King Air C90GTi, Pilot Training Manual

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s a t e s f o r f u e l t e m p e r a t u r e d e n s i t y va r i a -
tions. The left fuel quantity indicator, on
the fuel control panel, indicates the amount
of fuel remaining in the left-side fuel sys-
tem tanks when the FUEL QUANTITY se-
l e c t s w i t c h i s i n t h e T O TA L ( u p p e r )
position, and the amount of fuel remain-
ing in the left-side nacelle fuel tank when
the FUEL QUANTITY select switch is in
the NACELLE (lower) position. The right
fuel quantity indicator indicates the same
infor mation for the right-side fuel systems,
depending upon the position of the FUEL
QUANTITY switch. The gages are marked
in pounds.
The fuel quantity indicating system is a ca-
pacitance type that is compensated for sp-
cif ic gravity and reads in pounds on a linear
scale. An electronic circuit in the system
processes the signals from the fuel quan-
tity (capacitance) probes (Figure 5-12) in
the various fuel cells for an accurate read-
out by the fuel quantity indicators. A se-
l e c t o r s w i t c h , l o c a t e d b e t we e n t h e f u e l
quantity indicators in the fuel panel beside
the pilot, may be set in either the TOTAL
o r N AC E L L E p o s i t i o n s t o d e t e r m i n e
whether the gages indicate the pounds of
fuel in the nacelle and wing fuel cells of the
fuel system, or the pounds of fuel in only the
nacelle fuel cell.
COMPONENTS AND
OPERATION
Each side of the air plane has an indepen-
dent gaging system consisting of a fuel quan-
tity (capacitance) probe in the nacelle fuel
cell, one in the aft-inboard fuel cell, two in
the leading-edge fuel cell, and one in the
center-section fuel cell.
W h e n t h e f u e l s e l e c t o r sw i t c h i s l e f t i n i t s
TOTA L p o s i t i o n , p owe r i s s u p p l i e d f r o m
a 5 - a m p e r e c i r c u i t b r e a k e r ( o n t h e f u e l
p a n e l ) t h r o u g h t h e f u e l q u a n t i t y i n d i c a -
t o r t o a l l o f t h e c a p a c i t a n c e p r o b e s i n t h e
fuel system. When the fuel selector switch
is placed in the NACELLE position, power
i s t h e n s u p p l i e d t h r o u g h t h e f u e l q u a n t i t y
i n d i c a t o r t o t h e c a p a c i t a n c e p r o b e i n t h e
n a c e l l e f u e l c e l l o n ly.
Fuel density and electrical dielectric con-
s t a n t ly va r y w i t h r e s p e c t t o t e m p e r a t u r e ,
fuel type, and fuel batch. The capacitance
gaging system is designed to sense and com-
pensate for these variables. The fuel quan-
t i t y p r o b e i s s i m p ly a va r i a bl e c a p a c i t o r
comprised of two concentric tubes. The inner
tube is prof iled by changing the diameter as
a function of height so that the capacitance
between the inner and outer tube is propor-
tional to the tank volume. The tubes ser ve
as f ixed electrodes and the fuel of the tank
in the space between the tubes acts as the
dielectric of the fuel quantity probe.
The capacitance of the fuel quantity probe
va r i e s w i t h r e s p e c t t o t h e c h a n g e i n t h e
d i e l e c t r i c t h a t r e s u l t s f r o m t h e r a t i o o f
f u e l - t o - a i r i n t h e f u e l c e l l . A s t h e f u e l
l ev e l b e t we e n t h e i n n e r a n d o u t e r t u b e s
r i s e s , a i r w i t h a d i e l e c t r i c c o n s t a n t o f o n e
i s r e p l a c e d by f u e l w i t h a d i e l e c t r i c c o n -
s t a n t o f a p p r ox i m a t e ly t wo , t h u s i n c r e a s -
i n g t h e c a p a c i t a n c e o f t h e f u e l q u a n t i t y
probe. This variation in the volume of fuel
c o n t a i n e d i n t h e f u e l c e l l p r o d u c e s a c a -
p a c i t a n c e va r i a t i o n t h a t a c t u a t e s t h e f u e l
q u a n t i t y i n d i c a t o r.
KING AIR C90GTi/C90GTx PILOT TRAINING MANUAL
5-13
FOR TRAINING PURPOSES ONLY
5
FUEL
SY
STEM
FUEL PROBE
Figure 5-12. Fuel Probe