There are several major exceptions in the assignment of numbers in the six digit numbering system. In the 1960s, there were a block of six-digit numbers beginning with 00 that were assigned to an antisubmarine drone helicopter known as QH-50C and QH-50D. The reason for this odd system seems to have been lost in history. Another major exception is a set of non-sequential numbers beginning with 198003 and ranging up to 999794. These numbers were usually assigned to aircraft that came to the Navy from the Air Force, the Army, or even from foreign organizations. There doesn't seem to be any rhyme or reason to these assignments--in many cases they were derived by modifying the Air Force aircraft numbering system. In other cases, they were constructed out of manufacturer's construction numbers. For example, the Marine Corps F-21As that were leased from Israel in 1987 for dissimilar air combat training were given bureau numbers derived from the aircraft's three-digit construction number prefixed by 999.

The US Air Force has an entirely different serial numbering scheme, based on aircraft procurements within each fiscal year. Occasionally, aircraft are transferred from the USAF to the Navy. If the transfer is anticipated to be permanent, it is usually the case that the transferred aircraft are given brand new Navy bureau numbers. However, if the transfer is anticipated to be only temporary, the original USAF serial numbers are often retained in Navy service, but sometimes it happens that aircraft loaned to the Navy are assigned brand-new bureau numbers. Unfortunately, the system is not always consistent.

Not every assigned bureau number necessarily indicates an actual aircraft in Navy service, because program cancellations often took place before actual delivery. In the pre-war years, assigned bureau numbers that were cancelled before delivery were not reassigned, although this was quite often done with late World War II numbers. There are occasional situations in which a bureau number batch was successively allocated to two or even three separate aircraft orders, only to have them all cancelled before delivery.

A Bureau Number is assigned to an aircraft when it is initially ordered, not when it is actually delivered to the Navy. The total number of serial numbers assigned (by now over 165000) does not reflect the actual number of aircraft delivered, because of large-scale contract cancellations at the end of World War II and other program changes in recent years.

In 1940, so many aircraft were being ordered as a result of President Franklin Roosevelt's rearmament program that it was found necessary to terminate the second series at 7303 in order to avoid confusion with aircraft having late numbers in the original series that were still in service. A third (and final) series of Bureau Numbers was started with 00001 (using 5 digits from the beginning). When the third series reached 99999 in 1945, it was allowed to continue into six digits. This series of numbers continues in the present day. It has now reached over 165000.

In May of 1917, at the time of US entry into WW 1, this was changed to a sequential numerical list, irrespective of type. For some reason, the number 51 was taken as a convenient starting point. The old prefix letters were retained for a short time, but no longer had any relationship to the serial numbers. On May 19, 1917, the prefix letters were officially abandoned and replaced by the single letter A for Aeroplane. These numbers were initially termed "building numbers", but were later termed "designating numbers". In later years, they came to be known as Bureau Numbers (BuNos), since they were assigned by the Navy's Bureau of Aeronautics after that organization was established in 1921. The A-prefix was deleted from the numbers at the end of Fiscal Year 1930 (after OJ2, A-9204).

In March of 1914, this manufacturer-based designation system was abandoned and was replaced by a two-letter type and subtype classification scheme. The first letter was the type or class--A stood for Heavier Than Air, B for Free Balloon, C for Dirigible, and D for Kite Balloon. A second letter was used to designate the subtype--for example AH was a hydro airplane, AB was a flying boat, and AX was an amphibian. All of the surviving aircraft acquired under the original scheme were redesignated and reserialed. A separate sequence of serials was assigned to each type/subtype classification.

The system had to be quickly adapted to accommodate different types of airplanes obtained from the same manufacturer--Curtiss had to be given another letter, C, to distinguish its flying boats from its pontoon seaplanes, the letter D was assigned to Burgess and Curtis (no relation), and E was assigned to Curtiss amphibians.

The United States Navy acquired its first aircraft in 1911, a Curtiss Triad pusher floatplane. Originally, the US Navy assigned serial numbers to its airplanes sequentially by manufacturer, a separate sequence of numbers for each manufacturer. The serial number had two parts. The first part was a letter which designated the manufacturer-- A for Curtiss, B for Wright, etc. The letter was followed by a number to show the sequence of procurement from each manufacturer. For example, the first Curtiss-built Navy airplane was serialed A-1, the second A-2, etc.

In the Navy, these Block Letters or Numbers were affixed to the Bureau Number rather than being attached to the official designation as was the practice in the USAF. However, unlike in the USAF, the Block Letter or Number was most often used strictly on official records and was very rarely painted on the aircraft itself. When the Block Letters or Numbers do appear on the aircraft, they usually appear as a suffix to the the full six-figure BuNo that appears either on the rear fuselage or fin, with the letter or number appearing in smaller characters than the BuNo itself.

In addition to Block Numbers. the Navy also often used Block Letters to denote different production standards. although these don't seem to be related to any USAF-style designation. . There were various letter styles and combination of letters that were used. The differing styles appear to have no particular significance and probably existed because of the lack of any firm USN directive on the subject. Sometimes the sequence started with the letter 'A' or 'a", with the first change in production standard being denoted by 'B', then 'C', etc, until 'Z' was reached. If letters beyond 'Z' were required, it sometimes happened that the letters are started over from 'A', but on other occasions the next change was denoted by 'AA' to 'ZZ' . In order to avoid confusion with the number zero, the letter 'O' is skipped.

Block Numbers normally progressed in increments of 5 starting with -1, then -5, -10 and so on. Intermediate numbers were reserved to denote field modifications carried out after the aircraft's delivery, although the use of these seems to be exclusive to the USAF, and there is no known record of any USN aircraft having such intermediate numbers. Exceptions to the 'plus 5' rule of progression were fairly frequent, the prime examples being the McDonnell Banshee, Demon, and Phantom and the LTV Corsair II, whose Block Numbers progressed in single increments -1. -2, -3 and so on.

In the immediate postwar years, like the USAF, the US Navy/Marine Corps often used a production block system to keep track of minor production line changes that were not deemed sufficiently drastic as to call for a new aircraft configuration sequence number. This system continued after the adoption of the unified designation system in 1962. Just like the Air Force, the Navy/Marine Corps did not use production block designations for all of their aircraft, and there was considerable variation in the systems used from one aircraft type to another.

In the immediate postwar years, a scheme was adopted in which the designation and the BuNo of the aircraft were painted in very small letters on the rear fuselage. Since this number is seldom readily visible at any distance on most Navy and Marine Corps aircraft, the last four numbers of the BuNo were often painted in large characters on the fuselage sides or on the vertical tail, but there were occasions in which the entire BuNo was replicated. This number became known as the modex.

There is some controversy as to the origin of the name "modex". One possibility seems to be as follows. When naval aircraft fly in their own controlled areas, they are identified by their "Call Sign" -Nutgrass 122, Tigertail 012, Black Eagle 601, Lima Whiskey 05, etc. However, in civilian-controlled airspace these call signs mean little and don't meet Air Traffic Control Standards. So, instead, these naval aircraft are identified by NAVY (or MARINE) and the last four digits of the BuNo. Also the transponder identifies (or "Squawks) the aircraft on the Radar screen the same way. Since there are various "Modes" that the Air Traffic Control System uses, we get the name MODEX, plus the last four digits of the BuNo.

In addition, most Navy and Marine Corps aircraft are identified in squadron service and for maintenance purposes by something known as a side number or side letter. The side number is painted on the nose of each aircraft. I am not sure how the side numbers are derived--I have seen one-digit, two-digit, and three-digit displays, but never more than three. The side number sometimes consists of the last three digits of the BuNo, but on other occasions it has nothing to do with the BuNo--each Wing seems to have its own set of numbers and numbering schemes, and their is considerable variation in side number format from one aircraft type to another. In addition, there is a color coding scheme associated with the numbers.

According to one of my sources, the digits in the side numbers beyond the first one are in octal format (the digits going from 1 to 7, then from 10 to 17, 20 to 27, etc), with digits 8 or 9 never being used. This was supposedly done because maintenance actions were originally recorded on punched cards and had to be processed through IBM electronic accounting machines and computers which could only handle octal numbers. Curiously, the side numbers remain in octal format to this day even though computer systems have since been upgraded many times. However, I have seen side numbers beyond the first digit that do indeed have an 8 or a 9, so this restriction must not have been universal.

Another possible explanation for the origin of the octal side-number system is the nature of the early IFF (Identification Friend or Foe) systems. With the arrival of the Navy's E-2A airborne early warning aircraft, the side numbers changed from base-10 to base-8 numbering (e.g 500, 501, 502,..., 507, 510, 511, etc). The E-2As automatic tracking system required such a change, since its computers could only deal with base-8 numbrs. IFF is used by controllers for 'positive control'. There are several modes: two entered by the pilot at the direction of the controller, one for squawking altitude, and one reserved for military use. Early transponders were set by positioning toggle swithces ON or OFF, with there being four columns of 3 switches for a 4096 code or two columns of three for a 64-code. 4096 is the number of different four-digit numbers that can be created without using an 8 or a 9 and including 0000 which should never be used by airplane transponders. In such a scheme, there are 12 binary swiches. IFF can only handle three binary places, thus 8's and 9's are not used. IFF systems at that time (and I think they still do) used the octal system and the highest "squawk" in mode 2 was 7777, mode 1 and 3 was 77. Hence the appearance of octal numbering system on aircraft. There are no doubt a few reasons why octal rather than decimal is used, for one thing it accurately reflects the underlying "bit-oriented" nature of the codes. If decimals were used then for mode "A" (civilian, "3" military) only numbers ranging from 0 through 4096 would be useable, probably harder to understand than the ability to use any 4-digit number (0000-7777). Also, a previous military mode (mode "1") only allowed 6 bit codes (00-77), upward compatilbility is simplified if considered to be a doubling of the number of digits than if it were a case of suddenly being allowed to use numbers in the range 64 to 4096. The latest addition to radar technology, Mode-S, expands the squawk codes from 12 bits to 24 bits thus making it possible to permanently assign codes to individual aircraft without duplication. However, despite being a recent invention these codes are still represented using octal (as 8 digits) or hex (as 6 digits/letters).

The following is the side number scheme for carrier air wings. Generally, the first digit identifies the type of aircraft, and the remaining 2 digits represent the individual aircraft within a particular wing. Carrier-based fighter squadrons typically have three-digit side numbers 1XX or 2XX (both sets of numbers are used if there are two fighter squadrons in the carrier wing). Numbers beginning with 1 are colored red and those beginning with 2 are colored yellow. Usually, the squadron CO's plane is numbered 101 or 201, the executive officer's plane 102 or 202, and subsequent planes numbered in order of pilot seniority. Sometimes, 100 or 200 is used to represent the CAG's (Commander of Air Group) plane, if there is one. I even have heard of a side number of 000, which was supposedly applied to the Admiral's plane. Light attack aircraft have numbers begin with 3 (light green) or 4 (medium blue). Heavy attack aircraft have numbers beginning with 5 which are colored dark green. Fixed wing aircraft belonging to VAW squadrons have numbers 60X that are painted maroon, although they are fairly rarely used. VAQ squadrons used numbers 61X which were painted dark blue in color, but this was changed a few years ago to 5XX, usually starting at 500. The color is usually black, with the exception of CAG aircraft which can be almost any color. VS squadrons use numbers 62X and 63X, with a light blue or dark red color, but S-3 squadrons also used side numbers 70X or 71X with a solid black color. Helicopter squadrons use numbers 70X or 71X, with a green color. Some sources say that 3xx, 8xx, and 9xx are reserved for Fleet Replenishment Groups (RAGs), but this seems not always to have been the case, since at least one A-7 light attach squadron used 3xx. One of my sources has seen a picture of an F-4 with modex 3xx, A-4 with modex 2xx, F-14 with modex 4xx and F-model Super Hornet (which is usually brings the 1xx or 2xx modex) with modex 4xx. Another source said that those numbers are describing the first, second, third, fourth etc unit assigned to an air wing. It may very well be that the side number assignments are not always consiste.t

For example, all F-14 squadron carried either 1XX (and also 2XX if there are 2 F-14 squadrons in one air wing). The XX begins at 01 and increases from there. In some cases, the last 2 digits of this number are also painted on the tail and wings of each aircraft, as well as on the upper trailing edges of the flaps. This helps flight deck personnel on aircraft carriers in identifying different planes on the deck, especially when they are packed close together.

Marine aircraft normally use two-digit side numbers unless they are assigned to a carrier, in which case they adopt a three-digit code as listed above. However, when they are assigned to amphibious assault ships they still use a two-digit scheme.

Land-based Navy aircraft also use side numbers, but these are usually the last three digits of the BuNo. Less predictable were the side numbers assigned to Test Center, Air Station, and ship aircraft. However, ship aircraft usually have side numbers that are the ship's hull number. For example, when the carrier USS John F. Kennedy (CV-67) had a C-1 assigned to it, the aircraft had "67" as the side number.

I would appreciate hearing from anyone who has any additions or corrections to these modex and side numbers.