The first prototype, acting as an aerodynamic testbed only, flew on 21 October 1960. The first fully equipped aircraft followed it on 19 April 1961 and entered service with the US Navy as the E-2A in January 1964. By 1965 the major development problems delaying the E-2A Hawkeye got so bad that the aircraft was actually cancelled after 59 aircraft had already been built. Particular difficulties were being experienced due to inadequate cooling in the closely packed avionics compartment. Early computer and complex avionics systems generated considerable heat; without proper ventilation this would lead to system failures. These failures continued long after the aircraft entered service and at one point reliability was so bad that the entire fleet of aircraft was grounded.