There are not only passengers and shippers who can threaten aviation safety. The workforce in the airport such as, aircraft cleaning, crews, food service personnel, airline employees and airport merchants, are in a position to help smuggle explosives aboard an aircraft. Individuals inside the airport can unleash terror against passengers, as can vehicles near the terminal laden with explosives. Surface-to-air missiles can shoot passenger aircraft out of the sky. Various measurements have been proposed or implemented against such threats. Guarding against truck bombs, physical barriers have been erected outside some airport terminals. Prohibit parking, sometimes, within a certain distance from terminal. As well as, the interiors of vehicles are sometimes inspected and unaccompanied vehicles are almost never allowed to stand idle outside terminal building.