While air marshals aren't on every flight, they are authorized to fly on planes of any U.S. air carrier, Maryville University says. However, the 2020 GAO report notes that air marshals are not allowed to fly on planes of foreign flagged carriers, or airlines based in other countries.

No, Federal Air Marshals (FAMs) do not fly on every flight; in fact, they cover a very small percentage (estimated under 5%) of total commercial flights. With over 28,000 commercial flights daily in the U.S. and only a few thousand active marshals, a "100% coverage" model would require an agency of over 75,000 agents. Instead, the TSA uses a risk-based "threat matrix" to assign marshals to flights based on factors like the aircraft type, the departure and destination cities, the amount of fuel on board, and current intelligence reports. Air marshals travel in "deep undercover," typically sitting in an aisle seat in the back of the plane or in first class, often boarding last to observe the cabin. Their primary goal is to prevent hijackings and terrorist threats, not to intervene in minor passenger disputes. If you suspect someone is an air marshal, the peer-to-peer advice is to keep it to yourself; their effectiveness depends entirely on their anonymity and the "deterrent effect" of passengers never knowing which flight is protected.