T56 Fuel Systems Flashcards
Engine operating limits (ground) rpms (%) Start TIT Low rpm Normal rpm Flt Idle Take off Reverse
71.3-73.8
96.3-99.1
94.5-98.4
99-101
96.3-104.9
Maximum HP
Maximum TIT
4600SHP
1077
850, stop start
830, continue cautiously but investigate
Requirements of the engine fuel system
1) Provide the correct fuel flow for starting under all ambient conditions
2) Adjust fuel flow in response to power lever changes
3) Limit the maximum TIT obtainable
4) Regulate the fuel flow in the RPM range that the compressor is likely to stall or surge
5) Automatically maintain a selected power (TIT) regardless of aircraft speed, altitude, air temperature and BTU contents of the fuel
6) Control fuel flow in the event of propeller governor failure
7) Co ordinate fuel flow with blade angle changes in the beta (ground) range
8) Provide hydro mechanical operation
Purpose of the fuel-heather strainer
1) Heat the fuel
This is to prevent any minute water particles changing to ice and blocking the engine fuel filters
2) Strain the fuel
The FHS is the first engine fuel filter downstream of the AC fuel tanks booster pumps
What temperature does the vernatherm valve regulate fuel temperature at
4.5-21 *C
Occasions for servicing the FHS
1) During scheduled maintenance
2) After the first engine run following fuel tank maintenance
3) After 10 flying hours following fuel tank maintenance
What do you do before removing the FHS filter element from the housing?
E handle of the respective engine pulled.
This ensures the wing beam fuel shut off valve is closed prior, otherwise the wing tanks will gravity feed through the filter opening.
Fuel pump cluster contains 3 pumps known as:
1) Scroll pump. This operates on the screw type and centrifugal type principles 2) Secondary pump This is a gear type pump. 3) Primary pump
Major components of the low pressure fuel filter assemblies
1) Filter bodies/bowl
2) Filter elements
3) Filter bypass valve
4) Filter bypass pressure differential switch
AC fuel tank booster pumps supply fuel to the FHS at:
1) 30psi- no flow condition
2) 18 psi at 5000lbs/hr
The 2 gear type fuel pumps operate in either of the 2 following modes:
1) Series operation
From 0-16% and above 65%rpm
2) Parallel operation
16-65%rpm
Functions of the Secondary Check Valve
1) Prevents secondary pump discharge back to secondary inlet during primary pump failure
2) Provides alternative inlet to primary pump during secondary pump failure
3) Provides additional inlet to the primary pump during series operation due to its 10% greater capacity
4) Provide the inlet to the primary pump during paralleling operation
Functions of the Primary Check Valve
1) Prevents primary pump discharge back to primary pump inlet during secondary pump failure
2) Provides outlet from secondary pump during primary pump failure
3) Provides outlet from secondary pump during paralleling
Normal operation of the Parallel lights (1 light for each engine)
1) Lights are out from 0-16% rpm. Pump in series- no flow to fuel nozzles
2) Lights are on 16-65% rpm. Indicates paralleling mode in operation. It is normal for the primary pump failure lights to actually come on at approx 20% rpm. This is when the secondary pump pressure actually reaches 15- psi
3) Lights are out above 65% rpm. Pump in series for normal engine on speed conditions
Indications of Primary pump failure
1) Parallel light on between 65-100%rpm
Other faults which illuminate the paralleling light above 65% rpm
1) SSC 65% switch failure
2) Paralleling pressure switch US
3) Paralleling valve stuck open
Indications of Secondary pump failure
1) Paralleling light out between 16-65 % rpm on next engine start
Other faults which fail to illuminate the paralleling light between 16-65% rpm
1) Paralleling pressure switch US
2) Paralleling valve stuck open
3) Bulb US
Indications of Scroll pump failure
The amber ‘Pressure Low’ light on the fuel management panel illuminates. However, the scroll pump bypass valve opens at 2 psi. The low pressure fuel filters are bypassed. Fuel is ported directly to the secondary pump inlet.
What is the purpose of a scroll pump
1) Provide a positive head of pressure to the gear pumps to prevent cavitation &
2) Prolong their life
What is the Paralleling Valve and Paralleling Pressure Switch mounted on
High pressure fuel filter
Series fuel pump operation:
Paralleling operation:
Series: 0-16, 65-100%
Paralleling: 16-65%
4 inputs to the FCU
1) Throttle position
2) Engine RPM
3) Compressor inlet temperature
4) Compressor inlet pressure
Purpose of the FCU
Receives the total fuel output from the fuel pump cluster. It delivers 120% of the engine fuel requirements to the TD valve
Purpose of the bypass valve
Maintain a constant 20psi pressure drop across the metering valve. This feature ensures the flow through the metering valve is proportional to the size of the metering orifice only.
2 possible faults- eng starts, fuel flow but no ignition
ignition relay
ignitors
heiu
manifold drain valve open
Fault: Paralleling light stays on
Cause:
primary pump failure
paralleling valve on (seized)
lightbulb failed
ssc failure
what 2 sections of the fcu compensate for air density
temperature
pressure
what are things that alter the metering valve by rotation and linear movement
compressor air temp
compressor air pressure
throttle position
engine rpm
what section of the fcu controls fuel flow during start
speed servo system
2 precautions with the temperature probe when removing
comes off with the unit
dont damage
which cam schedules fuel flow upto 92%
acceleration cam
what component, during priming, is bypassed
metering valve
what is the purpose and rpm of the overspeed governor
1) provide overspeed protection in the flight range in event of prop governor failure
2) provide overspeed protection in the ground range
3) govern rpm when low rpm is selected
104. 5%rpm
what component terminates fuel priming and at what pressure
automatically terminated when fuel manifold pressure reaches 50psi
primer pressure cut out switch
what is the effect on HP, TIT and FF when anti icing is turned on
tit remains
hp drop
fuel flow drop
common reason for adjusting the null orifice on the TD valve
start TIT is too high
or too low
purpose of the 66* switch on the co ordinator
to change the td system from temp limiting to temp controlling and vice versa
purpose of the 1000ohm potentiometer
provide a temperature reference signal to the TD amplifier, in the temp controlling range
what must be done to the td valve after fuel control change
set to 0* after the null orifice adjustment
what are the 3 potentiometers in the temp control range
slope
bias
co ordinator
3 conditions for td valve to be capable of putting
auto
ssc >94%
throttle position 66*
maintenance precaution when fitting fuel nozzles
dont brush or probe. any damage can lead to fuel streaking which results in down stream burning then turbine failure
the inner sleeve is moved in a rotary direction by what?
it is sensed in the linear direction by what?
air pressure
the other 3 (temp, rpm, throttle position)
what are the 3 adjustments points on a fcu
overspeed governor rpm
low speed rpm
minimum flow stop
