Two E-2A test machines were modified as E-2C prototypes, the first flying on 20 January 1971. Trials proved satisfactory and the E-2C was ordered into production, the first production machine performed its initial flight on 23 September 1972. The original E-2C, known as Group 0, consisted of 55 aircraft; the first aircraft became operational in 1973 and serving on carriers in the 1980s and 1990s, until they were replaced in first-line service by Group II aircraft. US Navy Reserve used some aircraft for tracking drug smugglers. The type was commonly used in conjunction with Grumman F-14 Tomcat fighters; monitoring airspace and then vectoring Tomcats over the Link-4A datalink to destroy potential threats with long range AIM-54C Phoenix missiles.