Technology Explained- F-111 AUP Explained Part I

The longer term benefit of the RAAF’s F-111 AUP will be significant cost savings in the maintenance of the fleet, as the current generation of mission avionics is substantially more reliable than sixties technology. This will translate into better mission availability and a much lower demand for avionics maintenance time as well as spare components. As the mission avionics software will be maintained locally, the RAAF will gain the economies of local support as well as the flexibility of integrating weapons of choice.

The F-111 in the ADF Force Structure

The RAAF is in the uniquely fortunate position of having operated the F-111 family for two decades and thus having acquired substantial experience in modern defence penetration techniques.

The occasional public debate over the usefulness of the F-111 in ADF force structure demonstrates that there are still many individuals within the defence establishment and public who are sadly so technically inept to be willing to seriously question the role of the aircraft.

The F-111 is a long range tactical strike platform optimised for low level penetration to heavily defended targets. It has an unrefuelled payload/radius approximately three times greater than that of the F/A-18A fighter bomber (i.e. 5450kg [12,000lb] vs 1815kg [4000lb] at 600nm [1100km]), and utilising inflight refuelling (boom) can attack targets at substantial distance as demonstrated by the 1986 UK to Tripoli raid and the six weeks of sustained interdiction operations conducted during the Desert Storm campaign.

In the ADF force structure the F-111 has traditionally performed the role of the strategic deterrent, allowing the ADF the flexibility to strike at strategic targets with total impunity. Initially using radar/inertial bombing techniques and dumb bombs, the F-111’s role revolved largely about striking at high value targets such as airfields, aircraft, command/control/communication facilities, power installations, bridges, railway facilities, naval bases, docks, munitions dumps and other area targets. With a weapon system CEP (circular error probable) of about 45m (150ft) under all weather conditions, the F-111 had the accuracy to destroy all but the hardest point targets in a first pass strike. This was the role which the aircraft performed very successfully during the Linebacker II air campaign over North Vietnam.

By the early eighties the RAAF sought to expand the role of the F-111 to encompass both maritime strike and first pass strikes against hardened point targets, both of which required a major improvement in accuracy and thus the use of Precision Guided Munitions (PGM). AVQ-26 Pave Tack Flir/laser targeting pods were subsequently acquired; these capable systems improved the CEP of dumb bomb deliveries to about 25m (80ft), but significantly allowed the use of low cost Paveway 2 laser guided bombs with a CEP below 8m (25ft), or more accurate Paveway 3 bombs with a CEP of about 1.5m (5ft). The accuracy of the Pave Tack/Paveway delivery is such that virtually any target can be destroyed by penetration (GBU-24) or by the overpressure resulting from near direct bomb impact (GBU-10). At the today ridiculously low cost of several thousand dollars a round, Paveway 2 laser guided bombs have virtually supplanted unguided bombs as the bread and butter munition of the 82 Strike/Reconnaissance Wing.

The modifications carried out upon the F-111 force involved the installation of fuselage bay cradles for the Pave Tack pods, a cockpit display upgrade and the fitting of additional electronics to support the AGM-84 Harpoon anti-ship missile and the GBU-15 2000lb glide bomb. The addition of Harpoon to the F-111’s arsenal has vastly increased the strategic importance of the aircraft; its superlative operating radius, capable EW systems and high performance allow the use of Harpoon to best advantage. Supported by Jindalee OTHB and P-3C targeting, the F-111/Harpoon is the most potent anti-shipping weapon system in the Western World today.

The GBU-15 was one of the star performers in the Gulf War, used primarily to destroy heavily defended hardened point targets. This weapon carries a nose mounted television or thermal imaging camera which transmits a picture back to the navigator’s cockpit display via a Hughes AXQ-14 datalink pod. The navigator can then manually adjust the weapon’s aimpoint for maximum effect, his steering commands transmitted to the weapon by the datalink pod. With an accuracy and range much better than laser guided weapons, the GBU-15 was a potent addition to the RAAF’s armoury.

Land and maritime strike are thus the principal roles of the ADF’s F-111 force, although another role of major importance is supported. Four F-111C aircraft were modified during the early eighties to RF-111C strategic photo-reconnaissance platforms. These aircraft carry a range of visible and infrared photographic sensors in their fuselage bays, with dedicated cockpit controls and displays. Without the drag penalty of external stores, these aircraft offer the full aerodynamic performance of the basic F-111 airframe, which is even by today’s standards stunning. The recce aircraft are also strike capable.

With its successor the stealthy A-12 Advanced Tactical Aircraft cancelled even before prototype completion, the F-111 is an irreplaceable asset to its current users. There is no other tactical aircraft in existence today which offers its payload/radius and penetration performance. The currently popular use of sensor equipped air superiority airframes suffers from the radius and aerodynamic limitations of designs optimised for turning and acceleration performance in high altitude dogfights.

The success of the Allied air campaigns in the latter phase of WW2, and the recent Desert Storm campaign, stemmed largely from the superior tactical radius of Allied aircraft. There are endless instances which illustrate the simple strategic reality that there is no substitute for combat radius performance. Those who choose to ignore this fact will lose air wars, as has happened many times before and no doubt will happen again. If there is any doubt in somebody’s mind, consider the Battle of Britain, the North African campaigns, the Battle of Berlin, the entire Pacific Air War, the Korean MiG killing operations, Linebacker II over Hanoi, Israeli Air Force ops during the Eighties, and finally the Desert Storm campaign. Argument is futile in this matter.

The USAF F-111 Fleet

The upgrade programme for the RAAF’s fleet of F-111s was under discussion since the early eighties, when the USAF initiated its series of F-111 upgrades. The USAF operated four distinct tactical strike versions of the F-111: the analogue F-111A/E and the digital F-111D/F; supplemented by strategic FB-111As and tacjammer EF-111As, both analogue weapon systems. This resulted in major logistical problems over the years, as Tactical Air Command, and its successor, Air Combat Command, has had to support three distinct avionic suites: the fully analogue A/E, the fully digital D and the mixed analogue/digital F.

The history behind this situation is almost entertaining when we disregard the cost to US taxpayers and operational readiness, but serves as a case study in illustrating the disastrous consequences of allowing lay politicians and bureaucrats into the decision loop of a major systems design/definition project. (It never ceases to amaze this writer, a systems engineer and integrator by trade, how laymen can seriously seek to involve themselves in a process which systems engineers consider an extremely difficult task to do well. Perhaps they know something which we simple technical dummies don’t…).

The mid eighties saw the USAF proceed with its initial upgrade programme for the A/E series of aircraft, involving a new attack radar to replace the APQ-113, a new terrain following radar replacing the APQ-110, and a new nav/attack based on the AYK-18 digital computer. This upgrade differed from the early proposals from the late seventies which revolved about bringing all airframes up to F model standard. The late eighties also saw the USAF award contracts for the Pacer Strike D/F model upgrade, also involving mission avionics. The USAF never disclosed how much of the fleet was refitted.

The collapse of the Soviet Union, conventional arms reductions in Europe and the excessive US deficit have led the US DoD to mothball much of the F-111 fleet, much to the unhappiness of those who understand the vital role performed by this type.

The first to go were the F-111Ds, most troublesome to maintain and never flown in anger. Then the F-111A went. SAC transferred its FB-111As to TAC, which redesignated them as F-111Gs, but these have also been retired (the RAAF benefiting by acquiring no less than 15 low time airframes). The F-111G fleet was subjected to a General Dynamics factory upgrade when transferred from SAC to TAC. This upgrade was centred on mission avionics, and saw a new nav attack system fitted, built around the Honeywell H523 Ring Laser Gyro, and a new mission computer. SAC specific equipment, such as support for the nuclear SRAM defence suppression missile and the nose mounted ASQ-119 astrocompass, was removed. The unusual bulge on the nose of the RAAF F-111G is the only remnant of the astrocompass installation.

By 1994, the active fleet had been consolidated into the overstrength 27th TFW at Cannon AFB in New Mexico, with the 428th FS flying the F-111E (analogue like the older F-111A/C in RAAF service), the 522nd, 523rd and 524th FS the hybrid F-111F/Pave Tack and the 429th and 430th Electronic Combat Squadrons flying the specialised EF-111A Raven tacjammer.

Growing budgetary pressures have forced the retirement of the remaining strike variants in the 1995 to 1996 period. 26 Australian Aviation June 1995

The role performed by these aircraft will now be undertaken by the shorter ranging F-15E and F-117A. The USAF was however very reluctant to let go of its unique EF-111As, which albeit somewhat behind the Navy EA-6B in electronic capability (mainly due to contractor service infighting over upgrades), have far superior aerodynamic performance allowing them to fly escort jamming rather than just standoff jamming missions. Current schedules call for the aircraft’s retirement in the 1996 to 1997 period. Losing the EF-111A force is a major blow to the USAF as it is a capability which is near impossible to replace other than by a similar aircraft type.

Much has been said in this country about the USAF’s phase-out of the F-111 force, mainly in relation to support of our domestic F-111 fleet. The USAF held on to its F-111s for as long as was politically tenable; the ultimate choice the USAF faced was to forgo the vitally needed new F-22A Advanced Tactical Fighter, or the backbone of Air Combat Command’s strike fleet. The latter went, with much reluctance and after a long and intensive political battle. An interesting note here is that the USAF first disposed of much of its B-52 fleet, before finally letting the beancounters at the F-111 force, as it considered the aircraft’s combination of precision strike capability and payload radius performance to be one of the service’s most valuable assets.

The retirement of the USAF fleet should not significantly impact RAAF operations. The aircraft is now in the latter half of its operational life cycle, is technically mature, and what needed to be learned about its idiosyncrasies has been learned. The mothballed US fleet is a fallback source for structural, engine and non-avionic systems spares, almost certainly beyond the RAAF’s capacity to exhaust. The comprehensive upgrade being done on the RAAF F-111 avionics will mean that avionic spares will be readily available from the manufacturers, moreso since most of the components used are in wider military aviation usage, such as the RLG INS. The G model aircraft will share a considerable number of avionic components with the new upgraded A/C-model airframes, and thus their support in service will not be an issue, particularly given the abundance of mothballed airframes in the US.

The only alternative to upgrading the RAAF F-111 fleet to match the F-111’s capability was to acquire either the F-15E or F-117A, the former of which is about to cease production, the latter having been out of production for some years. Acquiring either type would have also required additional tanker support, with refuelling booms to provide the extra radius to match the F-111. The cost of the RAAF’s AUP programme, against the cost of acquiring a package of F-15Es, support facilities and two to four extra boom equipped tankers, is trivial.

As a result of the RAAF’s Avionics Upgrade Programme, the ADF will retain the most potent conventional tactical strike aircraft in the Western alliance, an asset which will serve Australia well into the twenty-first century.

The RAAF F-111G “G-Model Pig”

The government’s decision to acquire 15 former USAF F-111G aircraft was a major surprise for most observers of the Australian defence scene. Fortuitously for the ADF, the government did the proper thing and followed through on its promise, with the aircraft entering service over the last twelve months.

The primary reason for acquiring these aircraft was to reduce the number of hours flown on the F-111C airframes, which like all highly stressed airframes have a finite number of flying hours in their structures. Spreading the flying hours across the expanded fleet effectively extends the life of the F-111A/C airframes. The F-111G airframes have substantially lower accumulated airframe fatigue than their TAC and RAAF counterparts, as they spent most of their previous life in Strategic Air Command.

Former SAC nuclear armed FB-111As, the RAAF’s recently acquired F-111G aircraft have proven better than expected, with notably better payload radius performance than the established F-111C. These aircraft were acquired primarily to alleviate the flying load on the F-111C airframes, and thus reduce the cumulative fatigue load which limits the useful life of the airframes.

The issue of the F-111A/C fleet size has always been problematic, as the original purchase of 24 airframes to equip two essentially operational squadrons really fell somewhat short of the number of airframes properly required to provide full squadron strength, with an allowance for conversion training, depot overhauls and attrition reserves. The RAAF has always needed the extra airframes, but the US were loath to part with the aircraft during the Cold War and our government was always lukewarm on the subject as it involved spending a lot of money. The only additional airframes acquired were the four attrition replacement F-111As which were converted to C status and are with 6 Sqn. The acquisition of the G-model airframes has finally provided the airframe reserves to allow for full squadron strengths in a sustained operational environment.

The RAAF’s F-111Gs were built as General Dynamics FB-111A strategic bombers, serving with USAF Strategic Air Command (SAC) and tasked with suppressing Soviet strategic air defences and related C3 systems, using the AGM-69 SRAM missile. SAC had originally acquired 75 aircraft, with IOC being declared in October 1971. These aircraft equipped the 509th BW at Pease AFB, and the 380th BW at Plattsburgh AFB. In SAC service the aircraft would have carried 170kT W69 armed SRAMs or free fall 1MT B43, 10kT B57 or variable yield (10 to 500kT) B61 “special” devices. With a maximum payload of six SRAMs or freefall weapons, the FB-111A was truly a doomsday machine. Had the balloon gone up, the FB-111A force would have preceded the B-52G and H aircraft into Soviet airspace, carving access corridors through the PVO’s air defence SAM belts.

After a distinguished but thankfully uneventful career in SAC, the FB-111A wings were deactivated and the aircraft reassigned to Tactical Air Command, to bolster the 4 Wing TAC F-111 fleet. TAC redesignated the aircraft as F-111Gs and formed the 427th TFS at Mountain Home to operate the aircraft. The F-111G aircraft assigned to TAC went through a factory refit which substantially modernised the aircraft’s offensive avionics suite and communications equipment, to a standard very close to the RAAF’s AUP.

From a technical viewpoint the FB-111A was a distinct variant in USAF service, an optimised strategic penetrator built to defeat the massive Integrated Air Defence System (IADS) of the Soviet V-PVO. The airframe shared the “big” wing of the USN F-111B and RAAF F-111C, and the strengthened undercarriage of the latter subtype. The aircraft’s fuselage was closest to that of the F-111D, and employed the “stall proof” Triple Plow II inlet geometry shared by later model airframes. The FB-111A was fitted with the TF-30P-7 engine which delivered nominally 16% higher thrust than the P-3 used in the earlier A/C and E models. SAC had specified a wide range of detail design changes to the aircraft, including a stowable crew access ladder, jettisonable weapon pylons, wiring and plumbing to the outer wing stations, various cockpit ergonomic changes (primarily in switch locations), dual 285/300 USG internal weapon bay fuel tanks and a distinct avionics and electronic warfare fit.

The original offensive avionics system was built around an AJN-16 inertial nav-attack system and AYK-6 mission computer, both common to the F-111D Mk.II and F-111F Mk.IIB systems. With the D-model, the FB-111A also shared the AYN-3 cockpit display and the APN-167 radar altimeter. Unique to SAC were the newer APQ-134 Terrain Following Radar, the APN-185 Doppler navigation equipment and the ASQ-119 Astrocompass. The latter two items were needed to improve the aircraft’s navigational accuracy on long polar sorties into Siberian airspace.

The recent USAF avionics refit of the F-111G fleet replaced antiquated seventies offensive avionics system with a contemporary digital computer and dual ASN-41 (Honeywell H523 or Litton LN-39) Ring Laser Gyro Inertial Systems (INS), integrated with new APN-218 Doppler nav equipment, which replaced the obsolete APN-185. The integrated nav-attack system uses Kalman filtering techniques to yield the best possible position estimate from the dual INS channels and the Doppler velocity readings, providing very high accuracy. A newer TFR was fitted, replacing the seventies APQ-134. The elderly ASQ-119 astrocompass was removed and the provisions to fire the SRAM missile were deactivated, as this weapon was not used by TAC. Much of the new avionic equipment is identical to the RAAF AUP systems, albeit slightly older production versions.

The FB-111A defensive avionics suite was enhanced to provide for better survivability in the V-PVO’s hunting grounds. While the aircraft shared the ALR-62 Radar Homing And Warning System (RHAWS) with the other variants, the improved ALQ-137 defensive ECM system was fitted, with additional aft facing antennas, improved band coverage and more sophisticated jamming techniques generation, compared to the TAC standard ALQ-94 (used by the A/C/D/E/F models). The only other type to carry this sophisticated system is the high value EF-111A tacjammer. Unlike most aircraft defensive ECM systems, the ALQ-137 combines both track breaking and noise jamming techniques. The AAR-34 IR tail warning system (MAWS) is retained, as is the ALE-28 chaff/flare dispenser. The RAAF F-111G aircraft retain the USAF (SAC) EW suite, and thus have the most capable ECM fitted to any aircraft in the Southern Hemisphere. Compared to the basic F-111C airframe, the F-111G carries an additional 2215 litres (585 USG) of JP-4 fuel which is about 1590kg (3500lb) or 11% more than the basic capacity. The fuselage and wing tank capacity is identical, at 18,890 litres (4990 USG) or about 14,710kg (32,400lb). The engines deliver almost 20% higher thrust in service.

With the aircraft freshly out of the USAF avionic refit, the Doppler enhanced dual RLG INS provides superb accuracy; in fact the F-111G is the most accurate bomb delivery platform in RAAF service today, with blind radar dumb bomb delivery errors a fraction of the analogue F-111C’s sixties LN-14 system’s errors. It will be interesting to see whether the post AUP digital F-111C system matches the F-111G.

Australian Aviation recently had the opportunity to discuss aircraft performance with a number of 6 Sqn G Flight aircrew. The RAAF crews are very happy with the aircraft, which are easier to fly than the standard C-model aircraft due to the more robust intake, accelerate and climb much better due to higher installed thrust, fly further due to greater fuel capacity, and are so accurate with the Doppler enhanced INS that the navigators have to be selective about terrain features used for INS updates, so as not to fall foul of accuracy limitations in current maps.

Current RAAF intentions are to refit the aircraft with the digital flight control system to be used on the AUP F-111A/C and the AWADI/DSTO ALR-2002 Radar Warning Receiver also to be fitted to the F-111A/C. This would save maintenance costs on two of the most critical aircraft subsystems. There is no intention at this time to rework the remaining F-111G avionics system to the AUP standard. This is a sensible decision, as the F-111G offensive avionics are only a few years older than the AUP systems, of the same generation of technology, and share many common components.

An issue will be integration with RAAF specific weapons such as the AGM-84 Harpoon ASM. Such weapons, whilst not particularly technically difficult to integrate, do require some software and hardware changes, and it remains to be seen whether the RAAF will do so.

Even if the RAAF does not integrate its full suite of PGMs into the F-111G avionic system, the aircraft can still be most effectively employed in defence suppression and blind radar (dumb) bombing sorties, and as laser guided bomb or standoff weapon “carrier” aircraft, paired with an F-111C which would use its Pave Tack or datalink to guide weapons for both aircraft. This approach was used very effectively both by the USAF and the RAF in the Gulf, the latter having many more bombers than laser designator pods.

It is worth noting that the F-111G, with its better performance, endurance and defensive ECM capability, would make an excellent electronic combat platform to support the strike aircraft as anti-radiation missile (e.g. HARM) firing platforms. The RAAF would be well advised to consider this in the course of the upcoming ALR-2002 upgrade, and look at fitting the G-model 2002s with Emitter Locating System (ELS) facilities for this purpose. This would be a very cheap way of enhancing the RAAF’s electronic combat capability, as the incremental cost of adding an ELS capability to the 2002 is much lower than fitting out with additional equipment. To date electronic combat has not received the attention it properly deserves in the ADF force structure, and the 2002 upgrade is a golden opportunity to redress this deficiency at a minimal cost.

It is the author’s view that the F-111G acquisition was the single best value for money defence purchase in the last two decades, and has finally brought the 82nd SRW up to an operationally effective strength. We can hope the government will fully capitalise on this wise investment.

Part 2 explores in depth the F-111 Avionics Update Programme which will give Australia the most potent conventional strike aircraft in the Western World. This will feature in our August issue – don’t miss it!