The North Vietnamese Air Force began receiving the MiG-19 at the end of Operation Rolling Thunder, which ended in 1968. Despite their limited numbers, MiG-19s were involved in extensive combat during Operations Linebacker 1 and Linebacker 2 (a. k. a. the Christmas Bombing). The NVAF claimed only seven victories over U. S. aircraft, using the MiG-19, all of which were F-4 Phantom IIs. The MiG-19 was tested by U. S. pilots in the United States in 1969 after receiving a Chinese J-6 (F-6 exported model) from Pakistan. [N 1] In addition to finding the aircraft to have a good canopy allowing good visibility for the pilot, along with 3 hard hitting 30mm cannons, U. S. pilots found the MiG-19 (J6/F6) to be an excellent fighter, "like the MiG-17, it could easily out-turn the Phantom. . . and could out-accelerate the F-4 out to Mach 1. 2, but was slower than the MiG-21. ". However, the MiG-19's strongest fault was its extremely short range, as one U. S. test pilot remarked, "after going in full after-burner at low altitude for 5 minutes, the MiG driver will be looking for a place to land!" This, combined with the aircraft's twin engines, which were difficult to maintain, made the MiG-19 unpopular with North Vietnamese pilots.