15.11 - Fuel Systems Flashcards
What do the fuel nozzles do to the liquid fuel?
Atomise or vaporise the fuel
What are the three main processes of a fuel control system?
Scheduling
Metering
Limiting
What are the fuel to air ratios for:
- Accelerating
- Constant power
- Deceleration
Acceleration - 12:1
Constant - 15:1
Deceleration - 18:1
What are the components in the LP fuel system?
EDP Fuel heater FCOC Fuel flowmeter LP filter
What is the purpose of the LP fuel pump?
It prevents cavitation at the HP fuel pump.
Why is it necessary to heat the fuel up before passing it through the fuel filter?
It ensures water in suspension in the fuel doesn’t freeze and causes a blockage in the filter
What is the equation of the flow of a fluid through an orifice (jet)?
The area of the orifice multiplied by the square root of the pressure drop across it
What is the purpose of the fuel control unit (FCU)?
It maintains the correct air/fuel ratio under any running/flying conditions.
What inputs controls the FCU output?
Engine speed signal Inlet pressure Compressor discharge pressure Burner can pressure Inlet temperature
What are the four basic groups of fuel system types?
Pneumatic-manual
Hydromechanical
Hydromechanical/electronic
Full authority digital engine control (FADEC)
What is the primary purpose of trimming a turbine engine fuel control unit?
To obtain maximum thrust output when desired
What is the ASU?
Altitude sensing unit
With altitude… what happens to fuel flow to the nozzles?
It reduces
How do hydromechanical fuel control units (FCU) vary the fuel flow?
By using servo fuel as a hydraulic fluid
Where does FADEC get its air pressure and temperature signal from?
The air data computer (ADC)
What is CDP?
Compressor discharge pressure
As the temperature of turbine fuel decreases… what happens to its volume?
It decreases
How is a turbojet dashpot throttle used?
The pilots throttle mechanism is linked to the control valve via a rack and pinion mechanism.
What is an FFR?
A fuel flow regulator
What is the purpose of the combustor drain valve?
It prevents fuel accumulation in the combustion after a false start.
What type of fuel nozzle is used on modern gas turbine engines?
Duplex nozzles - air spray nozzles
How many fuel supplies are used in a duplex nozzle?
2, primary and main.
Does a FADEC system have a manual override?
No. If there is a total FADEC failure, the engine fails.
What is the main advantage of a FADEC system?
Highest possible efficiency of the engines.
What does the HMU control?
VSVs, VBVs and FMV
What is the HPSOV?
High pressure fuel shut off valve
What does the programming plug do?
It selects the proper software for the EEC for the thrust rating of the engine.
When will the channels of the EEC change?
On every engine shut down.
Standby to Active/Active to standby
What is the EIMU?
Engine interface monitoring unit.
What is the PMA?
Permanent magnet alternator
Where does the EEC get it primary power from?
The PMA
What directly provides the closure to the fuel supply?
The master switch/lever
In a kinetic valve, if the knife-edge is fully in…
The servo pressure is being bled off
After a fuel bag tank replacement, where should you disconnect the system to carry out flow checks?
At the engine
What happens to a FADEC computer if it loses it’s ADC input?
It goes into “soft reversion”
What is the main purpose of the boost fuel pumps?
Primary purpose is to provide fuel to the engines
What is the ADC?
The air data computer (Temperature & Pressure)
What is the purpose of the “attenuator” fitted between the HP fuel pump and the BPC in a fuel system?
It dampens out the pulsations in the fuel delivery to the BPC (barometric pressure capsule).
Why is the fuel heater located before the LP fuel filter?
It ensures the fuel does not freeze in the fuel filter and block the system
When re-light is wanted at altitude in a FADEC engine; how does the pilot select the igniter?
The igniter is selected automatically by the FADEC
What is the purpose of the BPC? (Barometric Pressure Capsule)
It takes P1 pressure (inlet pressure) and adjusts the fuel pressure to the combustion chamber ensuring correct RPM for a given throttle position at all altitudes and air speeds
Where is the BPC located within?
The altitude sensing unit (ASU) within the FCU.
When do the two channels on a FADEC system switch over?
On each engine start up
With an increase in altitude, what happens to the fuel flow?
It decreases to maintain the correct air to fuel ratio
What is the purpose of the silver strip on a fuel filter?
It indicates a presence of sulphur in the fuel
What is used to cool fuel boost pumps?
Fuel itself; cooling the bearings inside
What does a barometric pressure control unit (BPC) use as it’s sensing control?
Intake static air pressure
What does the hydro-mechanical governor maintain?
RPM irrespective to fuel SG
When a throttle is selected to increase power, what happens to the pressure drop across the FCU orifice?
It drops and then increases due to the pump outlet increase in pressure
