Decipher Air Force tail codes for me

I believe that number is the fiscal year of procurement

I just quick googled US Air Force  Aircraft serial numbers and one of the first hits is www.joebaugher.com/usaf_serials/usafserials.html.  That says that is the year the plane was ordered not necessarily delivered.  System has been in place since 1921.

...also the first 2 numbers in the serial number. You may already know this, but there was a website that listed tail letter codes as well, and which base they belonged to. Such as "SP" for Spangdahlem, and "SJ", for Seymour Johnson. I can't find the site right now...

in your hypothetical tail code the AF 78 refers to the fiscal year in which the aircraft was ordered. The 140 is the last 3 digits of the aircraft's serial number.  Two letter tail codes are a system that have evolved over time, with much variation in the early years of usage, while the serial numbers have been pretty much the same since they replaced the "Buzz Numbers" used on high visibility marked aircraft up until early in the Vietnam War.

This system only applies to USAF aircraft. USN and USMC aircraft have Bureau Numbers assigned by USN Bureau of Aeronautics. And their tail codes are assigned by air wing and/or geographical location.

US Army aircraft have  their serial number system with no tail codes.

The AF uses a 3 or 4 digit serial number in the modern era, so the example number would be 78-140 or 0140 on the aircraft. There is a lot of latitude depending on the squadron for the serial number display on the fin.  It seems that some  fighters use a full FY designator with a 4 digit number like a F-15E #90-0233. Others like the Arctic Flanker F-16C's of the 18th AGSRS of the 354th FW use 3 digits. Large aircraft like the C-5B can be seen using the single FY designator with a 4 digit number.  For example, C-5B #87-0039 has 70039 on the tail.  So it is a good idea to check your photographic sources as there is a lot of variation out there.  It used to be that the Federal Fiscal Year appropriations process started at July 1st of the previous year, so the FY of 1941 would have started on July 1, 1940 so the aircraft were purchased after July 1st to June 30th.  This was later changed to October 1st in the 60-70's I think.  For USAF tailcodes, try the USAF website or the USAF Museum.

One other variation you will see is when two or three digits in the serial number, and not necessarily the last two or three digits, correspond with the Squadron or Wing that currently has the aircraft assigned. In those cases, you sometimes see xx TFW/FW or xx TFS/FS in large letters and numbers while the remainder of the serial is in smaller numbers. These usually tend to end up as unit commander's aircraft while assigned to the unit, but will revert to normal when the airframe is transferred to another unit.

You guys are great. Thank you!

Normally the Wing "king" gets the pick of the litter for his aircraft (best paint finish, best maintenance records which means not being a hangar queen waiting for parts,etc).  Here is a good example of the example of the serial  number being used for the 18th FS commander's bird.  I was already retired so I didn't get to see the F-16, #90-0718.

http://www.ar15.com/archive/topic.html?b=1&f=5&t=590174

And here is the 354th FW CC's bird:

http://www.f-16.net/forum/download/file.php?id=9903&sid=dcb225ca35da45bb07ee9fbdb806c073&mode=view

Thank you Seasick for the Navy side of things which makes this a well-rounded, highly informational thread for people to use.

Having never been in service , I just learned a ton. Thanks guys.