Aircraft Serial Numbering:

1915-1919
During the First World War, Bulgarian military aircraft were exclusively of German origin (apart from a handful of captured enemy aircraft) and these retained their original serial numbers allocated by the Imperial German Military Aviation Service, eg: C.2362/16 for an Albatros C.III.

1925-1937
During the late 1920s and early 1930s, Bulgarian military aircraft operated clandestinely, and therefore carried civil registrations - B-Bxxx from 1923 and LZ-xxx from 1931.

1937-1968
In 1937 the Bulgarian Air Force began to operate openly. From June 1937, a new aircraft serial numbering system was introduced. This consisted of an aircraft Type Number (7000 onwards) plus an Individual Number allocated to each airframe was it was delivered (from 1). The serial was displayed within the outline of a triangle on the tail fin - with the Individual Number at the top and the Type Number positioned along the base, separated by a horzontal line.

The known type numbers are:

Military Register Number	Aircraft Type	Role	Service Entry
7013	Avia B.122	Trainer	1940
7021	Focke-Wulf Fw 44	Trainer	1938
7022	Kaproni Bulgarski KB-4	Trainer	1938
7024	Kaproni Bulgarski KB-6	Transport	1940
7026	Arado Ar 65	Fighter	1937
7032	Kaproni Bulgarski KB-5	Trainer	1939
7034	LAZ-7	Trainer	1948
7037	Avia B-534	Fighter	1939
7041	Bucker Bu 131	Trainer	1938
7042	Arado Ar 96	Trainer	1940
7047	Messerchmitt Bf 109E	Fighter	1940
7050	Messerchmitt Bf 108	Transport	1940
7051	DAR-9	Trainer	1940
7052?	Junkers Ju 87	Bomber	1942
7056	DAR-3	Trainer	1929
7057	Avia B-135	Fighter	1943
7067	Heinkel He 51	Fighter	1938
7071	Bucker Bu 181	Trainer	1944
7076	Kaproni Bulgarski KB-11	Recce	1940
7077	Dewoitine D.520	Fighter	1943
7086	DAR-10 & DAR-11F	Bomber	1941
7087	Yak-9	Fighter	1945
7096	Arado Ar 196	Seaplane	1943
7097	Bf 109G	Fighter	1944
7107	Yak-23	Fighter	1951
7117	MiG-15/15bis	Fighter	1953
7137	MiG-17PF	Fighter	1955
7138	Lim-5P	Fighter	1959
7139	PZL P.43	Bomber	1938
7151	MiG-15UTI	Trainer	1951
7200	MiG-19S	Fighter	1958
7201	MiG-19P	Fighter	1959
7202	Mi-4	Helicopter	1957
7205	MiG-19PM	Fighter	1960
7206	Il-28	Bomber	1955
7207	MiG-17/17F	Fighter	1961
7209	Aero L-29	Trainer	1964
7266?	MiG-21U	Trainer	1968
7274	MiG-21F-13	Fighter	1963
7276?	MiG-21PF	Fighter	1964
7294	MiG-21PFM	Fighter	1967
?	Aero A.304	Transport	1940
?	Aero MB.200	Bomber	1940
?	Avia B.71	Bomber	1940
?	DAR-8	Transport	1937
?	Dornier Do 11	Bomber	1937
?	Dornier Do 17	Bomber	1940
?	Fieseler Fi 156	Transport	1941
?	Focker-Wulf Fw 56	Trainer	1938
?	Focker-Wulf Fw 58	Trainer	1939
?	Focker-Wulf Fw 189	Recce	1943
?	Heinkel He 42	Seaplane	1938
?	Heinkel He 45	Fighter	1937
?	Heinkel He 60	Seaplane	1942
?	Heinkel He 72	Trainer	1938
?	Heinkel He 111	Transport	1944
?	Ilyushin Il-2	Bomber	1945
?	Junkers Ju 52	Transport	1938
?	Kaproni Bulgarski KB-2A	Trainer	1937
?	Kaproni Bulgarski KB-3	Trainer	1937
?	Letov S.328	Recce	1939
?	Petlyakov Pe-2	Bomber	1946
?	PWS.26	Trainer	1939
?	PZL P.24	Fighter	1938

1968-Present
After 1968, the triangle system changed. The type-number was reduced to only two digits and moved to the upper part of the triangle, with the bottom part taken up by the aircraft's construction number or manufacturers serial. There is normally only space for five or six characters within the triangle base, so the construction number is usually tuncated to the 'last five' significant figures.

The known type numbers are:

Military Register Number	Aircraft Type	Role	Service Entry
69	MiG-21UM	Trainer	1974