Ilyushin Il-18I

The Il-18I was the experimental link between the first-generation Ilyushin Il-18 passenger versions (A, B and V) and the last two production variants, the Il-18E and the Il-18D. Its designation appears in almost every reference source, but the aircraft is described unevenly: Western encyclopaedias present it as a "version" fitted with more powerful Ivchenko AI-20M turboprops and an enlarged cabin for 122 passengers, whereas Russian literature treats it as a test aircraft that never reached the assembly line. Both readings are compatible if the Il-18I is understood as the development prototype from which the Il-18E and Il-18D were derived.

In the early 1960s, when the Il-18V had just become Aeroflot's standard airliner, specialists at Ilyushin's OKB-240 proposed reworking that model for non-stop flights of over 5000 km. The answer was an additional 6300-litre integral tank in the wing centre section, combined with a higher-grade interior finish modelled on the official "salon" versions. The project also provided for cabin ventilation on the ground from external units with engines running, life rafts and on-board refrigerators.

According to Russian sources, factory testing of the second experimental aircraft, now designated Il-18I, began in December 1960. The most visible change was a 1.64 m fuselage extension that allowed up to 125 seats; English-language sources describe the same change as moving the aft pressure bulkhead rearwards by 1.64 m, giving 122 seats in a high-density layout. With the additional tanks the maximum range reached 6500 km. The aircraft passed trials at the state civil aviation institute (GosNII GVF) and in May 1962 was submitted for acceptance testing. The civil air fleet judged that it could be operated on the routes of the Soviet Far North and on the Moscow–Jakarta link, and flights to other distant countries were not ruled out.

The Il-18I was not produced in series as such. Its direct development was the Il-18E, which kept the prototype's airframe and its lighter, more modern equipment — including new seats and an improved air-conditioning system for hot climates — but without the extra fuel capacity; 23 were built in 1965-1966, initially with AI-20K engines and re-engined with the AI-20M during overhauls. The arrival of the AI-20M turboprop, with 4250 ehp for take-off (3169 kW) maintained up to +30 °C, was what allowed the Il-18's maximum take-off weight to rise from 61.2 to 64 t, the centre-section space to be filled with the integral tank, and the long-range Il-18D to appear, entering passenger service in early 1966. The new engine was accompanied by the Put-4M navigation system, with semi-automatic control, and the BSU-3P approach system. The German Wikipedia summarises this lineage by stating that the Il-18D was "the version originally designated Il-18I", produced from 1964; airwar.ru puts it the other way round: the Il-18D is "the production variant of the Il-18I" and the Il-18E "the production variant of the Il-18I without the fuel increase".

The figures that sources assign to the Il-18I match those of the production Il-18D: four Ivchenko AI-20M of 3169 kW (4250 shp), 122 passengers in summer — when the rear coat closet, essential in Russian winters, was removed and extra seat rows fitted — and a maximum range of 6500 km with the centre tank. The 125-seat figure for the stretched prototype comes from Russian historiography; the 122-seat figure from Western sources and airwar.ru, and no prototype-specific length different from the production 35.9 m is recorded.

The passenger lineage thus ran: Il-18A (first series, 75 seats), Il-18B (84 seats), Il-18V (Aeroflot standard from 1961, 90-100 seats, AI-20K engines), Il-18I (prototype of the stretched AI-20M generation) and, derived from it, the Il-18E (1965-1966, 23 built) and Il-18D (1965-1969, 122 built, the last delivered to Aeroflot in January 1969). With the Il-18D in service came the non-stop Moscow–Magadan flight (6270 km, October 1967) and the women's distance record Simferopol–Sakhalin (7661 km). The Il-18D also served as the basis for the military Il-20, Il-22 and Il-38 derivatives, so the work begun with the Il-18I ended up extending the life of the design far beyond its airline career.