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Technical Order, T.O. number, or AN numbers - What they mean and how to find them.

The definition of a TO or AN number is relatively simple, TO stands for Technical Order (or Tech Order), and AN stands for Army-Navy. The “TO” and “AN” on a document is followed by a series of numbers, separated by dashes, or the “document number. These document numbers can be found in the upper right hand corner of every military published manual during WWII. The TO or AN number on a manual contains quite a bit of information, and actually places it with like manuals within the Tech Order System. The Tech Order System was essentially the way that the military organized their library of information (much like the modern day Dewey Decimal system and ISBN numbers).

Tech Order, or T.O. Number on B-17 Table of Credit manual

AN, or Army-Navy number on P-51 pilot's manual

The first mention of a “TO” number was as early as 1919, but it wasn’t until several years later, in the early 1920’s that the beginnings of the document numbers as we know them started to take shape. As time went on, the Technical Order system (or Tech Order system) continued to expand in order to accommodate the massive numbers of manuals and other documents that were being published.

One thing that causes confusion is the terminology between calling something a manual, or Tech Order. The term “Tech Order” or “Technical Order” refers to what I call a “Short Tech Order”. These documents are often between 1 and 10 pages, and deal with very specific maintenance and operational topics related to WWII era aircraft and components. I also occasionally call them Service Bulletins, but only because they are similar in the type of information that they contain. Sometimes, regular manuals (such as parts catalogs or structural repair manuals) are referred to as Tech Orders because they will have a “T.O.” before their document number.

An “AN number” functions in the same way as a Tech Order number, and is the identifier for a document. The “AN” distinction on documents was used from about 1942 to 1953 and was the result of the military combining the organizational systems of both the Army and Navy to make the document numbering system easier, during one of the most chaotic times in American history.

At AirCorps Library we use the Tech Order System to organize all of our documents, and the TO or AN number on each manual is recorded as the documents “part number” of sorts. This means that you can find a manual by searching for it’s document number (if you know it) or, if you find a document by searching for a keyword, you will be able to easily identify the document number associated with that manual. A document number will also help you find related information, as similar manuals have similar AN or TO numbers.

AirCorps Library is that only website that is focused specifically on collecting only aviation related information from WWII. Because of this, it means that we are interested in both high level manuals (like flight manuals and parts catalogs), as well as single page Short Tech Orders, and making all of them available to the mechanics and enthusiasts that need them the most. Others might discard smaller documents because they are “too short” or “too specific”, but when performing maintenance, or working on a restoration, it is these short Tech Orders and service bulletins that often answer crucial questions, and help speed projects along. In short, we have many manuals that others do not!

The chart below is a very simplified and select list of some of the highest levels of the Technical Order System by document number. Even though I have only included several examples from the categories “00-” which is General information, through “07-” which deals with dope, paint, and related materials, there are many more specifics within the TO System, going all the way up to “31-” which deals with boats and marine engines. Notice as the document number progresses, the topics become increasingly specific.

Click on the links for each topic to view the manuals that we have available in each category, or CLICK HERE to get to our home page and try your own custom search!


Initial # ----Secondary # ----Modifier Letter ----Description of Topic ----
Boeing Aircraft
01-20EB-17 Flying Fortress
Curtiss Aircraft
01-25CP-40 Warhawk
Allison Engines
Pratt & Whitney Engines
Lycoming Engines
Jacobs Engines
03-Aircraft and Engine Accessories
Electrical Equipment
03-5AGenerators & Related Equipment
Fuel Systems
Oil Systems
03-15DOil Coolers
Wheels, Brakes, & Struts
03-25ANose & Tail Wheels
03-25CBendix Wheels & Brakes
04-Hardware & Rubber Materials
05-Aircraft Instruments
Airspeed Indicators
Aircraft Compasses
Flight Instruments
05-20GHorizon Indicators
05-20HDirectional Indicators

Dope, Paint, & Related Material

10-Photographic Equipment & Supplies