even more and at best without sick days, without vacations, the air marshals can cover only 5% of flights. the air marshal service tells cnn, additionally high trained officers are deployed on an increasing number of flights worldwide they say to keep air travel safe. >> mostly money down the drain as far as i can see. >> professor john mueller has completed a detailed cost-benefit assessment of aviation security in the u.s. he found of the 20 layers of security used by the tsa to protect air travel, the air marshals have been worthless since 9/11. >> we have seen with the underwear bomber, the passengers are not going to sit around waiting for somebody else to do something, because their lives are at stake and they will jump in. >> reporter: what should be done withir marshals, republican john duncan says to get rid of them.