Double Bar. The double bar symbol is used to denote certain kinds of norms in mathematics (e.g., or). It is also used to denote parallel lines, as in, and in an older notation for the covariant derivative. In this work, the notation or is reserved for matrix and function norms, respectively.
The F-4 Phantom II remained in use by the U.S. in the reconnaissance and Wild Weasel (Suppression of Enemy Air Defenses) roles in the 1991 Gulf War, finally leaving service in 1996. It was also the only aircraft used by both U.S. flight demonstration teams: the USAF Thunderbirds (F-4E) and the US Navy Blue Angels (F-4J).
After twenty-one years and 85,000 flight hours, VMFA-251 flew its last F-4 in November 1985. In January 1986, the Thunderbolts transitioned to the F/A-18 âHornetâ. The squadron stood up as the sixth Marine F/A-18A squadron the following August, and reported full combat ready in March 1987.