Cover Designations for Classified USAF Aircraft
Copyright © 2004-2012 Andreas Parsch
In the late 1960s the U.S. Air Force began to acquire Soviet fighter aircraft for the purpose of evaluation and air-to-air combat training. The acquistion and use of these aircraft was then highly secret, and operations were flown from the test facility at Groom Lake, Nevada (a.k.a. "Area 51"). The aircraft types flown included the MiG-17 and MiG-21 initially, and later also other MiGs (e.g. MiG-23) and presumably some Sukhoi types.
Just like any other USAF pilots, the men flying the secret aircraft had to log their flight hours in standard USAF forms (specifically, the "Form 5" flight records). These forms required the entry of the aircraft type flown, but of course you couldn't simply write "MiG-21" into that line! At some time early in the HAVE DOUGHNUT program (which evaluated the USAF's first MiG-21), someone had the idea to use "fake" cover designations, which looked like ordinary Air Force designations, but which were not used by any actual aircraft. It was decided to continue the old F-for-Fighter series of the USAF, which had reached F-111, but had been discontinued in 1962 when the DOD introduced a new joint designation system. Therefore, the numbers used were 110 (the original F-110A designation had been replaced by F-4C, so that -110B, -110C etc. were de facto unused), 112, 113, etc. All designations were prefixed by a "Y", indicating a test model. Over the years, the system of assigning YF-1xx designations was extended to U.S.-built secret aircraft as well.
Originally, the designations' suffix letters were not only used to designate different versions of the same basic aircraft type (as in standard USAF designations), but also to distinguish between individual aircraft of the same type (e.g. YF-113A, YF-113C and YF-114C were all MiG-17Fs). When the USAF's Form 5 was amended to allow the entry of the tail number of the aircraft flown, use of suffix letters was reportedly changed to the more conventional "one suffix per subtype".
Other than for standard designations, the numbers and suffix letters were not allocated in sequence, but more randomly. It was also not uncommon to use one number for several basic aircraft types, see e.g. 113 and 117 in listing below. Finally, the "F" or "YF" prefix doesn't mean "Fighter" as in normal designations (at least not anymore). Evidence for this is the TACIT BLUE demonstrator, which had a YF designation, but wasn't designed in any way to be a fighter. All said, it makes sense to hide the number of secret projects, as well as the mission or type of a certain project, by simply assigning the same designation prefix and a non-sequential number to all aircraft.
From the time frame of the YF-113G, it is clear that the system was still in use in the mid-1990s. It is not known what kind of "cover designations" (if any) are used nowadays, but the leaked YF-24 designator (used in the late 1990s and/or early 2000s) seems to indicate a deviation from the old "YF-1xx" scheme.
Apart from fake designations, fake USAF serial numbers were reportedly used as well. The latter are said to include 75-001, 75-004, 75-008 and 75-010, which are all officially listed as "cancelled serials".
Note: Although none of the above is officially acknowledged by the USAF, it isn't explicitly denied either, and there is enough evidence available to ascertain that the information given above is essentially correct. For example, the author's FOIA request in 2003 to the AFFTC (Air Force Flight Test Center) at Edwards AFB for "YF-xxx" designations for revealed projects (e.g. HAVE BLUE, TACIT BLUE, "Bird of Prey") did not return with a "No records found" response. Instead, records were forwarded by AFFTC's 412th TFW to the Pentagon office SAF/AAZ (Secretary of the Air Force, Directory of Security and Special Programs Oversight). Although the records were denied in full, their existence was not denied.
Even more importantly, the declassification of the Air Force's CONSTANT PEG program in 2006 has allowed former pilots of the USAF's "Red Eagles" and "Red Hats" squadrons (which flew the MiGs) to reveal first-hand information (presented in source  in list of references below).
Because the USAF so far doesn't officially acknowledge the use of said cover designations, all numbers presented here are technically rumours only. However, several numbers have been forwarded by reliable sources, and there can be little doubt that they are correct. This is especially true for data in source .
The "References" column indicates the sources of the alleged designations:
 Steve Davies: "Red Eagles. America's Secret MiGs", Osprey Publishing, 2008
 Peter Merlin. Peter is an aviation historian with some detail knowledge about operations at Groom Lake.
 Aviation Week & Space Technology. In 2000 and 2001, there were a few short notes in AW&ST about the alleged YF-113G aircraft. These notes included some information on the use of YF-1xx designations.
 Official USAF biography of Col. Joseph A. Lanni. At the time of this writing, it is not available online, but an archived version can be retrieved from the Internet Archive.
 Mr.Z. Z (real name withheld as requested) is a person, who has provided some information as well as the background story how he acquired his data (this story explains why he doesn't know the actual aircraft type associated with each designation). He asked not to reveal his name or background.
 Joseph Jones. Author and (self-proclaimed) "insider" on secret aircraft projects.
Designations shown in black can be regarded as identified with a reasonable degree of reliability, while those in grey are less certain.
|YF-110B||[1,2,5]||MiG-21F-13 "Fishbed-C/E".  quotes the serials 75-001, 75-004 and 75-010 for this type.|
|YF-110C||||Chengdu J-7B (MiG-21F-13 variant built in China)|
|YF-110E||||Designation used for unknown (probably foreign) type in the 1987/1995 time frame|
|YF-110L||||Designation used for unknown (probably foreign) type in the 1987/1995 time frame|
|YF-110M||||Designation used for unknown (probably foreign) type in the 1987/1995 time frame|
|YF-112||[1,2,3]||This is mentioned as "used for a MiG". It is likely that this remark only refers to the general use of "YF-112" for ex-Soviet aircraft, and not a specific type. Therefore a plain YF-112 designation (without suffix) probably didn't exist. Suggested candidates for a YF-112 designation include a Sukhoi Su-22 and (less likely) a MiG-19.|
|YF-112C||[2,5]||This might be the actual complete designation of the aircraft associated with the generic YF-112 code.|
|YF-113|||| explicitly claims the use of a plain (no suffix) YF-113 designation. However, this contradicts source , where all YF-113s are quoted with a suffix letter. Regarding all available information, the use of plain YF-113 designation for a specific aircraft appears unlikely.|
|YF-113A||||MiG-17F "Fresco-C" used in HAVE DRILL program|
|YF-113C||||MiG-17F (actually a Chinese-built J-5) "Fresco-C" used in HAVE PRIVILEGE program|
|YF-113C||||Designation reused for unknown (probably foreign) type in the 1987/1995 time frame|
|YF-113G||[2,3]||Possibly a secret USAF prototype developed and flown in the 1993/96 timeframe.
There was much speculation about this designation, and it was also said that the YF-113G was a MiG-23 (of unknown subtype).
However, the identification of the YF-113G as a U.S. prototype was made by reliable sources quoting a YF-113G pilot's official
biography, and must therefore be regarded as confirmed. The MiG-23 story apparently originated from confusion with other YF-113
designations (i.e. YF-113B/E).
Update: Recent analysis by  appears to indicate the possibility, that the identification of the YF-113G as a US prototype might actually be incorrect. However, this is far from certain.
|YF-113H||||Designation used for unknown (probably foreign) type in the 1987/1995 time frame|
|YF-114C||||MiG-17F "Fresco-C" (incl. the one used in the HAVE FERRY program)|
|YF-114D||[1,5]||MiG-17PF "Fresco-D".  quotes the serial 75-008 as used by a YF-114D.|
|YF-116A||||Designation used for unknown (probably foreign) type in the 1987/1995 time frame|
|YF-117A||(multiple)||Lockheed SENIOR TREND stealth fighter. This designation was eventually allocated as the official designation when the aircraft was revealed to the public, thus creating the only official post-111 F-designator.|
|YF-117D||||Northrop TACIT BLUE|
|YF-117E||||Reported to be unrelated to the F-117A.|
|YF-118G||||Boeing Bird of Prey|
|F-121||||This is claimed to be the designation for a pure-delta stealthy high-speed reconnaissance aircraft developed by General Dynamics (now Lockheed Martin). For details about this aircraft, see this page (based on info from ). However, it must be noted that the story of this aircraft and its F-121 designator is questionable at best.|
|YF-24||||Classified prototype, late 1990s and/or early 2000s. The YF-24 designation is a notable exception to the YF-1xx scheme, but the number 24 is apparently no coincidence since it follows on from the last known "official" F-series designator, YF-23A.|
One known formerly classified aircraft, which is missing from the list, is the Lockheed HAVE BLUE stealth demonstrator. However, source  says that this aircraft never received a YF-1xx style designation.
There are also reports which link the F-116 and F-118 designations to the MiG-25 and MiG-29, respectively. But these reports are rather vague rumours which do not claim any "first hand source" as a reference, and are therefore not included in the above listing.
Comments and corrections to: Andreas Parsch
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