In Portugal to identify individual aircraft, all military aircraft are allocated and display a serial number. A common serial number system is used for aircraft operated by the three military branches, the Portuguese Air Force, Portuguese Naval Aviation and Portuguese Army. Individual agencies have each their own system.
Some aircraft that were acquired and offered to Portugal, even before the organization of the military aviation as an entity, never received serials, being only identified by their own names.
History
Army Aviation
Sequential system by aircraft type
In 1914, with the foundation of the Escola de Aeronáutica Militar (School of Military Aeronautic) a serial sequential system by aircraft type was implemented. This system consisted in the assignment of a serial number to each aircraft of a certain type. For example, the Caudron G.III received serials from 1, 2, 3 and so on, as well the Farman MF-11 received the designation of MF11-1, MF11-2, and MF11-3. Aircraft that were the single existing example didn't receive serials, i.e. Morane-Saulnier Type H and Breguet 16Bn-2.
Block numbering system
In 1935, with the Administrative Regulation 172 of March 16, 1935 of the 3rd Division of the Direcção da Arma de Aeronáutica (Directorate of the Army Aviation), serials began to be assigned in blocks of numbers with three digits each. The blocks were non-contiguous and were related with the airfield where the aircraft were stationed.
However, this system was not well understood by some air bases which thought that a certain group of serials had been assigned to the airfield and not to the aircraft. For example, on November 12, 1935 when the Moranes 151, 152, 192 and 194 were transferred to the Tancos airfield, they were given at that location the serials respectively 185,186 183 and 184. On the same date the Moranes 183 and 184, which had been based at Tancos were transferred to Sintra where they maintained these same serial numbers. This lead that during 1936 and 1937 four aircraft used simultaneously two serials.
Also there were cases where two different types of aircraft were assigned the same block of serial numbers. This was the case of the Tiger Moth and Junkers Ju 52, both using the 100 to 110 block, and the range 125 to 137 being used simultaneously by the Avro 626 and Tiger Moth.
However, even with this mistakes this system wasn't immediately applied everywhere, as in October 1935, there were still aircraft flying with the previous sequence system.
2nd block numbering system version
After the military aeronautic restructuring of 1937, a new block numbering system was implemented with the Administrative Regulation 642 of June 3, 1938. This new system implemented the same block numbering system as the previous one, but more clearly defined, in which blocks of 50 serial numbers were assigned to each of the eight aircraft types in service. The blocks ranged from 101 to 450, being a last block of 501 to 549 assigned to special aircraft, either because they were bought for test or were single example of a certain type.
Also it was stated that the changes should be implemented as soon as possible, and that aircraft should maintain their serial numbers even when transferred to other bases.
During World War II a large number of aircraft was interned and incorporated in the Aeronáutica Militar, forcing the use of serial numbers that had been assigned to aircraft meanwhile retired. This resulted in assignment of blocks starting to 510 and 600 to the Hawker Hurricanes. The 300 block series used before by the Potez 25 (retired) was reassigned to the new Supermarine Spitfire I, P-38 Lightning and P-39 Airacobra. However, the Supermarine Spitfire V received still during the war and afterwards, was given as an oddity the serial number block 1 to 95.
The 700 block series was assigned to the AT-6 and Harvards that were received from 1947. The Airspeed Oxford was assigned the 500 block series which was also used by the de Havilland DH-84 Dragon (504 to 506).
Alpha-numeric system
A special alpha-number serial system was assigned during World War II to some aircraft that were forced to land in Portugal and were ultimately interned and later incorporated in the Aeronáutica Militar. This system was used to six Consolidated Liberators, which were given the serials L-1 to L-6; the Lockheed Hudson, H-1 to H-3; and the Douglas C-47 Dakota (D-1).