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History

37098/SE-DXN was built in 1977 she spent her entire military life with F 15 Wing at Söderhamn. She was retired in 1998 and put into storage. After a long period of restoration and maintenance, 37098 was placed on the Swedish civil register as SE-DXN and flew again on the 27th of March 2012.

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The Aircraft

Saab AJS 37 Viggen

The Saab 37 Viggen (Swedish for "the bolt" or "the tufted duck") is a retired Swedish single-seat, single-engine, short-medium range combat aircraft. Development work on the type was initiated at Saab in 1952 and, following the selection of a radical delta wing configuration, the resulting aircraft performed its first flight on 8 February 1967 and entered service on 21 June 1971. It was the first canard design produced in quantity. The Viggen was also the most advanced fighter jet in Europe until the introduction of the Panavia Tornado into operational service in 1981.

Several distinct variants of the Viggen were produced to perform the roles of strike fighter (AJ 37), aerial reconnaissance (SF 37), maritime patrol aircraft (SH 37) and a two-seat trainer (SK 37). In the late 1970s, the all-weather fighter-interceptor aircraft JA 37 variant was introduced. In November 2005, the Viggen was retired from service by the Swedish Air Force, the only operator, having been replaced by the newer Saab JAS 39 Gripen.

The Viggen was powered by a single Volvo RM8 turbofan. This was essentially a licence-built variant of the Pratt & Whitney JT8D engine that powered commercial airliners of the 1960s, with an afterburner added for the Viggen. The airframe also incorporated a thrust reverser to use during landings and land manoeuvres, which, combined with the aircraft having flight capabilities approaching a limited STOL-like performance, enabled operations from 500 m airstrips with minimal support. The thrust reverser could be pre-selected in the air to engage when the nose-wheel strut was compressed after touchdown via a pneumatic trigger.

The requirements from the Swedish Air Force dictated Mach 2 capability at high altitude and Mach 1 at low altitude. At the same time, short-field take-off and landing performance were also required. Since the Viggen was developed initially as an attack aircraft instead of an interceptor (the Saab 35 Draken fulfilled this role), some emphasis was given to low fuel consumption at high subsonic speeds at a low level for a good range. With turbofan engines just emerging and indicating better fuel economy for the cruise than turbojet engines, the former was favoured, since the latter were mainly limited by metallurgy development resulting from limitations in turbine temperature. Mechanical simplicity was also favoured, so the air intakes were simple D-section types with boundary layer splitter plates, while the fixed inlet had no adjustable geometry for improved pressure recovery. The disadvantage was that the required engine would be very large. In fact, at the time of introduction, it was the second-largest fighter engine, with a length of 6.1 m and 1.35 m diameter; only the Tumansky R-15 was bigger.

Saab had originally wanted the Rolls-Royce Medway as the Viggen's powerplant. Owing to the cancellation of the Medway, the JT8D was instead chosen as the basis for modification. The RM8 became the second operational afterburning turbofan in the world, and also the first equipped with a thrust reverser. According to aviation author Christopher Chant, the RM8 has the distinction of being the first engine to be fitted with both an afterburner and a thrust reverser. It had a bypass ratio of around 1.07:1 in the RM8A, which reduced to 0.97:1 in the RM8B. The RM8A was the most powerful fighter engine in the late 1960s.

The AJ, SF, SH and SK 37 models of the Viggen had the first version of the RM8A engine with uprated internal components from the JT8D that it was based on. Thrust was 65.6 kN dry and 115.6 kN with an afterburner. For the JA 37, the RM8A was developed into the RM8B, achieved by adding a third low-pressure compressor stage over the preceding model, increasing the turbine inlet temperature and fuel diffusion within the combustion chamber. Thrust is 72.1 kN dry and 125.0 kN with an afterburner. Owing to the increased length and weight of the RM8B engine over its predecessor, the airframe of the JA 37 was stretched in order to accommodate it. Onboard electrical power was provided by a 60 kVA generator. In the event of an in-flight engine failure, emergency power was provided by an automatically deploying ram air turbine (RAT), capable of generating 6 kVA.

In the early 1960s, it was decided that the Viggen should be a single-seat aircraft, Saab having recognized that advanced avionics such as a digital central computer and a head-up display could perform the workload of a human navigator and entirely replace the need for a second crew member. The use of a digital computer would reduce or entirely replace analogue systems, which had proven to be expensive to maintain and alter, as had been the case of the earlier Draken, in addition to accuracy issues. The computer, called CK 37 (central calculator Swedish: Central Kalkylator 37), was the world's first airborne computer to use integrated circuits. Developed by Saab, the CK 37 was the integrating unit for all electronic equipment to support the pilot, performing functions such as navigation, flight control, and weapon-aiming calculations. In practice, the CK 37 proved to be more reliable than predicted.

On later variants of the Viggen, from the JA37 onwards, it was decided to adopt a newer and more powerful computer, the American CD107, which was license-manufactured and further developed by Saab. The computing techniques and concepts, such as distributed computing, went beyond the use of the Viggen, in addition to civil-orientated derivatives, it directly contributed to the computers used onboard the Viggen's replacement, the Saab JAS 39 Gripen. Various electronic countermeasures (ECM) were installed upon the Viggen, these were typically provided by Satt Elektronik. The ECM systems consisted of a Satt Elektronik radar warning receiver system in the wings and the tail, an optional Ericsson Erijammer pod and BOZ-100 chaff/flare pod. Infrared warning receivers were also later installed. In total, the electronics weighed 600 kg, a substantial amount for a single-engine fighter of the era.

The aircraft's principal sensor was an Ericsson PS 37 X-band monopulse radar, which used a mechanically steered parabolic reflector housed in a radome. This radar performed several functions, including air-to-ground and air-to-air telemetry, search, track, terrain-avoidance and cartography. On the JA 37 fighter-interceptor model, the PS 37 radar was replaced by the more capable Ericsson PS 46 X-band pulse repetition frequency, which had an all-weather look-down/shoot-down capability reportedly in excess of 50 kilometres and continuous-wave illumination for the Skyflash missiles as well as the ability to track two targets while scanning. According to Ericsson, it had a 50 per cent chance of spotting a low-flying McDonnell Douglas F-4 Phantom II within a single scan and possessed a high level of resistance to interference from ECM.

Saab and Honeywell co-developed an automatic digital flight control system for the JA 37 Viggen, which has been claimed to be the first such system in a production aircraft. To assist low altitude flight, a Honeywell radar altimeter with transmitter and receiver in the canard wings was used. The aircraft was also fitted with a Decca Type 72 Doppler navigation radar. TILS (Tactical Instrument Landing System), a landing-aid system made by Cutler-Hammer AIL, improved landing accuracy to 30 m from the threshold on the short highway airbase system. In order to effectively enforce Sweden's air space, the Viggen was integrated with STRIL 60 national defence system. The JA 37 Viggen was also equipped with a Garrett AiResearch digital Central Air Data Computer, modified from the unit used upon the Grumman F-14 Tomcat.

Initially, only a single reconnaissance (S) variant was considered, but fitting cameras, as well as a radar, proved to be impossible. The SH 37 maritime strike and reconnaissance variant was very similar to the AJ 37 and differed mainly in a maritime-optimized PS 371/A radar with longer range, a cockpit air-data camera and a tape recorder for mission analysis. "Red Baron" and a SKa 24D 600 mm LOROP camera pods were usually carried on the fuselage pylons. The centreline fuel tank was converted for a short period of time to a camera pod with two Recon/Optical CA-200 1676 mm cameras. In addition to the reconnaissance equipment, the SH 37 could also use all weapons for the AJ 37. For the photographic SF version, the radar in the nose was omitted in favour of four SKa 24C 120 mm and two SKa 31 570 mm photographic cameras as well as one 57 mm VKa 702 Infrared line scan camera and air-data camera; all of which were integrated with and controlled by the aircraft's central computer. Additional equipment, such as more camera pods, fuel tanks, ECM pods, and self-defence air-to-air missiles could also be carried upon the fuselage pylons.

The fighter-interceptor version of the Viggen, the JA 37, featured various avionics changes, including the extensive use of digital electronics alongside mechanical technology. In 1985, the "fighter link" went into service, permitting encrypted data communication between four fighters; this enabled one fighter to "paint" an airborne enemy with guidance radar for the Skyflash missiles of the three other fighters in a group while they had their search and guidance radar switched off. This system was operational ten years before any other country's. The autopilot was also slaved to the radar control to obtain better precision firing the cannon. Once in service, Viggen's software was regularly updated every 18 months. In 1983, the mean time between failures (MTBF) was reported as 100 hours, a very high-reliability level for the generation of avionics systems involved.

The displays in the original cockpit were all of the traditional analogue/mechanical types with the exception of an electronic head-up display (HUD), which Saab has claimed makes the Viggen easier to fly, especially at low altitudes during air-to-ground strike missions. Unusually for a 1970s fighter, the JA 37 variant of the Viggen featured three multi-purpose cathode-ray tubes (CRT) display screens were fitted within the cockpit, in a system called AP-12, developed by Saab and Ericsson. These displays would be used to display processed radar information, computer-generated maps, flight and weapons data, along with steering cues during precision landings.

Between 1989 and 1992, the AP-12 display system was subject to a substantial upgrade. In 1999, a new tactical liquid-crystal display (LCD) system derived from the Saab JAS 39 Gripen, which replaced the CRT-based AP-12 system, began flight tests with the Swedish Air Force. On the twin-seat SK 37 trainer, the rear cockpit used by the instructor is only fitted with conventional instrumentation and lacks a HUD, computer controls and other features.

The ejection seat was the Raketstol 37 (Rocket chair 37) and was the last Saab designed seat in service. A derivative of the Saab 105 trainer seat, the seat was optimized for low altitude, high-speed ejections. Once activated by the pilot via triggers built into the armrests (on twin-seat models, the occupant of the forward cockpit position is able to initiate the ejection of both seats), the ejection sequence is automated, including the removal of the canopy; in the event of a malfunction, a reserve trigger can be activated. A combined parachute and seat harness is used, which features a barometric interlock to appropriately release the occupant and harness from the seat during the ejection sequence, a manual override handle is also provided for this function.

There were dedicated warning caption panels on each side of the pilot's legs. On the right console panel were numerous dedicated controls and indicators, including weapons and missile controls, nav panel, oxygen on/off, windshield de-fogging, IFF control, lighting controls. Situated on the left console panel were radar controls, canopy handle, landing gear handle, radio controls and the cabin pressure indicator. As per then-standard practice within the Swedish Air Force, all cockpit instrumentation and labelling were in Swedish.

With the performance requirements to a large extent dictating the choice of the engine, the airframe turned out to be quite bulky compared to contemporary slimmer designs with turbojet engines. The first prototypes had a straight midsection fuselage that was later improved with a "hump" on the dorsal spine for reduced drag according to the area rule. The wing had the shape of a double delta with a dogtooth added to improve longitudinal stability at high incidence angles.

A consequence of a tailless delta design, such as in the Viggen, is that the elevons, which replace more conventional control surfaces, operate with a small effective moment arm; their use adds substantial weight to the aircraft at takeoff and landing. Hinged leading edge surfaces can help counteract this, but an even more effective tool is the canard. The canard surfaces were positioned behind the inlets and placed slightly higher than the main wing, with a higher stall angle than the wing, and were equipped with flaps. The lifting canard surfaces act as a vortex generator for the main wing and therefore provide more lift. An added benefit was that they also improved roll stability in the transonic region. The canard flaps were deployed in conjunction with the landing gear to provide even more lift for takeoff and landing.

To withstand the stresses of no-flare landings, Saab made extensive use of aluminium in the airframe of the Viggen, which was constructed using a bonded metal honeycomb structure; the entire rear section of the fuselage, downstream of the engine nozzle, formed a heat-resistant ring composed of titanium. The main landing gear, manufactured by Motala Verkstad, was highly strengthened as well; each leg held two small wheels fitted with anti-skid brakes placed in a tandem arrangement. The design requirements imposed by the large anti-ship missiles employed upon the Viggen necessitated that both the undercarriage and vertical stabilizer be quite tall. To accommodate this, and to allow the main landing gear to be stowed outside of the wing root, the undercarriage legs shortened during retraction. The vertical stabilizer could also be folded via an actuator in order that the aircraft could be stored in smaller hangars, hardened aircraft shelters, and underground hangars, the latter of which were employed by the Swedish military to limit the damage of preemptive attacks.

The six tanks in the fuselage and wings held approximately 5,000 litres of fuel with an additional 1,500 litres in an external drop tank. The specific fuel consumption was only 0.63 for cruise speeds. The Viggen's consumption was around 15 kg/s at maximum afterburner. A pair of inlets placed alongside the cockpit feed air to the engine; simple fixed-geometry inlets were adopted, similar to the Draken, except for being larger and standing clear of the fuselage.

A weapons load of up to 7,000 kg could be accommodated on nine hardpoints: one centreline pylon, two fuselage pylons, two inner and two outer wing pylons and two pylons behind the wing landing gear. The centreline pylon was the only pylon plumbed for carrying an external fuel tank and was usually so occupied. A pair of air-to-air missiles were intended to be placed on the outboard wing pylons, which were more lightweight than the other attachment points. The pylons behind the landing gear were not used until the JA 37D modification when BOL countermeasure dispensers were fitted to them. The ground crew would enter the munitions fitted into the aircraft's central computer using a load-selector panel, which would automatically choose the correct values for fire control, fuel consumption, and other calculations.

The AJ 37 was typically equipped with a total of seven hardpoints, three underneath the fuselage and two under each wing, a further two wing-mounted hardpoints could be optionally fitted but this facility was rarely used. Various munitions could be carried, such as several types of rockets: the 135 mm M56GP 4 kg armour-piercing, the M56B with 6.9 kg of high explosives, and the M70 with a 4.7 kg HEAT warhead.

The AJ 37 was designed to carry two RB 04E anti-ship missiles on the inboard wing pylons with an optional third missile on the centreline pylon. The RB-04 was a relatively simple cruise missile that was further developed to become the more capable RBS-15, also integrated on the Viggen. An optional load consisted of two RB 05 air-to-surface missiles on the fuselage pylons. The RB 05 was later replaced by AGM-65 Maverick (Swedish designation "RB 75") television-guided missiles. In a ground-attack role, a combination of unguided 135 mm rockets in sextuple pods and 120 kg fragmentation bombs on quadruple-mounts could be used. Other armaments include explosive mines, and 30 mm ADEN cannon pods with 150 rounds of ammunition on the inboard wing pylons.

Self-defence measures included various ECM systems, as well as either the AIM-4 Falcon or AIM-9 Sidewinder air-to-air missiles. At one point, the AJ 37 Viggen was under consideration as a carrier of both a Swedish nuclear weapon and chemical weapons, although no nuclear or chemical weapons were ultimately adopted by Sweden.

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