To preclude compromising or embarrassing the United States Government in the event the aircraft were to be hijacked to an unfriendly country, a Service employee shall avoid carrying with him, on his person or in his luggage, any classified document or any Service manual, handbook, report, or file; whenever possible, such material needed for an assignment requiring air travel shall be sent ahead by mail. In every case, all doubts must be resolved in favor of security of Service material.