Ilyushin Il-62 - Development

Development

The Ilyushin OKB presented a proposal for a four-engined long-range jet airliner in February 1960, receiving the go-ahead from the Soviet Council of Ministers on 18 June 1960, with the Kuznetsov Design Bureau being instructed at the same time to develop the NK-8 turbofan to power the new airliner. The official specification required that the airliner, designated Il-62, must carry 165 economy-class passengers over a distance of 4,500 km (2,800 mi) or 100 first class passengers over 6,700 km (4,200 mi).

The Il-62 replaced the fast turboprop Tu-114 on long range routes. As the Tu-114 was just entering service when the Il-62 was on the drawing board, Ilyushin had time for an unhurried design, test, and development programme. This was useful, since the Il-62 did call for significant development.

The Il-62 and the British Vickers VC10 are the only commercial airliners with four engines fitted in twinned/paired nacelles by the sides of, and beneath, a "T" shaped empennage (T-tail), although the Lockheed JetStar business jet shares this configuration. In the case of Ilyushin, the configuration was dictated by TsAGI, the Soviet Union's aerospace agency, since Ilyushin's design bureau lacked the resources to engage in configuration studies. This layout allowed the wing design to be optimised for aerodynamic efficiency, without being cluttered by having to carry engines. In addition, the rear mounted engines reduced engine noise in the cabin and allowed smaller vertical tail surfaces (as the yawing moment in the event of an engine failure was reduced compared to wing mounted engines). These advantages are balanced by a number of drawbacks. The wing structure, without wing mounted engines to relieve the wing bending moment, need to be heavier, as did the rear fuselage structure, which had to carry the engines. In addition, aerodynamic wash (shadow) from the wing blankets the tail when the nose is pitched up (high angle of attack) leading the aircraft into a condition known as deep stall. This called for complex and (in the 1960s) unreliable automatic stall warning systems such as stick shakers and stick pushers to prevent the aircraft getting locked into the deep stall condition, although the Il-62's wing was designed to prevent deep stall.

Early aircraft (prototypes, pre-production and initial production aircraft) display an evolution from thin or thick kinked leading inboard edges to the ultimate thick and straight 1966 shape. The characteristic "dog tooth" also moved until fixed before production began. The engine installation also evolved, with the engines' longitudinal axes canted by 3 degrees from the horizontal; thrust reversers were added to the outer engines, and the entire installation was slimmed down as production began.

The prototype was grossly-underpowered. Its intended NK-8 engines were not ready and small Lyul'ka R-7PB turbojet engines had to be installed temporarily. The prototype with the R-7PB engines (registered СССР-06156) first flew on 3 January 1963. It crashed after clipping a perimeter fence during the development program. The production Il-62 was powered by the originally intended rear-mounted Kuznetsov NK-8-4 engines. The first Il-62 powered with NK-8 engines (registered СССР-06153) first flew in 1964.

The Il-62M variant (first flight in 1971, introduced in 1973) has more powerful and quieter Soloviev D-30KU engines and a fin fuel tank. Beneath the skin, the Il-62M has simpler and lighter single-slotted flaps and incremental aerodynamic improvements. Most important of these was the addition of spoilerons (spoilers or wing-mounted air brakes which act as ailerons by differential deployment in cruising flight) and the ability to use idle reverse thrust in flight during the final approach so as to shorten the landing run. Nearly all examples in service today are Il-62Ms. In 1978, the Il-62MK was further developed to seat up to 198 passengers and carry some two tonnes (4,400 lb) more payload and/or fuel than the Il-62M.

A version designated Il-62MK was designed as a much modified medium-range machine, though it never reached high production and was dropped from the programme by 1978 (although Germany operated two examples). Other versions were also planned, some "stretched" to seat up to 250 passengers and others suited to small airfields. None of those reached the detail design stage. No civil/military or military developments are known.

The Il-62 has a conventional landing gear with an additional trademark lightweight landing gear strut at the rear of the fuselage which extends when the aircraft reaches its parking position. Aircraft with rear-mounted engines are usually tail-heavy when sitting empty on the ground, and to prevent the aircraft from tipping up on its tail, various devices are used for supporting the tail - from simple "pogo stick" fixed struts on small aircraft, to light-weight extendable struts (Il-62). Aircraft like DC-9, MD-80/90 or Boeing 727 use an airstair door under their tail which serves the dual purpose of a tail support as well as an extra door for passenger loading.

The Il-62 is the largest airliner with manual flight controls, using steel cables and rods, pulleys, aerodynamic and weight balances, and trim tabs. There are also indications that the Il-62 has a forward-mounted tank for water ballast. This may be used when the aircraft flies empty or lightly loaded. If this is a fact, it would rank the Il-62 alongside other airliners that use ballast, notably the French Caravelle and the Soviet Tu-154. Due to the rear mounted powerplants, the wings are aerodynamically clean, and takeoff and landing aids are employed without the disturbing effect of engine nacelles, resulting in free airflow over the dorsal wing surface. Thus the aircraft can fly through air turbulence of 16–18 m/sec without affecting its stability (Thiel, 2001).

Another key Il-62 trademark is the "saw tooth" ("dog-tooth") on the wing leading edge. This prominent feature acts as an aerodynamic fence vortex generator (without which the wings would be almost vortex-free), and fixed leading edge droop/slat/flap. It ensures vice-free behaviour at high angles of attack and assists efficient long-range cruise. The saw tooth removes the need for hydraulic controls, stick shakers, and stick pushers. Interestingly, later models of the VC10 (for British United Airways and Ghana Airways) also adopted this feature, in their case closer to the wing tips.

Early NK-8-4-engined Il-62s suffered from indifferent performance as well as engine fatigue/overheating, sometimes leading to false fire alarms whereby the crew might be tempted to shut down paired engines to prevent contiguous engine and fuselage damage. Flying with only two paired engines on the same side would, however, render the aircraft unbalanced and difficult to control. Subsequent modifications to the Il-62 and the VC10 (which had a similar engine arrangement) largely rectified this problem. There were two fatal Il-62 losses involving engine failure, both occurring with aircraft owned by LOT Polish Airlines, which had also leased a number of Il-62s from Aeroflot and Tarom. The higher fatality accident (183) was a fully laden Il-62M Flight 5055 on 9 May 1987 which experienced a rear fuselage fire that possibly went unnoticed by the crew, hence their decision not to land at one of two nearby airports. Control of the plane was eventually lost on the return flight to Warsaw, most likely due to one of the auxiliary fuel tanks fitted to some LOT Il-62s having ignited. The other was an unmodified (early version) NK-8-4-powered Il-62 Flight 007 which crashed on 14 March 1980 with 87 fatalities after being fitted with an engine that had previously caused vibration problems when used on two other LOT aircraft (the investigation suggested that the turbine disc was damaged before its final installation).

Powerplant failure of the type that afflicted the LOT aircraft was extremely rare because bearing wear is generally identified by vibration tests during maintenance. At the time, however, LOT did not have equipment to test or fault-diagnose engines of the size used in the Il-62. Unfortunately this meant that any potential problem might not be identified between overhauls. There were other known instances of engine failure (e. g. YR-IRD & CU-T1283) but these did not result in loss of control. The LOT accidents involving different engine types (but same engine position) was a fatal crash-rate 30 times higher than the Il-62 average (2.8% vs 0.092%). At least one other LOT Soloviev D-30-equipped aircraft (Tu-154) also suffered a similar engine failure (non-fatal) around this time. After 1987, LOT introduced turbine vibration-detecting equipment and shortened the time between inspections. It also adopted the dual flight control system used on some other Il-62s and deleted the auxiliary engine-pod fuel tanks. These planes were subsequently onsold to Air Ukraine in 1991/1992 which operated them until the airline closed in 2000. In 2010, 30 years after the loss of Flight 007, an investigation of previously unreleased IPN archives at Instytut Pamięci Narodowej revealed that during the industrial unrest of the 1980s LOT had been instructed by the PRL authorities to effect operational cost savings by over-exploitation of service life of its jet engines (Flight 007 Causes of disaster).