988
FLIGHT International,
13 December 1973
The Jaguar will become the largest strike
element within Nato. This particular
aircraft,
seen at Lossiemouth recently, is orte of the
first to be delivered to the RAF. It is fitted
with the fin-mounted ECM pack but is without
the Ferranti laser range-finder and marked
target seeker
AVIONICS
digital computer which provides navigation and weapon-aiming information
and controls the data that the pilot
sees on his display (see Jaguar—A
New Look at Combat Effectiveness,
Flight, February 22, page 257). The
high level of integration associated
with Navwass is such that its computer and navigation control unit are
capable of testing the complete system
at first-line level. The MCS.920M computer organises ground testing exactly
as it operates in flight. Test results
and fault locations are shown as coded
annunciations on the navigation control unit.
The only additional test equipment
that is needed for first-line servicing is
a program loading unit which, by
means of a tape cassette, inserts the
test program into the Jaguar's digital
computer. Because the test program is
stored on tape any modifications to
the test sequencing that should prove
necessary are easily made without the
need to modify the loading unit.
The ability of a digital computer to
test itself is fairlv well known. In the
case of Jaguar what is perhaps less
well appreciated is the access such a
computer gives to all units within the
integrated system. This enables it to
detect and locate failures throughoi
the airborne installation. Marconi
Elliott has already used this technique
successfully in a number of applications, including digital navigation,
head-up displays, weapon-aiming systems, autopilots and, experimentally,
in engine-control systems. The company points out that about 2,000
digital systems have been sold and
these have now accrued nearly one
million flying hours.
Another important attribute of this
self-test technique is that it reduces
the influence of basic ground-crew
training on successful fault diagnosis.
It lowers the skill level required while
speeding up the whole, process of
fault-finding and rectification.
The effectiveness of testing is
greatly enhanced when it takes place
continuously
during
flight,
with
ground checks being used to confirm
a diagnosis. Jaguar's Navwass carries
out in-flight monitoring without compromising the normal navigation and
weapon-aiming modes—any indication
of a fault being stored for later readout by the rectification team.
The block diagram shows how the
computer has access to the whole of
stopping the check until the correct
selection is made.
To check the inertial velocity sensor
takes only 30min. While this is taking
place the system also tests other linereplaceable units. If both the inertial
velocity sensor and an individual linereplaceable unit are found to be
faulty, the control unit indicates the
fault in the replaceable unit, while the
computer remembers the sensor fault
for indication once the other secondary
fault has been rectified. The complete
Navwass can be checked out "end-to
end", including associated systems
such as the air data computer and
navigational aids, in less than one
hour.
The method of testing designed into
Jaguar's integrated Navwass system
eliminates the need to break into the
aircraft interconnector wiring which is
left undisturbed. This avoids the problems caused by disconnecting plugs
and sockets (with the inherent danger
of
self-induced
failures)
which
occurred with earlier designs. Furthermore, because the test stimuli are all
generated within Navwass there is
much less chance of damage being
caused to aircraft systems by faults in
ground test equipment.
the system once the ground test program has been inserted. The joint
flexibility of the computer and the
navigation control unit can be clearly
seen, with the same controls and displays that are used in flight also being
used for the test function. The indicators on the navigation control unit are
used during rectification to display
which line-replaceable unit is under
test—any fault being denoted by an
interrupted signal.
To carry out a ground check when
a fault has been reported, the ground
technician loads the test program into
the program loading unit and connects
it up to the Jaguar's computer. From
the cockpit he selects the test program
in "blocks"—each block relating to a
different line-replaceable unit and its
associated control highway. This enables him to test the suspect areas
first before rapidly checking out the
rest of the system, using the waypoint
selector on the navigation control unit
to call up the different test blocks.
The testing process is carried out by
the on-board computer and is entirely
automatic. The manual operations
needed are few and simple requiring
only that certain basic selection
sequences be adhered to. Should an
operator make an error, the test syste: automatically detects this and
." ish
the control unit display while
The portable program loading unit with its
test cassette is connected to the aircraft
when Navwass is being checked out on the
ground
Digital indicators
'(annunciates system serviceability)
Air data computer,
Head -up display,
Tacan, ILS,
Armaments,
Gyro magnetic compass
digital compute r
Computer
memory
1*
II
II
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Program
loading
unit
,
l^t™
i ^
"' Test
tprogram
]cassette