FUEL SYSTEM TESTING: THE WAY AHEAD
Present Situation
To understand the way
ahead it is necessary to know at least in general terms what the situation is
as of today. Fuel system test equipment
is at the present manual with the front panel covered in an array of switches
and buttons and connection to the aircraft fuel system is via one or a number
of switch boxes. The output from these
boxes connect to the aircraft fuel system using an aircraft specific set of
interconnect leads. The tester usually
needs adjusting/calibrating before work commences.
Normally the initial
measurements are concerned with measuring tank capacitances in pF, very often
followed by individual probe capacitances in pF, in both cases the tank(s) need
to be drained to be able to compare the measured values with the published
empty (wet) or empty (dry) values. Look
up tables are usually available to change the capacitance readings in pF to
cabin gauge units of gallons, Kg or pounds. During this process very commonly insulation
measurements on the tank/probe wiring harness are made. The purpose of these measurements is to check
that the fuel probes and interconnect harness are intact and within
specification.
Following this
procedure the tester is switched from a capacitance measuring mode to a
capacitance simulation mode, the purpose being to drive the amplifier, signal
conditioner and hence the gauge indicators (see Figure 1) to known readings,
normally empty and full. Errors in the
system can be negated by adjustments to the amplifier, signal conditioner.
So What’s The
Problem?
To start time,
to drain the tank(s) of an aircraft especially a large one is a complicated and
time consuming business, sometimes it is possible to transfer fuel from one
tank to others, but not always as there are rules concerning maintaining
balance and not over stressing the aircraft plus capacity restraints. Whilst this is going on, which could be hours,
testing is halted.
Secondly there is the
matter of complexity, under the conditions of time pressure and very
often difficult working conditions, the need to complete test sheets, be
constantly changing the tester and test box settings, moving the interconnect
leads and using look up tables puts the technician under considerable strain
and can lead to mistakes being made. This results in retest being necessary
under even greater time pressure.
Thirdly there is the
matter of traceability, as the aircraft moves from one part of the world
to another it is very difficult for paper maintenance records to keep up with
it, which is a pity since a history of fuel system faults can be extremely
useful to a technician especially when dealing with an intermittent fault.
Finally there is the
matter of fault location, this is reported to be the most time consuming
and labour intensive element of the process and mis-diagnosis of wiring faults
or probe faults is common, which leads to complicated and expensive removal and
replacement procedures which do not fix the fault.
Summarising the above
the existing situation can result in aircraft being grounded for a lot longer
than the operators would like them to be.
What a Processor
Brings
In a word a lot,
including automation, memory and computation.
Once the tester
includes a processor the situation described above can be dramatically changed
and most of the current problems can be addressed.
Processing power
brings the ability to make the measurements and simulations automatically
whilst knowing what the results should be with associated limits. This means the technician is guided through
the required series of tests with menu driven software which after each test
will give a result and a pass/fail report. Consequently it is not necessary to find a
written table with limits and calculate whether the result is satisfactory.
When a fail is
reported it is possible if instructed for the tester to go into a fault
diagnostic routine which leads the operator through a logical series of
measurements to isolate the fault in such a way that a replacement part or repair
can be efficiently carried out. These
two features alone would improve the efficiency of test significantly.
However it is possible
to build into the tester software the laws of each fuel tank within the
aircraft (Figure 2) and further the laws of each probe with the tanks. This means that the aircraft can be tested at
any state of fuelling, eliminating the time consuming task of draining the
tanks. This ability also has another not
so obvious advantage in that sometimes a tank probe or wiring will only show a
fault at a particular fill level as reported from the cabin gauges. In this case the fill level can be replicated
and the fault located.
Having memory in the
tester and an ability to transfer data to a stand alone computer not only
eliminates the tedious task of form filling but gives a transportable permanent
historical record, which is time stamped, and also gives location, operator and
tail number. This can be very valuable
to other maintenance staff either on a different shift or at another location.
With regard to
complexity of testing this can be greatly reduced from a technicians point of
view in that not only is the user guided through the test procedure leading to
a pass/fail result but the manual switch boxes can be replaced by relay
switched versions driven by the processor within the tester.
The testing of the
fuel system now becomes virtually fully automatic with perhaps a small amount
technician input as directed by the menu driven software.
In Summary
The use of a processor
built into a fuel system tester will bring a number of advantages, some of
which are listed below.
(a)
Time saving since aircraft at any state of fuelling can be tested.
(b) The user is guided through
the menu’s leading to a pass/fail result
(c)
Fault diagnostic routines make fault location more efficient.
(d)
Memory allows a transferable historical record.
(e)
Fully automatic testing becomes feasible.
(f)
The complexity of testing from the user’s point of view is much reduced.
Figure 1 Fuel System Schematic
Figure 2
Figure 3
