been visible changes in airport security. From more security guards to lots of new screening technology, catching a plane has certainly gotten more complicated since the tragic events that September day. What people don’t realize is that what you physically see is just part of the security policies employed in airports and airplanes.
For example, air marshals work undercover on airplanes. In all likelihood, you will never know if there is an air marshal on your flight. They dress normally, but they actually carry gun and have the authority to make arrests. They are there both as a preventative measure, since potential hijackers know they might be on the flight, and also in case of emergency to take action.
Since there are not enough air marshals to be on every flight, they are only on 5% of flights. The number 5% is a rough estimate because exact numbers are kept secret. The flights the marshals are on are also closely guarded secrets, so that no potential lawbreakers can plan knowing whether or not there is a marshal.