00:00:00
this lesson we'll look at fuel quantity
00:00:03
measuring systems fuel quantity can be
00:00:08
measured by one of two methods either
00:00:10
the volume of the fuel can be measured
00:00:16
mass or weight of the fuel can be
00:00:17
measured
00:00:21
volume measurement can be achieved by
00:00:24
having a float in the fuel tank but
00:00:30
mechanism to a variable resistor the
00:00:35
cockpit gauge is normally an ammeter
00:00:38
calibrated in gallons or liters in a
00:00:42
circuit with a resistor as the fuel
00:00:46
level decreases the resistance increases
00:00:50
this leads to a reduction of current in
00:00:52
the circuit so the indication on the
00:00:55
ammeter decreases the gauge is
00:00:58
calibrated to show the correct fuel
00:01:00
quantity with the aircraft in straight
00:01:02
and level flight this system is normally
00:01:08
restricted to light aircraft it has two
00:01:11
major drawbacks
00:01:14
firstly it is subject to maneuvering
00:01:17
error that is to say as the aircraft's
00:01:21
attitude changes the indicated fuel
00:01:24
quantity will change secondly this type
00:01:30
of system cannot compensate for changes
00:01:33
in the specific gravity of the fuel so
00:01:35
as the fuel gets colder the indicated
00:01:39
quantity will decrease and vice versa
00:01:42
should the fuel temperature increase
00:01:47
on modern gas turbine engine aircraft it
00:01:51
is important that we measure the fuel
00:01:53
onboard the aircraft in terms of weight
00:01:55
or mass rather than volume there are a
00:01:59
number of reasons for this firstly it is
00:02:04
important to know the weight of the fuel
00:02:06
in the aircraft for airframe performance
00:02:08
reasons and secondly the energy in the
00:02:14
fuel is measured by weight or mass not
00:02:17
volume so the pilot needs to know the
00:02:19
weight of the fuel that he has available
00:02:28
measuring the weight or mass of the fuel
00:02:31
is done by having variable capacitors in
00:02:34
the fuel tanks the capacitive method
00:02:39
by supplying the two plates of a
00:02:40
capacitor with alternating current the
00:02:43
current that flows in the circuit now
00:02:46
depends on four factors the level of the
00:02:50
voltage applied the frequency of the
00:02:53
supply
00:02:55
size of the plates and the dielectric
00:02:57
constant of the material separating the
00:03:00
plates
00:03:03
our circuit three of these factors are
00:03:05
fixed and the fourth the dielectric
00:03:08
constant is variable because the
00:03:11
dielectric consists of fuel and air a
00:03:16
dielectric consisting of fuel will allow
00:03:19
more current to flow in the circuit than
00:03:21
a dielectric of air will the level of
00:03:25
current flow will therefore be
00:03:26
proportional to the volume of the fuel
00:03:28
in the tank by measuring this current we
00:03:31
can know the volume of the fuel in the
00:03:33
tank
00:03:39
if the temperature of the fuel drops the
00:03:42
volume of fuel in the tank will decrease
00:03:44
causing a drop in the current the
00:03:47
opposite will happen if the fuel
00:03:49
temperature increases the dielectric
00:03:52
constant of fuel changes when its
00:03:54
specific gravity changes if the specific
00:03:58
gravity of the fuel increases more
00:04:01
current will be allowed to flow in the
00:04:02
circuit and similarly if the specific
00:04:05
gravity decreases the current flow will
00:04:08
decrease
00:04:10
when the volume of a fuel reduces
00:04:12
because of a reducing temperature it's a
00:04:16
specific gravity increases and vice
00:04:18
versa when the temperature increases
00:04:22
the system is designed so that these two
00:04:25
changes cancel each other out
00:04:28
the capacitor type of system is
00:04:30
therefore able to measure the mass
00:04:32
rather than the volume of fuel
00:04:36
there is however still the problem of
00:04:38
attitude error
00:04:42
to compensate for changes in attitude
00:04:45
the capacitive system has a number of
00:04:48
capacitor probes in the tank connected
00:04:50
in parallel to average the measurement
00:04:54
of the fuel in the tank in this way as
00:04:56
the attitude changes a fuel level
00:05:00
increase is sensed by one capacitor and
00:05:03
a decrease is sensed by another this
00:05:08
enables the system to give an accurate
00:05:11
indication irrespective of the aircraft
00:05:14
attitude
00:05:21
on older aircraft with analog fuel
00:05:25
quantity changes if the fuel tanks
00:05:27
capacitive gauging system fails it does
00:05:30
so in a manner to draw the attention of
00:05:33
the user a fail-safe circuit is
00:05:38
incorporated which drives the gauge
00:05:40
pointer slowly towards the empty
00:05:42
position in order to prevent the
00:05:44
indicator showing that there is more
00:05:46
fuel in the tank than there actually is
00:05:51
some systems also incorporate a test
00:05:53
switch utilizing the failsafe circuit
00:05:56
when the test switch is operated the
00:06:00
indication moves towards empty and when
00:06:03
the switch is released the pointer
00:06:05
should move back to its original
00:06:06
position
00:06:10
more modern aircraft the digital gauges
00:06:13
are controlled by a fuel quantity
00:06:15
indicating system or FQ is computer this
00:06:21
computer monitors the system for any
00:06:24
failures and generates appropriate
00:06:26
warnings
00:06:32
in the event that the electrical fuel
00:06:34
quantity measuring system fails or if
00:06:37
there is any doubt about the quantity of
00:06:39
fuel on board
00:06:41
aircraft are fitted with a simple manual
00:06:44
backup system to determine the quantity
00:06:47
of fuel on board before flight one
00:06:51
method is to use a dipstick in the top
00:06:55
of the tank but of course this exposes
00:06:57
the user to the dangers inherent of
00:06:59
walking on high and possibly slippery
00:07:02
wing surfaces
00:07:05
another method is the drip stick this is
00:07:10
a calibrated hollow tube which is
00:07:12
withdrawn from the undersurface of the
00:07:14
tank through a fuel proof aperture when
00:07:20
the top of the tube becomes lower than
00:07:22
the fuel level the fuel would drip
00:07:24
through the tube hence the name drip
00:07:27
stick the volume of the fuel in the tank
00:07:29
can be established by reference to the
00:07:32
calibrations on the tube the
00:07:37
much of this system is that the users
00:07:39
armpit soon becomes saturated with the
00:07:42
fuel dripping from the pipe a more
00:07:47
user-friendly version of this system is
00:07:49
the drop stick or magnetic level
00:07:52
indicator this system uses a rod
00:07:55
calibrated to show the level of the fuel
00:07:58
in the tank the rod is fitted within a
00:08:03
tube sealed from the fuel in the tank
00:08:09
the tube is a magnet supported on a
00:08:12
float the magnet moves up and down the
00:08:17
tube with the fuel the tip of the rod is
00:08:22
also fitted with a magnet when the rod
00:08:27
is lowered through the tube as the two
00:08:31
magnets line up their fields attract
00:08:33
each other and resistance to further
00:08:35
movement will be felt these systems all
00:08:41
establish the volume of fuel in the tank
00:08:43
the weight or mass can be calculated
00:08:46
provided the specific gravity of the
00:08:49
fuel is known many aircraft have tables
00:08:52
for doing this
00:08:58
the fuel system instrumentation on a
00:09:01
light aircraft will consist of contents
00:09:03
and pressure gauges has shown here on
00:09:08
large aircraft it is necessary to
00:09:12
provide more information than this to
00:09:13
the crew on older aircraft types the
00:09:17
fuel control panel is usually in the
00:09:20
form of a mimic diagram with flow bars
00:09:23
and lights to indicate flow
00:09:28
modern systems have electrically
00:09:30
presented schematic displays the diagram
00:09:33
here shows a typical air bus system
00:09:36
display modern Boeing aircraft have
00:09:39
similar displays the temperature of the
00:09:44
fuel in each tank is shown here this is
00:09:48
important information for monitoring the
00:09:50
system for fuel icing or fuel waxing the
00:09:56
fuel tanks are shown schematically with
00:09:59
the mass of the fuel in each tank being
00:10:01
displayed
00:10:03
the wing tanks are split into inboard
00:10:07
and outboard sections in this instance
00:10:10
the outboard tanks are empty as is the
00:10:16
center tank these symbols are used to
00:10:22
indicate the position of the main tank
00:10:24
outboard two inboard transfer valves in
00:10:27
this case they are open
00:10:31
when closed they look like this
00:10:36
these symbols are used to depict the
00:10:39
fuel booster pumps the
00:10:44
in the box is vertical when the pump is
00:10:46
operating as seen on the wing tank pump
00:10:49
indications and horizontal when the pump
00:10:55
is switched off as demonstrated by the
00:10:57
center tank pump indications the cross
00:11:03
feed valve indication is in line when
00:11:06
the valve is open and cross line when it
00:11:09
is closed
00:11:12
the engine and auxilary power unit
00:11:15
shutoff valve indications operate in a
00:11:18
similar manner the fuel used by each
00:11:23
engine since startup is displayed here
00:11:28
and finally there is a readout of the
00:11:31
total fuel on board here
00:11:39
that is the end of the lesson remember
00:11:42
that light aircraft may have a fuel
00:11:45
quantity measuring system consisting of
00:11:47
a float attached to a variable resistor
00:11:51
the two problems with this system are
00:11:54
that the indicated quantity of fuel will
00:11:56
change when the specific gravity of the
00:11:58
fuel changes and the system will give
00:12:02
erroneous indications during maneuvers
00:12:06
most large aircraft use a capacitance
00:12:09
type of system which uses the difference
00:12:12
between dielectric qualities of air and
00:12:14
fuel to measure the quantity of fuel in
00:12:17
the tank
00:12:21
system is unaffected by specific gravity
00:12:23
changes
00:12:25
and by using numerous detectors in the
00:12:29
tank it does not suffer from maneuvering
00:12:32
error
00:12:40
you
