AJ 37 VIGGEN ATTACK VARIANT

Written by: Greg Goebel

The AJ 37 attack variant of the Viggen, where "AJ" stands for "Attack Jagt / Strike Fighter", was the first of the series to go into service, and makes a useful baseline for further discussion.

The AJ 37 has a spikelike fuselage, with: fixed oval-shaped engine inlets alongside the canopy, the inlets set off from the fuselage to avoid ingesting stagnant "boundary layer" air; a large rear-mounted delta wing; canards alongside the engine inlets; and a tall tailfin. Overall construction is of aircraft aluminum honeycomb, with titanium where required, such as engine firewalls. The Viggen is arguably less elegant in appearance than its predecessor, the SAAB 35 Draken, or its successor, the SAAB 39 Gripen, but it has a certain solid, businesslike, and combatative style of its own. However, the extensive use of aluminum honeycomb makes it a surprisingly light aircraft for its size.

The wing has a somewhat complicated form, featuring a double delta with something of a "hoop skirt" appearance in planform and a dogtooth on each outer span. The dogtooth was added to improve logitudinal stability when carrying external stores. Each dogtooth is further marked by a bullet fairing for a radar warning receiver (RWR) antenna. There are two-section hydraulically actuated elevons on the trailing edge of each wing.

The canards direct turbulent airflow over the main wing at low speeds, reducing the stall speed on landings. They have no dihedral and are fixed at a few degrees of incidence, but have trailing-edge flaps to improve takeoff performance. The canards on the first prototype had a noticeable dihedral when the machine was rolled out, but the dihedral was eliminated before the initial flight. The tailfin is fixed, with a one-piece rudder. It folds to the left to allow the aircraft to be stowed in hardened shelters at field bases. There is a fixed ventral fin under the tail.

The AJ 37 features tricycle landing gear, with all gear featuring two wheels. The nose gear retracts forward. Each main gear assembly unusually features its two wheels in tandem to reduce its depth for storage in the wing, and retracts inward from the wing towards the fuselage, with the main gear struts telescoping during retraction for a tighter fit. The landing gear is heavily shock-absorbed to permit steep approaches for short landings. Landings are said to be "firm" and apparently are much along the lines of carrier landings, except that the landing strip isn't moving.

The RM8A afterburning turbofan is actually at the core a Pratt & Whitney JT8D-22 commercial turbofan used on the Boeing 727 and Douglas DC-9 airliners, built under license by Svenska Flygmotor (later Volvo Flygmotor) and fitted with a Swedish-designed afterburner and thrust reverser. It is said to be the first production engine to ever have such a combination of features. The engine provides 115.7 kN (11,800 kgp / 26,015 lbf) afterburning thrust. The thrust reverser features three "jaws" that snap shut when the nose gear touches down and blast the exhaust forward through three slots around the fuselage under the tail.

It is unclear how the engine is started; cutaways of the machine do not show anything that resembles an auxiliary power unit. The Viggen does have a ram-air turbine for emergency flight power, fitted to a door under the trailing edge of the left canard that pops out when hydraulic power is lost and (for some puzzling reason) when the landing gear is extended.

There is a fuel tank in each wing, a saddle tank over the engine, a tank on each side of the engine, and a tank behind the cockpit, for a total of six tanks. There is no provision for inflight refueling, as that would imply a "force projection" capability not in line with Sweden's defensive posture.

The pilot sits under a rear-opening clamshell canopy behind a one-piece windshield hardened against birdstrikes. There is a prominent humped dorsal spine running back from the canopy; most of the prototypes had a less prominent straight spine, but the enlarged spine improved transonic handling and was adopted for production.

The Viggen was originally fitted with a SAAB "Mark 2" rocket-boosted ejection seat with zero-altitude capability, but as demonstrated by the accident that killed Lennart Fryoe, the aircraft had to be moving 75 KPH (47 MPH) or more for the seat to work reliably. The "Mark 1" seat, incidentally, was the development version and was apparently never operationally fielded on any aircraft. It is an indication of the Swedish "do it ourselves" mentality that SAAB actually developed their own ejection seat, when a all-but-standard British Martin Baker seat would have probably been as good or better in technical and economic respects.

The control layout is analog, as would be expected of aircraft of its period, but features a Svenska Radio / Ericsson (SRA) head-up display (HUD) for primary flight data, including displays generated by the instrument landing system to help guide the aircraft down. The most significant element in the AJ 37's avionics suite is its Ericsson PS-37A monopulse X-band radar, which is primarily focused on air-to-ground and navigation, though it has limited air-to-air capabilities. The rest of the avionics is impressive, or at least it was for its era, and includes:

• Honeywell radar altimeter.
• Decca Doppler navigation system.
• A SATT RWR and countermeasures system.
• A SAAB CK-37 miniaturized digital air data and nav/attack computer.
• A Cutler-Hammer AIL microwave ILS receiver. The landing system was later upgraded to include an audio warning to tell the pilot if his approach was incorrect.

The AJ 37 has seven stores pylons, including a centerline pylon; a pylon under each engine inlet; and two pylons on each wing outboard of the main landing gear. There is provision for a third outboard pylon under each wing. Maximum external load is 7,000 kilograms (15,400 pounds). An external fuel tank is almost always carried on the centerline pylon. It does not appear that any of the other pylons are "wet". Primary stores initially included:

• Dumb bombs and unguided rocket pods. A typical bombload would be 16 120 kilogram (265 pound) fragmentation bombs plus a centerline tank, though arrangements of heavier bombs can be carried. The standard unguided rocket pod is the Bofors M70, which carries six 135 millimeter (5.3 inch) rockets. These rockets can be fitted with high-explosive or fragmentation warheads and the AJ 37 can carry four such pods, with two on the pylons under the inlets and one on each wing.
• The SAAB Rb-04 solid-fuel antiship missile. "Rb" stands for "Robot", indicating a guided missile.
• The SAAB Rb-05 air-to-surface missile (ASM), a dartlike weapon somewhat along the lines of the US Bullpup ASM.
• Although the AJ 37 has no built-in cannon armament, it can carry two cannon pods built by FFV of Sweden, each pod containing a single British-designed and license-built Aden 30 millimeter revolver-type cannon with 150 rounds.
• For self-defense, the AJ 37 can carry Sidewinder short-range heat-seeking air-to-air missiles (AAMs), built under license in Sweden. Swedish Sidewinders were originally designated "Rb-24", originally equivalent to the AIM-9B and later the AIM-9J Sidewinder variants.

  SAAB also manufactured the old Hughes AIM-4 Falcon AAM as the "Rb-28" and the Viggen could carry heat-seeking Falcons, but this weapon was obsolescent and gradually phased out. The AJ 37's radar system does not have the capabilities needed to control longer-range radar-guided missiles. The Sidewinder can be used on the AJ 37 because it is the missile's seeker itself that acquires a target, with a target lock indicated by a tone in the pilot's headphones.

• Defensive countermeasures can be carried as well, including an Ericcson EriJammer 200 jammer pod and a Philips / Bofors BOX-9 (AKA BOZ-100) chaff-flare pod.

As mentioned, the Viggen was designed to be easy to maintain. It has over 100 access panels, with the nose cone sliding forward to give access to the radar system, and the rear fuselage can be quickly pulled off to give access to the engine. Most of the access panels can be reached without use of a ladder or service scaffold. Systems are modular to ease servicing and upgrades. A "built-in test system" was integrated into the aircraft, with specialized servicing vehicles produced to help find and fix faults.

A total of 109 production AJ 37s was built, with the type equipping six squadrons. Although prototypes and early production aircraft flew in natural metal finish, in service AJ 37s are painted gray on the bottom and with a "splinter" camouflage scheme on top, featuring a four-color pattern of tan, light green, dark green, and dark blue. The Flygvapnet refers to this color scheme as "fields and meadows" and it is meant to help conceal the aircraft at their dispersed bases. All Viggen variants except the JA 37 interceptor feature this color scheme.

There were a number of losses of AJ 37s in 1974 and 1975 due to wing spar failures. As it turned out, the first 27 machines had been built with lighter wing spars, and the 21 survivors were rebuilt with stronger spars. Oddly, though the Viggen is easier to fly than the Draken, the Viggen's accident rate has been higher. One interesting accident took place on 28 September 1982, when a pilot was taking off from Norrkoeping and suddenly encountered a herd of elk that had wandered onto the runway. There was a collision with one of the elk, with the aircraft suffering minor damage while the elk ended up as venison. One wonders if the pilot was at least tempted to put a "kill marking" on his machine.

In the early 1990s, a decision was made to update a number of AJ 37s and some other Viggen variants to the "AJS 37" multirole configuration, of which more is said later. The last operational flight of a true AJ 37 was in March 2000.