Air-to-air combat testing suggested "The Six" was a reasonable match for the F-4 Phantom II in a dogfight, with superior high-altitude turn performance and overall maneuverability (aided by the aircraft's lower wing loading). However, the Phantom had better radar – operated by an additional crewman – and could carry a load of up to four radar-guided AIM-7 Sparrow and four infrared AIM-9 Sidewinder missiles, while the AIM-4 Falcon missiles carried by the F-106 proved a disappointment for dogfighting over Vietnam. The F-4 had a higher thrust/weight ratio with superior climb, better high speed/low-altitude maneuverability, and could be used as a fighter-bomber. Air combat experience over Vietnam showed the need for increased pilot visibility and the utility of a built-in gun, which had been added to the "E" variant of USAF Phantoms.