After the Civil War , in 1921 Soviet government switched from “Military Communism Policy” to much more liberal “New Economic Policy”, allowed some field for private initiative and enterprise. Of course, the most important areas of state economy were kept under governmental control.

Some of state-run enterprises were re-organised as “joint-stock”, or something like. Air communications were proclaimed as a wery important field of economy and all airlines were organised with big governmental participation. In 1923 the first airline, DOBROLET, was started using “free donation of workers and peasants”. DOBROLET began recreational and commertial flights over Moscow and between Moscow and other cities. Two years later, first state-controlled ukranian airline UKRVOZDUKHPUT (Ukrainian Airways - UVP) began commertial flights, and first international airlines DERULUFT (German-Russian Airline) began flights between Moscow and Koenigsberg.

Due to weak domestic aircraft production, these airlines operates mostly foreign-built aircraft, dsuch as Dornier “Komet”, Junkers F-13 and Fokker III. But Soviet government made a strong efforts to stimulate soviet aircraft production. In that time K.A. Kalinin using production facilities of RVZ (Aircraft Repair Workshop) in Kiev built his first passenger aircraft, founding financial and technical support from UVP and government. First three aircraft types (K-1 - K-3) were produced in extremly small numbers, but next typeK-4 was wery successful and was ordered not only by UVP, but by DOBROLET too.

K-4 was a high-wing monoplane, wing and tail plane was elliptical. Fuselage was of wielded steel tube construction without bracing wires, front part was covered with aluminium, aft (behind passenger compartment) - with fabric.. Wing was of wood, with fabric cowering. Aircraft could be powered by wide variety of liquid-cooled engines, rated 240 - 300 h.p. First aircrafts were equipped with BMW-iV, liquid-cooled in-line engine, rated 240 h.p., but later soviet-built M-6 Vee liquid-cooled in-line engine, rated 300 h.p. or Junkers L-5 liquid-cooled in-line engine, rated 300 h.p. were used. Later engine become standart.

All versions had a crew of two : pilot and engineer, seating side-by-side. Normal payload was about 450 kg (1000 ibs).

K-4 was primarely ordered by ukranian government as a paramedical aircraft, carrying two patients on stretched cases and one medician with all necessary equipment. But after the publication of aircraft data, DOBROLET ordered a batch of K-4 aircrafts in the variant of airphoto aircrafts, which was to be equipped with two large hatches in fuselage bottom and two photocameras. Special compartment for camera reloading and film handling was installed. Airphotographjic varian has a crew of three (pilot, engineer and photo operator). Airphoto variant got a highest priority and these aircrafts was first to enter active service.They are widely used for terrain mapping for TURKSIB (Turkestan-Siberain Railroad) construction, and later - for all major construction in Soviet Union in pre-war period.

Paramedic variant (flying ambulance) was second to enter service. One example of K-4 was presented at III Berlin airshow in October, 1928. This aircraft was made exclusively from soviet materials and was powered by M-6 engine. K-4 was awarded with a gold medal as best paramedic aircraft.

Due to limited production facilities, passenger variant was last to enter service. Passenger K-4 started in a strong competition with foreign-built aircrafts, but it looks fawourabli in comparison with Dornier ”Komet”, which were operated by UVP in late 20-th. Soon K-4 in service of UVP were joined by K-4 ordered by DOBROLET. Normal payload of K-4 could be encreased on short routes by replacing of engineer for fifth passenger. Easy to fly, stable and reliable, K-4 was liked by its crews and ground personnel. K-4 were used for recreational flights over Crimean peninsula, for mail and passenger flights over Siberain region. In Central Asia passenger K-4 were oftenly used even for ammunition and food supply routes to isolated garrisons. One aircraft and its crew was lost due to bandits gunfire. K-4 piloted by I.Babushkin were used on the route from Moscow to Tashkent, another K-4, along with other aircraft, served at international route of DOBROLET Kharkiv - Baku - Teheran.

K-4 participate in a number of long-range flights. At May 1-st, 1929, first seven K-4 with 35 young ukranian communists flew from Kharkiv to Tbilisi via Rostov and Mineralnye Vody. This flight was named “air expedition of ukranian youth to Caucasus”. It was “the first in USSR flight of serial-production passenger aircrafts”.

Another long-range flight was carried out at August 21 - September 23 1929. Aircraft, named “Chervona Ukraina” (The Red Ukraine), flew the route Kharkiv - Moscow - Kazan - Kurgan - Novosibirsk - Krasnoyarsk - Irkutsk and back to Kharkiv, covering 10400 km in 73 hours in extremely heavy weather conditions.. Aircraft was piloted by M.A.Snegirev (pilot) and I.T.Spirin (navigator).

K-4 were used for experiments with radionavigation and air-to-groung communications. In 1929 Soviet Airforces tested K-4 in a variant of “flying tanker”, carrying 420 kg (approx. 1000 ibs) of fuel to ground refuelling stations.

K-4 was replaced by more advanced design K-5, ten passenger single-engined airliner, which made its first flight in 1930. But some K-4 remains in active service for a long time. May be the last paramedical K-4 participate in Winter War 1939 -1940, saving a lot of lives of wounded soviet soldiers.

According to recent data, 39 K-4 were built, but no one survived. K.A. Kalinin was arrested and disappeared during Stalin’ “Great Purge”.

FSFS model of K-4 in this package represents K-4 powered by Junkers L-5 engine, in service with DOBROLET, 1930.

V.B. Shavrov “The History of Aircraft Constructions In USSR”. vol. 1, 1985 (in Russian)

V.S.Savin “Kalinin-4 aircraft”. AeroKhobby Magazine, 1994, No 3(7). (in Russian).

Reg D. Davies “Aeroflot : An Airline and Its Aircraft”.