King Air C90GTi, Pilot Training Manual

Page: 162 / 557

sure at the fuel nozzles for a proper spray
patter n during all modes of engine opera-
tion. The high-pressure pump supplies fuel
at approximately 800 psi to the fuel side of
the FCU.
Two va l v e s i n c l u d e d i n t h e F C U e n s u r e
c o n s i s t e n t a n d c o o l e n g i n e s t a r t s . W h e n
t h e i g n i t i o n o r s t a r t s y s t e m i s e n e rg i z e d,
t h e p u rg e va l ve i s e l e c t r i c a l ly o p e n e d t o
c l e a r t h e F C U o f va p o r s a n d b u b bl e s . T h e
ex c e s s f u e l f l ow s b a c k t o t h e n a c e l l e f u e l
t a n k s . T h e s p i l l va l ve , r e f e r e n c e d t o a t -
m o s p h e r i c p r e s s u r e , a d j u s t s t h e f u e l f l ow
f o r c o o l e r h i g h - a l t i t u d e s t a r t s .
Between the FCU fuel valve and the engine
comb ustion chamber, the minimum pres-
surizing valve in the FCU remains closed
during star ting until fuel pressure b uilds
suff iciently to maintain a proper spray pat-
ter n in the combustion chamber. About 80
psi is required to open the minimum pres-
s u r i z i n g va l ve . I f t h e h i g h p r e s s u r e f u e l
p u m p s h o u l d f a i l , t h e va l ve wo u l d c l o s e ,
and the engine would flame out.
The fuel cutoff valve is located downstream
from the minimum pressurizing valve in the
FCU. This valve is controlled by the condi-
tion lever, either open or closed. There is
no inter mediate position of this valve. For
starting, fuel flows initially through the flow
divider to the 10 primar y fuel nozzles in the
combustion chamber. As the engine accel-
erates through approximately 40% N
1
, fuel
pressure is suff icient to open the flow di-
vider to the 4 secondary fuel nozzles. At this
time all 14 nozzles are delivering atomized
fuel to the combustion chamber. This pro-
gressive sequence of primary and secondary
fuel nozzle operation provides cooler starts.
During engine star ting, there is a noticable
increase in ITT when the secondary fuel noz-
zles are activated.
During engine shutdown, any fuel left in the
manifold is forced out through the fuel noz-
zles and into the comb ustion chamber by
purge tank pressure. As the fuel is bur ned,
a momentar y increase in N
1
r pm may be ob-
ser ved. The entire operation is automatic
and requires no input from the crew.
FUEL CONTROL UNIT
The fuel control unit (Figure 7-17), which
i s r e f e r r e d t o a s t h e F C U, h a s m u l t i p l e
f u n c t i o n s , b u t i t s p r i m a r y p u r p o s e i s t o
meter proper fuel amounts to the fuel noz-
z l e s i n a l l m o d e s o f e n g i n e o p e r a t i o n .
F C U o p e r a t i o n w i l l b e s i m p l i f i e d a n d d e -
s c r i b e d b r i e f l y h e r e . F o r d e t a i l e d d e -
s c r i p t i o n a n d o p e r a t i o n , r e f e r t o t h e P r a t t
& W h i t n e y
M a i n t e n a n c e M a n u a l w h i c h
a p p l i e s t o t h i s e n g i n e .
T h e c o n d i t i o n l ev e r s e l e c t s i d l e s p e e d s
b e t we e n L OW I D L E ( 5 8 % t o 6 2 % N
1
) t o
H I G H I D L E ( 7 0 % N
1
) , wh i l e t h e p owe r
l e v e r s e l e c t s s p e e d s b e t w e e n i d l e a n d
m a x i m u m , 1 0 1 . 5 % N
1
. T h e s e c o n t r o l
l eve r s i n f l u e n c e t h e N
1
g ove r n o r a n d c o n -
t r o l N
1
s p e e d . T h e g ov e r n o r u s e s p n e u -
m a t i c a i r ( P
3
) p r e s s u r e t o c o n t r o l e n g i n e
s p e e d . T h e g ove r n o r c o n t r o l s t h e a i r p r e s -
s u r e i n t h e f u e l c o n t r o l u n i t by va r y i n g
t h e P
3
l e a k r a t e .
T h e P
3
a i r c h a m b e r a n d f u e l c h a m b e r a r e
separated by a diaphragm, which has a nee-
dle valve mounted on it which is called the
m e t e r i n g va l ve . A s t h e d i a p h r a g m i s i n -
fluenced by var ying air/fuel pressures, the
m e t e r i n g va l ve i s r e p o s i t i o n e d t o a c h i eve
the desired fuel flow. The N
1
gover nor con-
t r o l s f u e l f l ow by a l l ow i n g s o m e P
3
p r e s -
s u r e t o b e l e a k e d o f f a t v a r y i n g r a t e s ,
d e p e n d i n g o n t h e d e s i r e d f u e l f l ow.
I n a n u n d e r s p e e d c o n d i t i o n , t h e N
1
g ov -
er nor acts to increase P
3
air pressure. This
r e p o s i t i o n s t h e m e t e r i n g va l ve , a l l ow i n g
m o r e f u e l t o e n t e r t h e c o m b u s t i o n c h a m -
b e r, i n c r e a s i n g N
1
.
KING AIR C90GTi/C90GTx PILOT TRAINING MANUAL
7-17
FOR TRAINING PURPOSES ONLY
7
POWERPLANT