The nation's largest airport police association Monday advocated that all law enforcement officers be allowed to carry firearms aboard domestic airline flights. The president of the Los Angeles Airport Peace Officers Association, Marshall McClain, says additional armed, trained law enforcement personnel aboard aircraft is a prudent measure to be taken at this time. The proposal, sent to the White House, asks that all law enforcement officers, active and retired, and certified by the Law Enforcement Officers Safety Act, to carry firearms aboard domestic flights as an added line of defense against terrorists.