A cover letter is just why you feel you would best be fit for the job compared to everyone else. Resume shows past experience that your employer can benefit from, a cover letter tells the employer how they will benefit from you after hiring you. Talk about why you are applying for airport security and not something else, do you have a lot of experience with handling people diplomatically or, if you've had to, forcibly? Find out the shortcomings that your resume has and try to help your employer fill in the links between the resume and how it applies to him hiring you. Maybe you had some previous experience where you outran a purse snatcher and took him down. Thats not usually in a resume but it'd be something your future employer would be interested in hearing about. You should be highlighting traits such as athleticism and people skills in your cover letter, things that are important in a security position, as well as the kind of employee-loyalty and work ethic that any employer would want to hear about.