Pilot's Post - Antonov Design Bureau - The Big, The Heavy and The Bold

03.04.2022

Introduced in 1954, the IL-14 was the Soviet Union's main passenger aircraft and was made as a replacement for the Li-2 (Soviet DC-3) mentioned in the previous article. It was designed to meet both military and commercial requirements just like the An-8, which was also expecting to replace the Li-2, but unfortunately it did not.

Aeroflot Ilyushin Il-14. Photo: © Lars Söderström, CC BY-SA 3.0, via Wikimedia Commons.

Modernisation
With the never-slowing advancement of technology and engineering, the fleet of Il-14s was ageing rapidly. To modernize the Aeroflot fleet, the USSR Council of Ministers released Enactment No. 1417-656 on the 18th of December 1957. This specified the development of an airplane with a payload of 4000 kg, a flight range of up to 400 km and a cruising speed of 450 km/h (242 Kts). Its design had to include, in typical Soviet fashion, the ability to operate from small unpaved airfields and that flight characteristics and power plants should be such that it could be used between points with considerable variations in altitude and/or temperature.

Without hesitation, the Antonov OKB initiated development and came up with a 32 to 40 seat semi-monocoque structure with high mounted wings and 2 turboprops mounted in pods beneath the wings. The pressurized and climate-controlled fuselage was 23m long and slender with a rounded nose featuring a stepped cockpit in the front. It had a wingspan of 29m and a back-tapered fin with angular fairings on the underside of the tail. The Antonov OKB's real ingenuity was however shown not in the simple and effective exterior design, but in the unnoticeable internal design, such as being able to adjust tire pressures in flight or on the ground, allowing operation on a variety of different surfaces. It also boasted an inbuilt TG-16M Auxiliary Power Unit (APU) to provide ground power which meant the An-24 needed minimal ground equipment, allowing it to fly to the remotest of areas. The most impressive part of the design was the way the aircraft was constructed and which used modular sub-assemblies of chemically milled aircraft aluminium and which permitted better control over the thickness of skin panels. For the first time in the world, the use of adhesive bonded/welded joints was introduced, instead of traditional riveting. This reduced labour-intensity in aircraft manufacturing and improved structural strength and durability.

An An-24 parking at Kupol airport, a remote and unpaved airport serving one of Russia's biggest gold and silver mine. The airport is placed in the permafrost zone with a subarctic climate. The cold season lasts about 8 months. Photo © Evgeniy Trufanov, CC BY-SA 4.0 via Wikimedia Commons

There was no serial production engine that could meet Antonov's requirements at the time, so the A.T. Ivtchenko Design Bureau, who produced the AI-20 engines for the An-8, An-10 and An-12 stepped in. With more advanced simulation methods and with the experience gained from the AI-20, they were able to create the Al-24, a scaled-down turboprop engine rated at 2,550 horsepower. Each engine nacelle included a fire extinguishing system that could be set to an automatic operation mode and an APU in the right engine nacelle. The main advantages of this engine were its simple design, high reliability, long service life, easy maintenance and good performance at all power settings, altitudes and flight speeds.

The APU outlet can be seen at the tip of the right-hand engine nacelle. Photo: © Georgiy Shvetsov, CC BY-SA 3.0 GFDL 1.2, via Wikimedia Commons.

The first prototype had a very successful test flight on 20th of October 1959 after 2 years from the initial Publication of the Enactment. This first prototype differed from the original design with a capacity changed to 44-seats. The final approved production model was changed to 50 seats and dubbed An-24A. It was approved for serial production two years later on the 19th of August 1961 at the Kyiv aircraft manufacturing facility No. 473 at Sviatoshyn Airfield, the site of the An-2 production. The An-24A had a single door on the left rear with a folding two-section airstair and seated 50 passengers in a four-across configuration. There was also an emergency exit on each side of the fuselage, forward of the wing on the left side of the fuselage and rear of the wing on the right. It also had baggage compartments fore and aft, with a large cargo door forward on the right and a smaller service door to the rear on the right along with toilet, coat closet and galley in the rear. On the 29th of July 1962 before commercial service had begun, an An-24 crashed at Ternopil Airport during a test flight while simulating a right-engine failure. All eight crew on board survived, but the aircraft was sadly written off.

The first passenger flight took place with Aeroflot between the modern-day Ukrainian cities of Kyiv and Kherson in September 1962. The second variant, the An-24B, was converted from an An-24A and flew for the first time on the 16th of November 1965.The main purpose of the An-24B was to make it more flexible as per customer requirements. For example, some customers wanted additional range, so the An-24B could be built with 4 additional fuel bladders in the centre wing section or could be configured for an all-cargo or mixed passenger-cargo ("combi") operations. The new and improved An-24B was even easier to build, offered better performance and it was approved for production at manufacturing facility No. 99 in Ulan-Ude, Eastern Siberia where 174 aircraft were produced between 1965 and 1971 and at Kyiv with the An-24A.

The Military was not forgotten and they got their own variant, the An-24T (Transportnyy - Transport) with a ventral loading hatch, cargo winch and escape hatch aft of the nose landing gear. One hundred and sixty-seven of these were produced between 1965 and 1971 at the Irkutsk State Factory Number 39, where the An-12 was produced. Production in Kyiv ran between 1961 and 1979 with 1,028 airplanes produced. The Chinese also got their hands on the An-24 and were able to negotiate production licenses to produce it locally as the Y-7, of which 103 were built from 1977 and are still in production.

Antonov An-24 landing at the Ust-Kut Airport in Irkutsk Oblast, Russia. Photo © Wikipedia

The An-24 was a reliable and rugged success. It unsurprisingly branched out into numerous variants such as the Nuclear, biological and chemical warfare reconnaissance version, Search and Rescue, fire bombers, cloud seeders, airborne command posts and many more. There was one special variant however, the AN-24RV (Reaktivnyy V - boosted V), which was basically an An-24V with turbojet boosters. This variant was responsible for accommodating 39 Women's World Records. Among them, on the 7th of May 1982, the ceiling of 36000ft was reached and on the 7th of July 1982, the cargo of 8,096 kg was lifted to an altitude of 6561ft. Manyna Popovich and Galyna Korchuganova were the alternate captains of the record setting female crews.

Despite all of its success, the An-24 had a reputation as an unsafe aircraft due to a high number of crashes:- 159 accidents with a total of 2,134 fatalities to be exact. Taking a closer look at the aircraft's accidents, you'll notice 3 trends:- many accidents happened due to pilot error and/or during approach or landing stages and they occurred in more unconventional conditions, i.e rough airstrips, bad geography or unfavourable weather and flying to dangerous areas which posed dangerous consequences. In addition, a lot of accidents occurred during hijackings, especially during the 80s. Now, did the aircraft deserve its reputation or not? I'll leave it to the reader to decide for themselves.

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Antonov Design Bureau - The Big, The Heavy and The Bold - Part 3

By Amith Babu