United States Tri-Service aircraft designation system
On the 18 September 1962, the United States Department of Defence introduced a unified designation system for the aircraft of the United States armed forces. Prior to this date, each service used their own nomenclature system. The 1962 system was based on the one used by the US Air Force between 1948 and 1962. Since it was introduced the 1962 system has been modified and updated; in 1997 a revised form of the system was released  (http://www.designation-systems.net/usmilav/afji16-401.pdf). Almost all aircraft operated by the USAF, USN (including the USCG and USMC) and the US Army are assigned a designation under this system. Experimental aircraft operated by manufacturers or NASA are also often assigned numbers in the X-series.
The designation system produces an MDS (or Mission-Design-Series) designation of the form:
Of these components, only the Design Number, Series Letter and Basic Mission are Mandatory. In the case of 'special' vehicles a Vehicle Type symbol must also be included. The options and usage of each designation elements will be discussed below.
The Vehicle Type element is used to designate the type of aerospace craft. Aircraft not in one of the following categories (most fixed-wing aircraft) are not required to carry a type designator. The type categories are:
Interestingly, a UAV Control Segment is not an aircraft, it is the ground control equipment used to command a UAV. Only in recent years has an aircraft been designated as a Spaceplane, the proposed MS-1A.
These prefixes are attached to aircraft not conducting normal operations, such as research, testing and development. The prefixes are:
A temporary special test means the aircraft is intended to return to normal service after the tests are completed, while permanent special test aircraft are not. The Planning code is no longer used but was meant to designate aircraft 'on the drawing board'. For example, using this system an airframe such as the F-13 could have initially been designated as ZF-13 during the design phase, possibly XF-13 if experimental testing was required before building a prototype, the YF-13; the final production model would simply be designated F-13. Continuing the example, some F-13 during their service life may have been used for testing modifications or researching new designs and designated JF-13 or NF-13; finally after (many) years of service, the airframe would be permanently grounded due to safety or economic reasons as GF-13.
All aircraft (special and normal) are to be assigned a basic mission code. In some cases, the basic mission code is replaced by one of the modified mission codes when it is more suitable (eg. MH-53E Pave Low III). The defined codes are:
Of these code series, the no normal aircraft have been assigned a K or R code in a manner conforming to the system. The rise of the multi-role fighter in the decades since the system was introduced have created some confusion about the difference between attack and fighter aircraft. According to this designation system, an attack aircraft is only capable of ground attack missions (eg. the A-6 Intruder and A-10 Thunderbolt II), while a fighter may only possess minor air-to-air combat capabilities (eg. the F-111 Aardvark).
Aircraft which are modified after manufacture or even built for a different mission to the standard airframe of a particular design are assigned a modified mission code. They are:
The Multimission and Utility missions could be considered the same thing, however they are applied to multipurpose aircraft conducting certain categories of mission. M-aircraft conduct combat or special operations while U-aircraft conduct combat support missions, such as transport (e.g. UH-60) and electronic warfare (e.g. UC-12).
According to the designation system, aircraft of a particular vehicle type or basic mission (for manned, fixed-wing, powered aircraft) were to be numbered consecutively. Numbers were not to be assigned to avoid confusion with other letter sequences or to conform with manufacturers' model numbers. Recently this rule has been ignored, and aircraft have received a design number equal to the model number (e.g. the KC-767A) or have kept the design number when they are transferred from one series to another (e.g. the X-35 became the F-35).
Different versions of the same basic aircraft type are to be delineated using a single letter suffix beginning with 'A' and increasing sequentially (skipping 'I' and 'O' to avoid confusion with the numbers '1' and '0'). It is not clear how much modification is required to merit a new series letter, e.g. the F-16C production run has varied extensively over time. The modification of an aircraft to carry out a new mission does not neccessarily require a new suffix (eg. F-111C's modified for reconnaisance are designated RF-111C), but often a new letter is assigned (eg. the UH-60A's modified for Search and Rescue missions are designated HH-60G).
Non-systematic aircraft designations
Since the 1962 system was introduced there have been a number of non-systematic aircraft designations and skipping of design numbers
Further details on the reasons for these designations can be found at Andreas Parsch's non-standard designation (http://www.designation-systems.net/usmilav/nonstandard-mds.html) page.
Skipped design numbers
The design number '13' has been skipped in many mission and vehicle series for superstitious reasons. Some numbers were skipped when a number was requested and/or assigned to a project but the aircraft was never built. More information on the reasons behind the apparent skipping of design numbers can be found at Andreas Parsch's "Missing" USAF/DOD Aircraft Designations (http://www.designation-systems.net/usmilav/missing-mds.html) page.
The following design numbers in the 1962 system have been skipped:
Other Designation Systems