Technology Explained

The software running on the EXPDG is unique to the RAAF, and about twice as complex as that used in the Pacer Strike upgrade, with about 80 display pages stored. It is coded in Jovial. The EXPDG has 32kByte of EPROM and 32kByte RAM, substantially more than the earlier model PDG used in Pacer Strike, and uses two internal processors, one to manage the device and one to generate display symbology.

While the existing AUP MFDs are monochrome CRT based, Honeywell has recently released an active matrix colour LCD version of the MFD. Whether the RAAF chooses to later replace the monochrome MFDs with colour remains to be seen, as this would incur some software overhead to integrate. A colour MFD could be used for facilities such as a moving map display, previously fitted only to the F-111D (see photo), a most useful aid in low level navigation.

The Collins built Integrated Communications Navigation Identification Set (ICNIS) is derived from the design initially used in the Grumman F-111A/E AMP upgrade. This equipment provides a central shared user interface to the aircraft’s comms equipments, navaids and IFF. It allows the Navigator to select equipment modes and enter frequencies from the console Control Display Unit (CDU) or the MFD push buttons. The RAAF AUP aircraft will carry a comprehensive suite of HF, VHF and UHF radios. This suite includes the secure UHF ARC-164 Have Quick, and the Selcal KY-58 and KY-75.

The ICNIS is another software intensive subsystem, its embedded Intel 8085 processor running about 30,000 lines of PLM-80 code. The ICNIS CDU can display about 40 pages, which is twice that of the Pacer Strike version. Visible change to the aircraft cockpit are the pair of MFDs, one in the middle of the auxiliary instrument block and one replacing the AJQ-20A Bomb Nav System controls, beneath the Navigator’s Pave Tack/radar display. The pilot’s primary instruments are unchanged, and the ASG-23 Lead Computing Optical Sight (ie gunsight) is retained. While a HUD would have been nice, it is not essential for a heads down night IFR environment and the RAAF thus did not specify one.

The Pave Tack display, which is retained unchanged in the upgrade, provides a two screen Virtual Image Display (VID), using optics to enlarge the Navigator’s view of the screens. One screen displays FLIR imagery and the other a radar image, the upper screen has a 15cm (6″) diagonal size, the lower screen 10cm (4″).

The Navigator may swap these images at any time, and the larger screen will be typically used for radar imagery during navigation and blind bombing, or FLIR imagery during the delivery of laser guided bombs or TV/FLIR imaging guided weapons such as the GBU-15 or the new standoff weapon (either the AGM-130 or AGM-142). A combination of any two of the three video sources thus may be displayed at any time. The avionic architecture also allows the MFDs to display radar and FLIR imagery redirected from the Pave Tack VID, which provides further failsafe redundancy.

The post AUP avionic system will provide a wide range of navigation and delivery modes. These will include modes for automatic level bombing, Continuously Computed Impact Point (“death dot” visual mode) and air-to-air modes for use with AIM-9 Sidewinder missiles (the M-61 gun is now deleted). The system will allow the aircraft to level bomb, dive bomb, loft bomb, and support the wide range of Pave Tack delivery modes for laser guided and dumb bombs.

The weapon system will also support the GBU-15 glidebomb, the Harpoon anti-shipping missile and later the new standoff weapon. Existing support for the AIM-9L/M missile will also be retained, giving the aircraft a potent self defensive capability and limited offensive air-to-air capability.

Terrain Following Radar and Digital Flight Control System

The Terrain Following Radar and Flight Controls are mission critical items which are tightly integrated. The analogue F/RF-111C employed the J-band AN/APQ-128 TFR (common to the F-111D), which was a derivative of the early model USAF AN/APQ-110 (refer AA June 1984 for details). This equipment is to be replaced by the dual redundant multimode Texas Instruments AN/APQ-171 TFR which is used in all USAF F-111 models subjected to the late eighties AMP programme.

The APQ-171 was developed by Texas Instruments in the mid eighties with the objective of replacing the four different TFR models used across the USAF F-111 fleet with a single type. While the old TFR had an MTBF cca 20 hours, the new TFR was to deliver better than 50hrs MTBF and a useful in service life until 2010.

The new TFR was a substantial redesign. The analogue TF computer LRU (Line Replaceable Unit) was replaced with a new digital computer LRU, the transmitter LRU was replaced, the antenna/receiver subsystem redesigned (90% new), and the amplifier/power supply rebuilt (50% new). The mechanical antenna pedestal and waveguides (ie microwave plumbing) were retained. The new transmitter uses a modern tunable long life magnetron with a magnetic modulator, replacing the original two stage design and providing ECCM via frequency agility.

The new receiver is fully solid state and has Automatic Frequency Control (AFC) to match the agile transmitter. It replaces a Klystron and tube based design. The new digital computer accepts INS inputs and radar returns and calculates the proper aircraft flightpath in relation to the TF equation, generating pitch command outputs for the flight control system. The cockpit TF display was retained with internal modifications, and the original control panel is also retained.

While the new TFR provides a modest performance and capability improvement, its maintainability and reliability are outstanding. The retired analogue TFRs typically had to be individually adjusted for each airframe, with a total of 115 shop adjustments required to deliver nominal performance. The new TFR allows for line maintenance by swapping arbitrary LRUs between airframes, and only 27 in shop adjustments are required to calibrate the system. The in service MTBF in USAF operation was measured at 163 hours, which is three times the design requirement and eight times better than the analogue APQ-110/128.

Understanding Us Military Electronic Equipment Designations

A common source of confusion among the uninitiated is the US military scheme for designating electronic equipment. This scheme has been in usage since the end of World War 2, when it was adopted to provide some measure of commonality in designating equipment. Until then, the Army (and USAAF) employed separate designations to the US Navy and this resulted in numerous problems. The ‘new’ scheme solved much of this, but requires a decoding table to be intelligible to those not familiar with it.

JETDS – Joint Electronics Type Designation System

Often called the ‘AN’ system, the JETDS designation scheme was created to unify the service specific designations of the US Army and Navy. The AN/ prefix to all three letter platform/equipment designators stands for Army-Navy. The JETDS scheme forms designations in the format AN/XYZ-### (V), which decodes in the following fashion:

AN/ – device designated under JETDS X – platform installation Y – type of equipment Z – function or purpose of equipment

(V) – variable configurations are available

As an example, consider the F-111C attack radar set, the APQ-169. A decodes to piloted aircraft, P decodes to radar set and Q decodes to combined purpose. Hence all APQ designated equipments are airborne combined purpose radar sets. Another example is the ALR-62(V) RHAWS on the F-111, which decodes to airborne countermeasures passive receiver.

The specific decode letters are as follows: the F/RF-111C analogue flight controls are being replaced with the triply redundant Lockheed (formerly General Dynamics) Digital Flight Control System used in Grumman AMP upgraded F-111A/E aircraft. Because of the similarity in aerodynamics and performance between the F-111A/E and the RAAF F/RF-111C, this equipment is virtually identical to the USAF standard with no design changes required.

The new DFCS is not only a state of the art technology design, but was also built to eliminate many of the known reliability problems inherent in the analogue FCS interface to the aircraft flight controls and flight instrumentation. This has resulted in a system which is 18 times more reliable than its predecessor, with an expected in service abort rate due DFCS failure of one in over 200000 flight hours.

The nucleus of the DFCS are a set of three Lear Astronics flight control processors, each using a 20 MHz Pace Semiconductor Mil-Std-1750A CPU chip. The processor modules have dedicated memory as well as dedicated I/O processors to handle communications between the three DFCS channels and the interfaces to instrumentation and flight controls.

The new DFCS will provide autopilot performance previously seen only in the SAC FB-111A, as well as ground collision avoidance (GCAS) with voice warning when following terrain manually or automatically. In the inherently hazardous high speed low flying environment characteristic of F-111 operations, the DFCS offers a major improvement in flight safety by virtually eliminating systemic failures as a cause of accidents.

The RF-111C Strategic Recce Aircraft

The RF-111C strategic reconnaissance aircraft have gained considerably from the upgrade programme. The early eighties RF-111C upgrade saw the installation of an Honeywell AN/AAD-5 thermal imaging linescanner, the Fairchild KA-56E panned tactical camera, the Fairchild KS-87C stereo framing camera, the KA-93A4 high level panoramic camera and a high altitude radar altimeter (this suite shares many components with the USN F-14 TARPS pod, but is substantially better). Hardware to support the recce system included facilities to insert geographical coordinates and time stamp information into camera imagery. The cockpit was modified to accommodate a TV monitor for the recce cameras, which forced the removal of the centre console throttles.

These aircraft did not receive the Pave Tack/APQ-165 system upgrade, retaining the APQ-113 with its analogue display, and thus could not support either the GBU-15 or weapon. They therefore became unique against the remainder of the fleet. The AUP will see the RF-111C brought up to a high standard of commonality with the F-111C, and will support all weapons used by the standard aircraft, with the exception of self designated laser guided bombs due the absence of the Pave Tack pod.

The differences in the post AUP RF-111C will be confined to the recce camera pallet in the fuselage bay, the installation of SC-DDS recce system controller equipment and its associated cockpit controls, and some recce specific software in the aircraft’s computers. It is expected that this will significantly ease both maintenance and transition training to the aircraft.

Summary

The F/RF-111C AUP avionic system is a well thought out and robust design, built with a very high emphasis on mission reliability, functional integrity and maintainability.

The RAAF’s choice of system components capitalised on existing USAF avionic upgrade technology, thereby minimising the cost and time delay overheads of designing new system components. This strategy means that the only truly type specific components of the upgrade are the unique system software, firmware and detail components required to tie the system together.

The benefit of this approach will become very apparent over time, as the reliability of mature subsystems coupled with the flexibility of locally maintained software will substantially reduce support costs, and contribute to the longevity of the aircraft. As many of the system components are used over a wide range of aircraft types, spares will be readily available and in many instances, competitively priced.

Australian Aviation understands that the RAAF in the initial phases of the project produced a highly detailed design specification, which constrained the contractors involved to using the specific equipment types as used in the USAF upgrades. The production of such a design specification is no mean engineering feat, and reflects particularly favourably on the RAAF’s engineering and operational skills. Very few air forces are technically competent to produce such documents.

Indications at this stage are that the RAAF’s specified performance criteria for reliability and accuracy will be bettered by an appreciable margin. It is expected that the post AUP avionic system will provide an order of magnitude better system level Mean Time Between Failure (MTBF) than the Mk.I analogue system, which had an MTBF measured in hours. This translates into a post AUP MTBF of the order of tens of hours (USAF experience has been cca 30 hrs MTBF with the earlier AMP suite). Since the system has a high level of redundancy, the MTBF for faults which would result in non-mission capable aircraft will be substantially better.

In practical terms, this translates into much improved operational readiness as well as a vast reduction in required maintenance manhours per flight hour, which results in turn in a major reduction in support costs.

At the time of writing Rockwell was in the process of starting the installation of production avionic system kits at RAAF Amberley, after the completion of development work and prototype testing in the US. The installation and testing production kits will be carried out in Australia, thus enabling the base of acquired experience to be retained in this country.

The F/RF-111C Avionic Upgrade Programme is the most complex avionic upgrade programme ever carried out on a RAAF tactical aircraft, or any USAF F-111 variant, and will provide the F-111 with a comprehensive and reliable offensive avionic suite in the latter half of its life cycle. Like all projects of this type, it is the end product which is of most importance, and observers of the project should never lose sight of this.

It is the author’s view that the F-111 AUP represents very good value for taxpayer’s money, and the end result will be a credit to the RAAF. We can have no doubt that the RAAF’s F-111 force will be widely envied in years to come.

Acknowledgements:

Attempting a technical review of a system as complex as the new F-111 avionic suite is not a trivial task, and could best be described as a team effort. The author would like to thank the RAAF F-111 AUP Project Team, the AUP Production Office, the EW Project Office, the aircrew of 82 Wing, Rockwell Australasia, Honeywell Australia, Hadland Photonics (TEAC) and Martin International (TI) for their respective contributions in the preparation of this three part analysis.