Exceptions to the Tri-Service System

As previously discussed, the Tri-Service System was adopted by the Army, Air Force, and Navy/Marine Corps to provide a common, simpler naming convention for both aircraft and weapons. However, the system has not always been faithfully applied and the two cases you mention represent only a sample of the inconsistencies.

The first case that you mention is the F-19. At least officially, there never was any aircraft designated as the F-19, but there have been persistent rumors that this designation was given to some sort of secret stealth fighter that still remains classified. Or perhaps the designation was set aside for some secret fighter development program that was later cancelled. Another possibility is that the number was purposefully skipped to confuse potential adversaries like the Soviet Union and convince them to waste resources spying on some secret program that never actually existed. A more down-to-earth explanation is that Northrop requested the designation F-20 for its radically updated variant of the F-5 in order to make the aircraft seem more revolutionary. When the F-20 designation was approved by the Air Force, F-19 was skipped. The only other designation previoulsy skipped in the fighter series was F-13, but this is thought to be due to the military's somewhat superstitious nature and the perception that 13 is unlucky.

As for the F-35, the explanation is far less mysterious. The two prototypes developed under the Joint Strike Fighter program were the Boeing X-32 and Lockheed Martin X-35. These aircraft were numbered under the Experimental X-plane series rather than as fighters. However, it was decided at some point that the winner of the JSF contract would be designated by the X-plane number rather than renumbered under the fighter series. Thus, the winning X-35 demonstrator became the F-35 fighter as opposed to being redesignated the F-24, which would have been the next available value. While F-24 might have been a more logical choice, who are we to argue with the Pentagon.

Some other examples of illogical aircraft designations include:

There is also a great deal of confusion in the Trainer series which was not restarted in 1962 but continued to number new planes consecutively under the pre-1962 system until at least the late 1980s. The only exceptions were the Lockeed T-1 SeaStar, a designation applied to a Navy version of the T-33, and the North American T-2 Buckeye. Yet inexplicably, the Trainer series was restrated in the 1990s with the introduction of the Beech T-1 Jayhawk. Yet even more inexplicably, several numbers have been skipped in this new series which includes only the T-1, T-3 Firefly, and T-6 Texan II. The latter case is most likely due to the legacy of the original Texan trainer of World War II fame which was also known as the T-6.

In addition, the Experimental X-plane series has never been restarted. Though the first member of this series, the X-1 predates the 1962 Tri-Service system, subsequent aircraft have continued to be numbered according to the pre-1962 values ever since.

It should also be pointed out that the suffix letters that appear at the end of aircraft designations are often not sequential. For example, certain foreign customers are typically given the same suffix letters even if that is not the next available letter. Examples include the F-4K sold to the United Kingom, the F-15J for Japan, and the F-16I sold to Israel. Many aircraft have also been given unusual suffixes such as the AV-8B+, F-14A+, and F-16CJ. These are typically for unofficial use only.

Finally, for the truly picky, here is the full text of the official Tri-Service MDS system as specified by Air Force Joint Instruction 16-401 of 1 September 1997.
- answer by Jeff Scott, 23 June 2002

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