AN Unmatched combat radius and a capability for deep penetration against heavily defended targets has established the Royal Australian Air Force's (RAAF) General Dynamics (now Lockheed Fort Worth) F-111 fleet as Australia's prime strategic deterrent, and one well-matched to its defence scenarios.

With over 25 years of F-111 operations behind it, the RAAF first broadened the type's role with the development of tactics to allow its use in interdiction and maritime-strike as well as land-strike roles.

To those capabilities it has now added air control, strategic and tactical reconnaissance and air support to ground forces. Among the earlier progressive airframe and weapons-system modifications implemented to provide for these expanding roles, was the incorporation of General Dynamics-designed Pave Tack laser-designator pods (allowing the F-111s to deliver laser- guided bombs), the television-guided GBU-15, and the Harpoon anti-shipping missile.

To retain a long-range tactical-strike capability and to extend the life of the type, the RAAF ultimately had no affordable alternative to undertaking the current A$500 million ($400 million) upgrade of its F-111s' ageing systems. The 30-year-old analogue avionics and related electromechanical modules were suffering increasing unreliability and logistical-support problems, with some module components so difficult to source that any acceptable level of combat availability had become increasingly difficult and costly to assure.


Modification programme

Of Australia's fleet of 21 F-111Cs, four are strategic photo-reconnaissance variants, designated RF-111C. The remaining 17 are the tactical-strike version. All 21 are now being modified under the current avionics upgrade programme (AUP), which finally won approval in the face of considerable opposition from defence experts and from some areas of politics. As the US Air Force had already gone through several modernisation programmes, there were considerable data available on availability, performance and cost of systems, which helped define and cost the available options. Four years ago the RAAF acquired 15 ex-USAF F-111Gs to extend the life of the type by reducing the flying hours on its existing airframes, particularly in training functions. The final role of those aircraft is still being defined, but, because they already have considerable similarity with the upgraded F-111Cs through previous avionics-modification programmes, they will not enter the Australian AUP yet.

The AUP does not in itself affect the missions that the F/RF-111C can undertake or the threats it can counter, apart from enhancing its performance by reducing crew workload and improving systems reliability. That is because, in the main, the aim has been to improve reliability and maintainability, rather than operational capability. The selected modernised avionics are integrated into a system which interfaces them with the retained avionics. It incorporates new digital architecture which was also designed to minimise development costs and avionics-system fault vulnerability, as well as making fault isolation easier and providing accommodation for growth. One of the main areas of difference is the integration of the Harpoon capability, for which the USAF had no role because the Harpoon was a US Navy weapon. Other differences are the KY58 secure-voice UHF, and the Have Quick II, a frequency-hopping HF secure-voice system.

In the offensive avionics suite, the new bombing/navigation system is built around dual Honeywell H423 ring-laser-gyro inertial units originally developed in the USAF's standard inertial-navigation unit (SINU) programme. They are augmented by a single Rockwell- Collins 3M global-positioning-system (GPS) receiver, which updates the SINUs. The mission computer (MC) runs a "hierarchy" in which the optimum navigation mode is called GPS/I1 (Inertial 1) in which the primary inertial and GPS systems are mixed together to derive a system-navigation solution. The next degraded mode is GPS/I2; and, if neither SINU is operating, the next mode is GPS mode, which mixes GPS and air-data computer inputs. Below GPS is inertial-only mode, degrading down to air-data computer mode. The TACAN is separate and not part of the cross-referencing regime.


New system

The new system incorporates dual-redundant MIL-STD-1553B Multiplex databuses (mux buses); a digital computer complex (DCC) consisting of dual fully redundant MIL-STD-1750A IBM AP-102 MCs; an avionics converter unit; and more-supportable digital avionics. The updated system is controlled by the DCC, in which each MC has the capability to perform both primary navigation and weapons-delivery functions, and as a MIL-STD 1553 mux bus controller. One MC, designated as the primary, assumes bus control when the system is powered up, while the other (secondary) MC acts as a remote terminal with full-redundant capability to become the primary MC in the event of a failure.

Transition from normal to back-up operation is automatic when the primary MC detects its own failure and sets its GO/NO GO output discrete to the NO GO state. (Navigation and weapons delivery functions are maintained throughout the automatic transition.) The MCs, the functions of which are primarily navigation and bombing, do not interface with the flight-control system (FCS). They are responsible for all navigation functions, including interacting with the autopilot system; all weapon-delivery modes, including guided and unguided bombing; and Harpoon anti-shipping and air-to-air missile modes. On the reconnaissance aircraft, the controller is the strike-camera data-display system, providing 1553 muxbus communications between the mission computer and that system, rather than through its own panel with separate switches. Camera operation is now achieved via the display system, which then communicates with the reconnaissance system to set up the cameras.

The existing stores-management systems (SMS) were replaced in both models by a standardised F/RF-111C SMS, using the same hardware as that on the RAAF's McDonnell Douglas F-18 SMS update, but controlled by an F/RF-111C AUP-specific stores-management systems operational flight programme.

Equipment deleted includes the navigational computer unit, ballistics computer unit, multiple-offset aim panel, weapons-control panel, SMS computer-processing unit and Harpoon data-processing unit (F-111C only); and reconnaissance control panel and reconnaissance digital-data inserter unit (RF-111C only).

The dual-redundant multi-mode Texas Instruments APQ-171 terrain-following radar was upgraded with kits supplied by the Government, as was the attack-radar set, with kits supplied by the Ocean, Radar & Sensor Systems division of Lockheed Martin .

In the cockpit, the most visible change is the installation of two interchangeable multi-function displays (MFDs) and the control display unit, providing the equivalent of a glass cockpit on the navigator's side. All navigation-type data and communication frequencies can now be pre-programmed via those screens, giving the ability to plan the whole route on the ground, and to download it from a cartridge into the mission computer.

There are few differences, mostly transparent, on the pilot's side and the AUP aircraft is treated as the same type for the purposes of crew training, with a "transition" course to cover differences training. The AUP makes no changes to the Pave Tack virtual-image display (VID), which provides the navigator with a two-screen optically enlarged display of forward-looking infra-red (FLIR) and radar images, which are interchangeable. Diagonal screen sizes are 154mm and 100mm, the larger screen normally being used for radar imagery in navigation and blind bombing. FLIR imagery is substituted during laser-guided bombing. Datalinked weapon imagery is available for the delivery of TV/infra-red-imaging guided weapons, or of the new stand-off weapon, the AGM-142, a derivative of the Rafael Popeye. Further redundancy is provided by the ability of the system to switch FLIR imagery from the VID to the MFDs. The F-111's capability to support the ALM-9L/M missile is also retained.

The original analogue flight controls have been replaced by triple-redundant General Dynamics digital flight-control systems, as used in the USAF F-111A/Es upgraded by Grumman in the mid-1980s. The installation is built around three Lear Astronics flight-control processors. Being virtually identical to the USAF standard, it requires no design changes, and has demonstrated reliability some 18 times better than that of its analogue counterpart, which suffered unreliability in the FCS interface with the flight controls and flight instruments.


Flight tests

The first prototype aircraft was modified and flight tested in California, for proving the design-navigation performance, weapon-delivery performance and functionality, and all other aspects, apart from reliability. The functional test was in accordance with a Rockwell flight-test plan, with the performance-evaluation flight test detailed in an RAAF-developed plan. The remaining 20 aircraft are being modified at the RAAF's base at Amberley, Queensland. The Government has accepted three aircraft since the programme started in January 1995. One more is in flight test and approaching completion and, over the next two years, the remaining 16 are also scheduled for completion, with each aircraft out of service for about six months.

Reliability of the overall avionics system is defined in the specification as 20.9 flying hours mean time between failures. The RAAF is still gathering data and cannot yet confirm reliability to specification. Better accessibility for maintenance is linked with the mean time to repair specified in the contract. On average, that figure is 1h, with 90% of cases inside 1.5h for all the items introduced by the AUP. Also, because most modules are now digital, their built-in test function substantially improves troubleshooting effort. System faults are recorded by nature, time of occurrence and frequency, and logged on the data-transfer system with the code broken down to identify the fault. The cartridge downloads the data on to the flightline-maintenance computer.

Source: Flight International