M15.11 Flashcards
What is a rich blow out
If the quantity of fuel becomes excessive the limiting temperature of the turbine blades can be exceeded, or a compressor stall could occur
Gas turbine engine fuel control systems are generally divided into three categories:
- Hydromechanical
- Hydromechanical/electronic
- Full Authority Digital Engine (or Electronics) Control (FADEC)
The hydromechanical/electronic fuel control is a hybrid of the two types of fuel control but can function solely as a hydromechanical control. In the dual mode, inputs and outputs are electronic, and fuel flow is set by
Servo motors
The FADEC-type control gives the electronic controller (computer) what type of control
Full control
What 2 sections do fuel controls have
Computing and metering
Does a hydro mechanical fuel system have any electronic interface in computing or metering fuel flow
A pure hydromechanical fuel control system has no electronic interface assisting in computing or metering the fuel flow. It is generally driven by the gas generator gear train of the engine to sense engine speed
What parameters do hydro mechanical fuel systems sense
compressor discharge pressure, burner pressure, exhaust temperature, and inlet air temperature and pressure
How is fuel metering achieved through a hydro mechanical fuel system
When the computing section determines the correct amount of fuel flow, the metering section through cams and servo valves delivers the appropriate quantity of fuel to the engine fuel system.
Is a hydro mechanical fuel delivery system as accurate as electric
No
Why is the fuel control unit sometimes mounted on the fuel pump
The fuel control unit is sometimes mounted on the vane fuel pump assembly. It provides the power lever connection and the fuel shut-off function. The unit provides mechanical overspeed protection for the gas generator spool during normal (automatic mode) engine operation.
What type of filter is fuel supplied through
A 200 micron inlet screen
Is the fuel manual mode solenoid valve energised or de energised in the automatic mode
The manual mode solenoid valve is energised in the automatic mode.
Fuel sub systems
- Distribution
- Control
LP fuel pump pressure
175psi
HO fuel pump max pressure
900psi at max power
Overpressure relief valve pressure
1250psi
Types of HP fuel pump
- Gear type
- Piston type
Most common type of fuel pump in turbofans
Gear type
Heat exchangers that can be found in the engine fuel system
- Engine oil cooler
- Servo fuel heater
- IDG oil cooler
What prevents fuel leaks in the hot environment of the HPC and combustion case
Fuel lines protected by shrouds
Methods to protect the connections of fluid lines against leaks
- Shrouded connections
- Double sealed connections
Disadvantage of ding,e flow nozzles
Only has good atomisation at a small fuel flow range
How do we overcome the disadvantages of single flow nozzles
- Having a primary and secondary set
- Primary for start and idle
- Secondary for max
How are primary and secondary fuel nozzle usually supplied
Independent fuel manifolds
What do we use in modern engines in regards to fuel nozzles
Duplex fuel nozzles
What prevents residual fuel and carbon build up on duplex fuel nozzles
- Combustion airflow
- Shut off valves
Can the air spray nozzle be a duplex type
Never
Why is the air spray nozzle always simplex
It can provide suffice t fuel at all speeds
Operating conditions important fuel metering
- Steady state
- Transient
Fuel metering demand limits
- Over temp
- Over speed
- Compressor stall
- Flame out
N2 speed for most engine idle
50-65%
How does the FCU switch minimum idle to approach idle
Air config via air-ground logic or other config switches
What is the N2 command signal
Signal from thrust lever to the FCU
Types of FCU
- Speed governed
- Constant thrust FCU
What signals do hydro mechanical FCUs have
Hydraulic, mechanical and pneumatic
FCU major sections
- Metering
- Power control
- Engine protection
Where is the PMC (Power Management Computer) usually located
On the fan stator case of the engine
PMC function
Calculates required N1 based on throttle lever input
Where does the PMC send the control signal
Torque motor on the FCU if the required is different to the actual
FADEC
Full Authority Digital Engine Control
What is FADEC
Digital computer that has full authority over engine control functions
First and second main component of FADEC
1 - Digital computer
2 - FMU
Engines with FADEC do not have what component installed
Hydromechanical FCU
Typical FADEC systems have what components
ECU
FMU
Electrical sensor
Control of other engine sub systems
How many ECU computers are there
2 which are A and B
How many sensors are at the T12 sensor
2 one for A and B ECU
What concerns pressure signals to electric signals
Pressure transducers
Power management calculates the thrust command based on what
- Thrust lever angle
- Ambient air temp
- Pressure such as T12 and Ps12
What does the FMU servo valve control
- VSV
- VBV
- HP and LP turbine clearance control valves
EIU
Engine Interface Unit
How many EIUs are there
One for each engine
What supplies the ECU with power
PMA
Advantage of EIU in a FADEC system
- Saves weight
- Reduces risk of wiring faults
What is the only signal FADEC needs to operate
Trust Lever Angle
Can FADEC operate with out signals from the aircraft
Yes it only uses the additional signals to improve accuracy
