by Carlo Kopp
published in Australian Aviation, May, 1990
(c) 1990, 2005 Carlo Kopp
One of the most significant yet
least publicised capabilities to be acquired by the RAAF in this decade
is that of inflight refuelling. The inflight refuelling tanker is
a potent force multiplier in nearly all defensive and offensive
scenarios, providing fighter and strike aircraft with substantial gains
in effective payload radius performance. In practical terms this
translates into greater time on station for air superiority aircraft and
larger weapon loads at greater radii for strike aircraft.
The first major application of inflight refuelling in a real conflict
took place during the Vietnam conflict, where USAF SAC tankers supported
not only strategic aircraft, but also tactical fighters which were based
in Thailand and South Vietnam. It was here that the tanker proved its
worth in tactical air warfare, allowing fighters and strike aircraft to
enter and exit hostile airspace along tactically favourable routes with
substantial weapon loads. SAC KC-135s saved many tactical fighters from
an unenviable fate when their pilots ran their tanks dry in afterburning
dogfights deep inside hostile territory. In purely defensive scenarios
the tanker has acquitted itself equally well, allowing extended combat
air patrol at substantial distances. Both the RAF's North Sea and North
Atlantic air defences and the USAF TAC's Alaskan air defences are
largely dependent upon inflight refuelling, given the wide areas to be
defended. In both of these situations the tankers fly out with the
fighter aircraft which will top up their tanks as required throughout
the mission. This is particularly relevant when investigating poor
quality target tracks, as the fighters may spend a lot of time searching
for their quarry. The alternative of using unrefuelled fighters is
simply not viable, as their time on station and radius of action would
be constrained, thus forcing the use of many more aircraft to maintain
a continuous presence in a given area.
The Australian requirement for inflight refuelling stems largely from
this scenario. Defending the air and sea space of the North and North
West imposes constraints not unlike those faced by the RAF and the USAF
in the Northern hemisphere. The RAAF will defend Northern Australia from
a chain of airfields stretching from Western Australia to Cape York.
Learmonth, Curtin, Tindal, Gove and a planned bare bones base on Cape
York would provide basing for F/A-18 aircraft, with Tindal serving as a
base for squadrons deployed from Williamtown. Forward deployed F/A-18s
would then refuel inflight as required by the mission and
tactical/strategic situation in place at the time.
The RAAF's specific requirement for an Air-Air-Refuelling (AAR)
capability has existed for at least two decades, but has been repeatedly
preempted in the acquisitions queue by other programs which were deemed
to have a higher immediate priority. By the mid eighties the requirement
had firmed up and approval was granted for the RAAF to proceed with a
specific program to modify four Boeing 707 transports to a dual role
tanker/transport configuration. The RAAF had sought a very modest
capability with the principal objective, in common with many other RAAF
capabilities, to provide a core force for operational training which
would provide a limited operational capability if required. This core
capability with four airframes and several qualified crews could then
be expanded to a full operational capability, were the requirement to
ever arise.
The RAAF tanker force will support the F/A-18 force in training, on
long range deployments, during exercises with Allied air forces and when
the opportunity arises, also refuel Allied aircraft during exercises.
The latter would hopefully help repay those of our Allies who have in
the past generously provided the RAAF with opportunities to practice
AAR.
At this time the RAAF program will involve only a hose/drogue
capability to support the F/A-18, there are no immediate plans to
install a refuelling boom to support the F-111, or to refuel other
tanker/transports.
The RFT for the RAAF tanker program was issued in June, 1986, with
source selection taking place in late 1987 after a detailed evaluation.
The contract was subsequently awarded on the 23rd June, 1988, to Israel
Aircraft Industries (IAI), who nominated Hawker de Havilland as
subcontractors. The proposal involved a close derivative of an existing
system design which IAI had successfully supplied in the past.
HdH would, as subcontractors, carry out the installation of upgrade
kits supplied by IAI. To that effect, HdH made a major financial
commitment in constructing a new hangar at Tullamarine airport, adjacent
to the existing Ansett facility.
Pic 1
Cameras clicking, A20-629 rolls
out of the new HdH Tullamarine hangar to an audience of dignitaries,
journalists and senior RAAF officers. This is the first aircraft to be
modified and was at the time being prepared for ground testing of the
fuel system installation. Following ground tests and further avionic
installation work, the aircraft entered its flight test program.
Cutaway of Mk.32B pod.
Pic 2
Detail view of the port wingtip
Mk.32B pod installation. Note that at this time many panels were
removed from the aircraft to provide access to systems for initial
ground testing. The recess in the aft lower part of the pod conceals
the pod status indicator lights.
Pic 3
Detail view of the ventral TV
camera turret. Note the 360 degree field of view enjoyed by the camera,
which is used by the Flight engineer to view the progress of the
refuelling operation.
Pic 4
A20-627 awaiting its
conversion. The RAAF will be converting four of its six B-707-338C
aircraft to dual role tanker/transport configuration.
The RAAF Tanker/Transport
Upgrade
The upgrading of the four B-707-338C aircraft to a dual role
tanker/transport configuration involves the installation of AAR hardware
and a substantial exercise in installing and integrating AAR mission
avionics. The AAR hardware installation includes pumps, plumbing,
hydraulics, pods and associated instrumentation and control hardware,
while the avionic upgrade involves the fitting and system integration of
Tacan beacons, IFF equipment and some additional systems.
All hardware fitted is fully Milspec qualified and therefore has a full
operational capability. The stated limitation in force operational
capability results from the tanker/transports' MTOW limit given the
runway lengths available, ie the internal fuel capacity of up to 158,000
lb held in the B-707's wing and centre section fuel tanks is comparable
to the typical load of dedicated KC-135 tankers. The aircraft are
powered by four 18,000 lb class two spool JT3D-3B (civilian derivative
TF-33) low bypass ratio turbofans. Higher takeoff weights could be
achieved with CFM-56 high bypass ratio fans as fitted to current build
E-3C/E-6A/KE-3A and reengined KC-135R airframes, but this would be
rather expensive and is not currently under consideration. While the
fitting of fuselage fuel cells and/or bladders is possible, it would
offer little in return given the performance limited MTOW constraint.
Furthermore, it would restrict the role of the aircraft to AAR alone
thus limiting the utility of the otherwise dual role aircraft. Fitting
fuselage fuel tankage would bring the fuel capacity up to about 180,000
lb were it ultimately required.
To place the matter of force capability in to context, an F/A-18A with
4,000 lb of bombload (ie 4xMk.83 low drag) and three 315 gal tanks (ie
6,000 lb fuel) and full internal fuel (ie 11,000 lb) has a combat radius
of about 550 NM. Refuelling inflight at 450 NM on the outbound leg and
receiving about 8,000 lb of fuel, its strike radius is increased to over
800 NM. With about 50,000 lb of fuel to offload at this radius, a single
B-707 tanker can refuel a strike force of six F/A-18A aircraft. Assuming
three tankers available for operations at any given time, one of which
is being used as a backup, the RAAF ought to be capable of flying a 12
aircraft F/A-18A strike force against a target at 800 NM. To place this
in to further context, an F-111A/C with 4,000 lb of bombload has an
unrefuelled combat radius of about 1,100 NM. At about 800 NM combat
radius an unrefuelled F-111A/C can deliver up to 8,000 lb of bombload.
In an operational situation, assuming the RAAF were to have 12 F-111A/C
aircraft available for strike operations, the use of the tankers and
F/A-18A aircraft would increase the RAAF's medium range strike
capability by about 50 %. That is excellent value for taxpayers' money.
The AAR hardware installation is based upon a design produced by the
Bedek Division of Israel Aircraft Industries for the Israeli Defence
Force, which has several tanker aircraft in service. The system design
has had some detail changes to meet the RAAF's engineering requirements,
with installation work being carried out by HdH under the supervision,
where appropriate, of IAI staff. Most of the hardware is sourced in the
US and UK, with remaining components supplied as upgrade kits by IAI.
The refuelling pods are manufactured and supplied by Flight Refuelling
Ltd in the UK.
Internal modifications to the B-707 systems are necessary. The AAR
system uses hydraulically powered fuel pumps to drive fuel to the pods,
which in turn feed the fuel via hose to the receiver aircraft. Four
submerged J.C.Carter fuel pumps are situated in the centresection fuel
tank and these feed fuel into 3" pipes via a crossfeed valve arrangement
which allows either pod to be fed by any pump. This functional
redundancy was adopted to minimise the likelihood of fuel pump failure
interrupting an AAR hookup. Under operational conditions one pod will be
supplied by a selected pump. The 3" pipes are installed through the
wing main spar box using attachments designed to decouple mechanical
loads from the wing structure. The pipes then attach to mounting flanges
within the wingtip pylons, the pylons are structurally attached to the
forward and aft main spars. The fuel management strategy used during
AAR operations differs from that adopted for regular B-707 operation,
as fuel from the inboard and outboard wing tanks is pumped into the
centresection tank from where it is offloaded to receiver aircraft.
Hydraulic fluid for the pumps is supplied via 1 1/4" pipes and hoses
from two redundant utility hydraulic systems, designated UT1 and UT2.
UT1 is the basic B-707 hydraulic system which is powered by two Abex
engine accessory drive hydraulic pumps fitted to inboard engines #2 and
#3. Typically one fuel pump will be driven by UT1, together with
remaining aircraft systems such as flight controls. UT2 is a new
installation carried out as part of the upgrade and involves, other than
the necessary plumbing, the installation of another two Abex hydraulic
pumps on outboard engines #1 and #4. Again, under operational
conditions, UT2 will in turn supply the second pod. This highly
redundant strategy is designed to allow system operation with full or
partial capability in the event of hydraulic or fuel pump failures.
The installation employs the latest Flight Refuelling Ltd Mk.32B pods.
The RAAF will be the first operational user of the Mk.32B, which is
currently under evaluation on USAF KC-10A tanker/transports and is
planned for testing on USAF KC-135 tankers.
The Mk.32B pod is the latest in a long line of refuelling pods built by
FRL. It is a direct descendant of the Mk.32/2800 pod in use with the RAF
since the 1980s. Unlike other pod designs which rely upon parent
aircraft high pressure hydraulic fluid to drive pumps and drums, the
Mk.32B uses a clever combination of fueldraulic and mechanical hardware
to achieve the same capability with substantially greater reliability
and robustness. In addition, pod control is carried out by a
microprocessor based digital controller, which allows pod performance
characteristics to be tailored to a particular customer's requirements.
The pod is fully self contained and derives its power (ie fuel working
pressure) from a variable pitch ram air turbine which is controlled by
the pod computer. Fuel fed from the parent aircraft at pressures as low
as 6 psi is charged to a working pressure of about 50 psi by a
centrifugal pump, which feeds the fuel into a multiport rotary valve,
which directs the fuel either back into the parent aircraft fuel system
or into a vane pump/motor. The valve position and fuel flow are
determined by pod operating mode. Three modes are employed, trail,
transfer and rewind. In trail mode, when the pod deploys the hose/drogue
assembly, the vane motor acts a pump and thus brakes the drogue as it is
dragged out of the pod by the slipstream. Once the drogue is in its
proper trailing position, receiver aircraft may connect.
Receivers must insert their probes into the drogue reception coupling
to connect their fuel line in and then must push the drogue forward to
automatically initiate fuel transfer. Proper hose mechanical loading is
ensured by a set of tensator springs in the pod, these store mechanical
energy as the drogue unwinds the hose off the drum and once in transfer
mode, remove hose slack. When refuelling is complete, the energy in the
springs is used to wind the hose back on to the drum.
While fuel transfer is under way, the high pressure fuel flow produced
by the centrifugal pump is fed into the hose and hence receiver fuel
system, with flow rates of nominally 2800 lb/min (FRL have quoted better
figures achieved in testing).
Once refuelling is complete, the receiver falls behind the tanker and
disengages its probe when the hose reaches full trail position. In the
event of the receiver damaging the drogue such that it becomes a
flailing hazard to the tanker, the hose/drogue assembly can be
jettisoned.
Pod status is signalled to the receiver aircraft with a set of shrouded
yellow, red and green lights on the rear of the pod. These together with
other pod functions are controlled by the pod computer. The self
contained Digital Refuelling Control Unit monitors pressure,
temperature, flow rate, position and speed sensors and controls in turn
the various valves and actuators required to control the pod. The
software continuously monitors pod health and signals its status to a
flight deck monitor panel via an ARINC 429 serial databus. The bus also
carries commands down to the pod.
The pod design had to meet both Milspec and more stringent US FAA
requirements for reliability and design integrity, this in turn will
translate into very favourable life cycle costs. The software based
control system allows rapid adaptation to various types of tanker and
receiver airframes, over a wide range of airspeeds (ie 160-325 kt IAS).
The pod may be operated at altitudes up to 35,000 ft, it weighs in at
1190 lb.
Additional mission support hardware has also been installed to support
AAR operation. Floodlights will illuminate the aircraft's tail surfaces
to provide receiver aircraft with a clear view of the tanker's tail
during night operations. In addition, infrared floodlights illuminate
the receiver aircraft so that the tanker Flight Engineer can view the
operation, day and night, with a remote television camera. The TV
camera, fitted with a servo driven zoom lense, is mounted on a vibration
damped turntable assembly inside a ventral rear fuselage turret. This
installation allows the Flight Engineer to view the aft lower hemisphere
about the aircraft. The camera installation is fully qualified for
B-707 operating conditions and also feeds a video tape recorder which
is used for postflight debriefing.
Flight deck modifications specific to the AAR function include an
additional column at the flight engineer's station, mounting control and
indication panels for the pods, pumps and lights and a small TV monitor
and joystick controller for use with the TV camera system. The
navigator's station is transformed into a Mission Coordinator station,
with panels mounting a CRT display and controls for the mission
avionics.
The mission avionics fit is comprehensive. The aircraft will be fitted
with new dual redundant inertial navigation equipment (INS), Tacan, IFF
and upgraded communications. The INS will be Litton LN-92 ring laser
gyro equipment. All aircraft will be equipped with dual redundant
Collins SIT-421 IFF transponders, and a forward facing Hazeltine
AN/APX-76B(V) IFF interrogator with dipoles mounted on the existing
weather radar antenna. A Collins 150 Tacan system will be installed,
this consists of AN/ARN-118 and APN-139 subsystems which allow receiver
aircraft to locate and rendezvous with the tanker.
Additional communications equipment includes a Magnavox AN/ARC-164 UHF
transceiver and a Collins DF-301E/F UHF DF set.
The Mission Coordinator's console is equipped with a Bendix
MultiFunction Display (MFD), which is used for displaying mission
control information and weather radar imagery. The aircraft's existing
weather radar installation was substantially revised by HdH, who added
in facilities for timesharing the radar between the pilots and Mission
Coordinator. This was a non-trivial engineering exercise which involved
hardware changes to route radar video to the alternate displays. In
timeshare mode the refresh rate of the respective displays is halved.
Functionally, the MFD will be employed to display radar images overlaid
with navigational and IFF symbology, this allows the Mission Coordinator
to evaluate weather conditions in a given area and reposition waypoints,
tracks and rendezvous positions accordingly. Raw data for this function
is provided by the INS, Tacan and IFF interrogator subsystems.
In summary the RAAF's tanker hardware upgrade is a lean yet effective
combination of state-of-art hardware and existing systems, with
sufficient system redundancy to provide a fully operational capability.
Project Management
The project management structure is divided into the management teams
of the RAAF, the prime contractor and the principal subcontractor. The
leader of the RAAF group is the part time project director, Group
Captain P.J. Rusbridge, to whom the full time project manager, Wing
Commander Trevor Couch, reports. The project manager is supported by
three RAAF engineers on a full time basis, and by other RAAF personnel
from Logistics Command and Air Headquarters as required. These include
the Life Cycle Support group, Airworthiness, 486 Sqdn (B-707
maintenance) and 33 Sqdn (B-707 aircrew). During the flight testing
phase, flying operations will be under the control of Wing Commander J.
Foley, the program flight test director.
The prime contractor, IAI, have a small team of senior personnel on
site who perform an essentially supervisory role, overseeing the work
carried out by HdH personnel. This is particularly important with work
being carried out on the first airframe to be upgraded and it is
expected that HdH will assume greater responsibility for supervision on
subsequent airframes. Interface with IAI's design office and corporate
management in Israel takes place via the local team.
The HdH organisation is more comprehensive, with various groups from
manufacturing, design, assembly and installation making their respective
contributions. Installation work takes place at the Tullamarine
facility, with manufacturing and design work carried out at the
Fisherman's Bend site.
At the time of writing installation work was close to completion on the
first airframe, A20-629, the roll-out for development and acceptance
flight testing taking place on the 12th February. The project has
proceeded very smoothly for a systems integration exercise of such
complexity involving three major parties. Conventional management wisdom
suggests the potential for problems increases with the increasing number
of organisations involved in a given task, it was therefore most
refreshing to see a program successfully defy the odds.
Needless to say, this is no accident, and results from a sustained
effort by the management teams of all three organisations. Group Captain
Rusbridge indicated that 'The project is characterised by a strong sense
of cooperation between all parties involved and a clear commitment to
achieving program objectives. This results largely from a firm project
management policy based upon harmonious work relationships - being able
to openly state your point of view while accepting the other party's
point of view...' This strategy has clearly paid off and contrasts
sharply with the adversarial contractor/customer relationships which
often accompany complex defence programs. The large commitment of HdH
upon entering the program is clearly appreciated by the RAAF, the
program director stating ' It is a particularly welcome event that HdH
showed confidence in the program by making a major capital investment
which should ultimately be useful in other applications'.
The next major phase of the project is the acceptance testing which
will take place from February to April, and as with all projects is the
phase which will determine how well the systems integration exercise
turned out. Significantly, there is agreement between all parties on
testing procedures, a traditional area of disagreement in many projects.
Upon completion of acceptance testing the aircraft will be handed over
to the RAAF and subsequently returned to operation. No doubt it is
eagerly awaited by 33 Squadron.
The tanker/transport upgrade program is an interesting case study in
project definition and management, an instance where a system has been
carefully defined and tightly matched to its role, and where its
implementation has been carried out in a relatively painless fashion,
largely as a result of solid management practice and good engineering.
The final comment is left to
the program director, Group Captain Rusbridge: 'The RAAF hopes that at
the end of the project people will find lots of good things to emulate.
This will ultimately be for the overall good of the ADF and Australian
industry.'
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