in addition to interdicting the road and rail system of north vietnam, "linebacker" also systematically attacked its air defense system. the north vietnamese air force, with approximately 200 interceptors, strongly contested these attacks throughout the campaign. navy pilots, employing a mutually supporting "loose deuce" tactical formation and many with topgun training, enjoyed a kill ratio of 6:1 in their favor in may and june, such that after that the north vietnamese rarely engaged them thereafter. in contrast, the air force experienced a 1:1 kill ratio through the first two months of the campaign, as seven of its eventual 24 "linebacker" air-to-air losses occurred without any corresponding north vietnamese loss in a twelve-day period between 24 june and 5 july. air force pilots were hampered by use of the outdated "fluid four" tactical formations (a four-plane, two element formation in which only the leader did the shooting and in which the outside wingmen were vulnerable) dictated by service doctrine. also contributing to the parity was a lack of air combat training against dissimilar aircraft, a deficient early warning system, and ecm pod formations that mandated strict adherence to formation flying. during august, however, the introduction of real-time early warning systems, increased aircrew combat experience, and degraded north vietnamese ground control interception capabilities reversed the trend to a more favorable 4:1 kill ratio.