Chp 3 (Fuel System) Flashcards
What are the functions of a fuel system?
To provide the engine with fuel in a form suitable for combustion and to control the flow to the required quantity necessary for combustion.
What are the fuel requirements for an engine?
- Ease of flow under all operating conditions
- Quick and easy starting of the engine
- Complete combustion under all operating conditions
- High Calorific Value
- Non-Corrosive
- No harmful by-products to flame tube or turbines
- Minimum fire hazard
- Provide lubrication
What are the corrosive properties of fuel?
- Water Content
- Sulphur compounds
What is volatility in fuel systems?
Being able to change readily from a liquid to a gas by evaporation.
What is calorific value?
The amount of energy produced by complete combustion of a fuel.
What are the purposes of additives in fuel?
- Reduce contamination levels and microbiological growth
- Minimise corrosion
- Enhance lubrication
What is the purpose of Fuel System Icing Inhibitor (FS II)?
Inhibits the fuel system against icing and fungal growth.
What is the normal fuel type for aviation?
Normal - AVTUR
AVTUR is preferred to AVTAG as a fuel because of the reduced fire risk.
What is the emergency fuel type for aviation?
Emergency - AVTAG
Always refer to the AMM for the correct fuel for the aircraft type being maintained.
What are the identification codes for AVTUR?
AVTUR/FSIL, F-34, F-35.
What is a fire hazard associated with aviation turbine fuels?
Aviation turbine fuels give off a ‘Heavier than air vapour’ which is highly flammable.
What are the three main sources of fire hazard in aviation?
- Fuel spillage with subsequent ignition of vapour from a spark (static).
- Fuel spillage on to a hot surface causing self-ignition.
- The existence of flammable or explosive mixtures in the aircraft tanks.
What are the functions of a fuel system?
To provide the engine with fuel suitable for combustion and to control the flow to the required quantity necessary for combustion, starting, acceleration, and stable running under all operating conditions.
What are the fuel requirements?
- Ease of flow under all operating conditions
- Quick and easy starting of the engine
- Complete combustion under all operating conditions
- High Calorific Value
- Non-Corrosive
- No harmful by-products to flame tube or turbines
- Minimum fire hazard
- Provide lubrication
What are the corrosive properties of fuel?
- Water Content
- Sulphur compounds
What is volatility in the context of fuel?
Being able to change readily from a liquid to a gas by evaporation.
What are the purposes of additives in fuel?
- Enhance calorific value and heat produced by combustion
- Reduce contamination levels and microbiological growth
- Minimise corrosion
- Enhance lubrication
What is the purpose of Fuel System Icing Inhibitor (FS II)?
Inhibits the fuel system against icing and fungal growth.
What is the normal fuel type used in aviation?
Normal - AVTUR
What is the emergency fuel type used in aviation?
Emergency - AVTAG
Although AVTAG is classed as an emergency fuel, always refer to the AMM for the correct fuel for the aircraft type being maintained.
Why is AVTUR preferred over AVTAG?
AVTUR is preferred to AVTAG as a fuel because of the reduced fire risk.
What are the identification codes for AVTUR?
AVTUR/FSIL, F-34, F-35
What is a significant fire hazard associated with aviation turbine fuels?
Aviation turbine fuels give off a ‘Heavier than air vapour’ which is highly flammable.
What are the three main sources of fire hazard in aviation fuels?
- Fuel spillage with subsequent ignition of vapour from a spark.
- Fuel spillage on to a hot surface causing self-ignition.
- The existence of flammable or explosive mixtures in the aircraft tanks.
What is the purpose of the high-pressure fuel pump?
To provide a fuel supply to the engine at sufficient pressure for satisfactory atomisation of the fuel.
How is the fuel pump driven?
The pumps are driven from the compressor shaft via the accessory gear train.
What is the relationship between pump output and engine demand?
The pump’s output is always greater than engine demand.
What is a PRV and its function?
A PRV (Pressure Relief Valve) may be fitted in parallel with the pump to maintain system pressure.
What are the two types of fuel pumps used?
- Piston Type Pump
- Half-Ball Spill Valve
How does the piston type pump operate?
Output is dependent on the angle of the swashplate; a greater angle results in a greater stroke of the pistons and thus output.
What is the function of the half-ball valve or spill valve?
It varies servo outflow and thus servo pressure, altering the piston pump swash plate angle and piston stroke.
What effect does the half-ball valve have on engine RPM?
It increases or decreases engine RPM by altering the piston pump swash plate angle and piston stroke.
What is a Spur Gear Pump?
A gear type pump that is lighter and simpler than the piston pump, used on the majority of modern engines.
How does fuel flow in a Spur Gear Pump?
Fuel flows between the gear teeth and casing to the outlet side.
What maintains system pressure in a Spur Gear Pump?
A Pressure Relief Valve (PRV) maintains system pressure and allows excess fuel back to the inlet side.
What is the role of a Centrifugal Governor?
The management of the fuel supply is critical to the efficient operation of the fuel system, automated through a centrifugal governor.
What components make up a Centrifugal Governor?
It consists of an engine-driven shaft, flyweights, a datum spring, and a throttle valve.
What is the state of the governor at rest?
No drive, weights held in under spring pressure, fuel valve fully open.
What happens during the running up phase of governor operation?
Drive increases, weights move out under centrifugal force against spring pressure, valve starts to close; fuel flow starts to be restricted/metered.
How is governed speed achieved in governor operation?
As the speed of the drive increases, the weights move further out against the spring until the centrifugal force is balanced by the spring tension, providing the required fuel flow for the governed speed. Adjusting the force on the datum spring changes the governed speed.
What occurs during a load change when the load on the drive increases?
The speed will decrease, weights move in under spring force, the fuel valve opens wider to admit more fuel, allowing the engine to accelerate back to governed speed.
What happens when there is a decrease in load on the drive?
The reverse of the load increase occurs: the speed increases, weights move out, and the fuel valve restricts fuel flow.
What is the purpose of the LP Filter in a Free Turbine Fuel System?
Provides protection for metered fuel.
What does the Fine Filter do in a Free Turbine Fuel System?
Protects the pump against debris.
What is the function of the HP Pump in a Free Turbine Fuel System?
Increases fuel pressure for system operation.
What is the role of the FCU & Throttle in a Free Turbine Fuel System?
Automatically controls fuel flow from start up, acceleration up to governor speed.
What does the PRV maintain in a Free Turbine Fuel System?
Maintains system pressure.
What is the function of the FCU 60psi?
Feeds fuel to the torch igniters during the start cycle, whilst fuel pressure is below 60psi.
What does the Solenoid valve & NRV do in a Free Turbine Fuel System?
Allows fuel to the torch igniters during the start cycle, then P2 air purges the Torch igniter after the start is terminated.
What is the purpose of the Torch Igniter?
Maintains flame until rotary atomisers can take over.
What does the HP Cock control in a Free Turbine Fuel System?
On/Off Cock. Used to allow fuel flow during start and normal operation, and for stopping the fuel to shut down the engine.
What is the function of the Rotary Atomiser?
Provides an atomised fuel spray into the combustion chamber.
What does the Free Turbine Governor maintain?
Maintains Constant Rotor RPM.
What is FADEC?
Full Authority Digital Engine Control. It reduces the complexity of Hydro-Mechanical and pneumatic engine systems.
What does a typical FADEC system consist of?
A central processing unit called an Electronic Control Unit, a Hydro mechanical unit, and various sensors.
What are the benefits of a FADEC system?
- Reduces fuel consumption by substituting Hydro mechanical control system components, which reduces weight.
- Automation brings reduced pilot workload.
- Optimised control allows the engine to work more efficiently, reducing the risk of surge or engine flame out throughout the entire operating range.
How is the FADEC system designed for redundancy?
It is built around two independent channels (dual inputs, dual outputs) and automatic switching from one channel to the other eliminates any dormant failure.
What determines the Active Channel in the system?
The system selects the healthiest channel based on a fault priority list.
How often do channels determine their state?
Every 30 milliseconds.
What factors do channels compare to determine their state?
Each channel compares its own health to the health of the cross-channel.
What happens if a channel’s health is better than the cross-channel’s health?
The channel can become active.
What happens if a channel’s health is not as good as the cross-channel’s health?
The channel will become standby.
What occurs if both channels have equal health status?
The channels will alternate on each engine shutdown.
What happens to the standby channel on the next start?
The standby channel will become the active channel.
What must be read and understood before servicing the fuel system?
Safety and Maintenance Notes and Safety Data Sheets.
Why is cleanliness essential after component removal?
To avoid the ingress of foreign matter.
What must be done after the throttle control has been disturbed or replaced?
The system must be RIGGED to ensure correct engine datum’s and unrestricted fuel flow.
How should filters be maintained?
Filters are cleaned or replaced in accordance with relevant maintenance manuals.
What should be examined when re-fitting a filter?
Examine for damage and change ‘O’ rings.
How are rigid pipelines secured?
They are secured at set intervals along their length to prevent fracture and vibration.
What should be ensured about flexible pipelines?
They should not be twisted.
What should be checked before re-connecting pipes?
Examine for cleanliness and damage, especially at flared ends and nipples.
What must be done to seals before re-connecting pipes?
Seals should be lubricated if necessary, and union nuts must be correctly locked.
What should be done after disturbing a fuel pipe or filter?
The system must be run at normal operating pressure and checked for leaks.
Where are fuel flow tests conducted?
At the Airframe Fuel System connection to the Engine.
