Yes indeed, the BuAer numbers, C/Ns, and S/Ns can become confusing at times, especially when you are trying to track down exactly which Model 75 Stearman you own, only to find that someone has removed all the data plates, even to the extent of making up their own serial number and filing it as the official serial number for that airplane. It almost seems as though someone who does this is trying to destroy the history of the Model 75 airplane, however, there is still 99 chances out of 100 that there still remains on the airplane evidence that will give you the information.

Perhaps first of all we should look into how the BuAer numbers, C/Ns and S/Ns situation came into being.

As the Model 75 Stearman airplane contracts were granted by the U.S. Government to the Stearman Aircraft Co. and the Boeing Airplane Co. at Wichita, Kansas, the company assigned a manufacture’s serial number (75-????). But the military, in order to avoid confusion, would list the manufacture’s serial number as the company number or C/N (also referred to as the Construction Number by some) and the military would assign its serial number to the airplane.

At this point it might be noted that today on official Boeing Airplane Co. records at Wichita, Kansas, the Model 75 airplane records still list the 75-???? number as the manufacture’s serial number, while the military records still list it as the company number, C/N.

The U.S. Army’s method of assigning serial numbers, S/N, tied in the fiscal governmental year as well as reflecting how many U.S. Army airplanes had been purchased during that time frame.  US Army S/N 41-25801 would show the air plane as a PT-17, with the contract procurement being signed for by the U.S. Government and Boeing in the 1941 fiscal year (1 July, 1940 – 30 June, 1941) and would identify this PT-17 as being the 25,801st U.S. Army airplane procured during the 1941 fiscal year. The U.S. Army S/N had nothing to do with delivery date as such. S/N 41-25801 was delivered to the U.S. Army in July, 1942.

The model number and S/N would appear on the left side of the fuselage under the cabane struts and would read:

U.S. ARMY PT-l7, AIR CORPS SERIAL NO. 41-25801

While a minor detail, yet an interesting item, is why this airplane is listed as “Air Corps” on the technical data legend block. The U.S. Army Air Corps service was established 2 July, 1926. However, on 20 June, 1941 it was changed to U.S. Army Air Forces. Until the last PT-13D rolled out of Boeing at Wichita in February, 1945, the technical data legend continued to list the U.S. Army Air Forces as the US. Army Air Corps. On 18 Sept., 1947 the U.S. Army Air Forces became the U.S. Air Force.

It seems as though more Model 75 owners have problems with the U.S. Navy’s method of handling serial numbers, which are known as BuAer Numbers. During the time span of August, 1940, when the U.S. Navy received its first Model 75 trainers, the N2S-1, until the last of the N2S-5s went to the Navy, the U.S. Navy was sectioned off into bureaus. There was the Bureau of Ships, the Bureau of Yards and Docks, to mention a couple, and, yep, you guessed it! The Bureau of Aeronautics.

The Bureau of Aeronautics handled the Navy’s airplane serial numbers which were known as Bureau of Aeronautics Numbers and in U.S Navy code appeared as BuAerNo, hence C/N 75-1171, a N2S-l, would be listed with the Navy as BuAerNo 3394 on Navy records.

Today the Navy Bureau of Aeronautics has been re-designated as the Naval Air Systems Command, however, the Naval Air Systems Command still assigns the U.S. Navy airplane serial numbers as BuAerNos. (Thanks goodness some old traditions still exist and are hard to break!)

Now that we know about BuAerNos, C/Ns and S/Ns, let us find out how we can find these numbers on the Model 75 airplane. But perhaps first we might look at some of the most common pitfalls of this.

The Stearman Aircraft Co. and the Boeing Airplane Co. belonged to the Manufacturers Aircraft Association, which was an association which decided how the aviation industry would standardize on different items. Plus the M.A.A. was heavily involved in lobbying on behalf of the aviation companies.

On the Model 75 trainer, a M.A.A. tag, sometimes an oval shape, sometimes a diamond shape, was attached to the landing gear sponson horizontal tube, (part number 75-2624) or to the front lower horizontal tubing of the fuselage frame assembly. Usually this M.A.A. tag will have engraved in it, “S/N 3250” and the owner thinks he has serial number 3250 which normally would be C/N 75-1027, a U.S. Navy N2S-1. But sad to say, “It just ain’t so!”

In this case, the M.A.A. tag has listed the sales order number (S/N) and sales order number 3250 had the most Model 75s sold through it to the military. Hence more M.A.A. tags have S/N 3250 on them and the M.A.A. tag sort of becomes as fool’s gold to the owner/researcher.

Data plates were installed on every Model 75 and there is engraved the C/N and the S/N or BuAerNo. The data plates were installed at four different locations on various Model 75 Stearmans and at three locations you cannot miss them.

On some Model 75s, the data plate was located on the rear instrument panel base, right hand corner, or on the left hand or right hand corner of the rear cockpit cowl bulkhead assembly. (part number 75-2368)

Another place the data plate was attached, particularly on Navy Model 75s, was on the rear cockpit elevator tab control assembly. But unless the Model 75 owner has the fabric off of his airplane, it almost requires one to stand on his/her head to read this data plate.

Over the years data plates off any type airplane, especially Model 75s, have become the target of collectors, plus due to wear and tear, the ones mounted on the rear instrument panel, rear bulkhead, have been lost. Again there are those people who take the data plates off in order to file their own made up serial number with the FAA. Normally the FAA lists the C/N or the manufacture’s serial number, 75-????, as the official civil serial number for

the Model 75 Stearman.

Now that you have looked for these data plates and none can be found on your Model 75 airplane, plus the FAA records show your serial number as XYZ-13, is there anywhere on the airplane that you can still locate the C/N?

YES!

As the Stearman Aircraft Co. and the Boeing Airplane Co. manufactured the fuselage frame assembly of the Model 75, the C/N 75-???? was stamped into the mid-section area of the fuselage frame assembly. The C/N was engraved in the diagonal tubing that supports the rear cockpit control stick socket assembly. The C/N was stamped on the bottom side of this diagonal tubing, usually on the right hand side tube. (see drawing)

Quite often the C/N was stamped very lightly and primer paint filled the engraving, however, a little light sanding should reveal the C/N. Once the C/N is found, the Model 75 owner can find the BuAer No., S/N and the military model number from the Model 75 BuAer No, C/N, S/N model listing.

Another related subject matter that fits into this article is the often asked question, “How many Model 75 airplanes were built?”. As all SRA members are St. Louis Cardinal fans and are used to box scores, to a baseball fan the answer would appear as:

  • X-75     1 each
  • Static Test Airplane         1 each
  • Customer Assembled Airplanes     8,416
  • Spare parts, production equivalent airplanes      1,918
  • TOTAL       10,346

The box score of Model 75 airplanes requires a couple of explanations. The static test Model 75 airplane that went to Wright Field, Dayton, Ohio, was a bare frame, no covering. It carried no C/N and although Boeing Airplane Co. records show it as U.S. Army S/N AC-36-1. This is in error. S/N AC-36-1 was assigned to a Douglas C-32 and no other record yet recovered shows any S/N being assigned to this static test Model 75 airplane. What ever happened to this static test airplane remains a mystery. It met all tests and was not destroyed, but was never returned to the Stearman Aircraft Co. at Wichita, Kansas.

The X75 was assigned manufacture’s serial number 75-000. It might seem as though it should have been C/N 75-001, but Stearman Aircraft Co. records show otherwise. The X75 was sold to the Aviation Manufacturing Corporation.