Chapter 5 - Aircraft Fuel System Flashcards
What is the official name for aviation gasoline?
Avgas
What is the official name for Gas Turbine Fuel?
Avtur (aviation kerosene - paraffin)
What fuel is created by mixing Avtur and Diesel?
Avcat
What fuel is created by mixing Avtur and Avgas?
Avtag
‘Wide-cut’ fuels
What is Avtur mixed with diesel to create Avcat?
Diesel has a lower flashpoint, thereby lowering the flashpoint of the fuel and increasing safety for use on carriers.
(Avoids vapour build up within the confined spaces)
What fuel is used in aircraft with spark ignition internal combustion engines?
Avgas
What is the standard UK military AVTUR fuel?
F-35
F-34 is also used, and is the same as F-35 but with the addition of FSII.
What is Avtag/wide-cut fuel?
A combination of gasoline (Avgas) and kerosene (Avtur).
Lower temp range of flammability and greater fire hazard than Avtur so not first choice. However, means has a lower freezing point than AVTUR so is used by the UK military in cold climatic conditions.
What is the NATO equivalent of AVTAG?
F-40
What is AVCAT also known as?
High flash kerosene
What is the NATO equivalent of AVCAT?
F-44
What is density?
Mass per unit volume
Gives an indication of the concentration of matter in a material. Paraffin typically 800kg per cubic metre
What is Specific Gravity (SG)?
The ratio of a substance’s density compared to the density of water.
Varies inversely with temperature, i.e. filling up with cold fuel means you can fit more in the set volume of the tanks (more dense).
What is the freezing point of a fuel?
The temperature in the ‘WARMING UP’ process where the waxy crystals disappear.
What does FSII stand for and what is its purpose?
Fuel System Icing Inhibitor
Inhibits fuel system icing and supresses fungus growth.
What is a fuel’s flashpoint?
The lowest temperature at which fuel begins to form a combustible vapour.
At temps below the FP, fuel will not vapourise enough to provide vapours capable of combustion.
What is the volatility of a fuel?
Cannot be expressed as a single figure.
A volatile liquid is one that can change from liquid to vapour when heat is applied or contact is made with a gas into which it can evapourate.
What is viscosity?
A substance’s ‘resistance to flow’.
What are the most significant properties of AVGAS?
Anti-knock value
Volatility
Vapour locking tendency
Stability
Solvent and corrosion properties
What is anti-knock value?
The resistance the fuel has to detonation.
A fuel with a good anti-knock value combusts smoothly throughout the combustion chamber, providing a regular rise in pressure and push on the piston.
What is vapour locking tendency?
A measure of a liquid’s tendency to evapourate.
The standard used for the measurement of vapour pressure for fuels is the ‘Reid Vapour Pressure’.
What is the stability of a fuel?
Its tendency to form a ‘gummy’ substance when stored for long periods.
What are the key properties required of an AVTUR fuel?
Ease of flow under all operating conditions (including low temperature)
Quick starting of the engine - Low viscosity and high volatility are easier to atomise and thus facilitate starting
Complete combustion under all conditions
A high calorific value
Non-corrosive
Minimal fire hazard
Provide lubrication of the moving parts of the fuel system
What is the calorific value of fuel?
A measure of the heat potential of a fuel.
Important as the combustion chamber is trying to provide maximum heat for minimum fuel expenditure.
What are the rich and weak ratio limits of AVTUR?
Kerosene rich limit = 5:1
Kerosene weak limit = 25:1
What are some typical additives to aviation fuel?
AL 41 - FSII
AL 61 - Enhances lubricity and prevents pipeline corrosion
AL 48 - Blend of AL 41 and AL 61
TEL (Tetraethyl Lead) - Raises anti-knock value. Allows for the use of higher compression ratios for greater efficiency and peak power in gasoline-fuelled engines.
What are the two distinct sub-systems of an aircraft fuel system?
Engine fuel system
Airframe fuel system
What is the function of the airframe fuel system?
Store fuel until it is required and then deliver it to the engines (or auxiliary power unit), at a set pressure and quantity.
What is the boundary between the engine and airframe fuel systems?
LP (low pressure) Fuel Cocks
How is a positive pressure applied to the fuel and why?
Applied using engine air.
When going to altitude, the low pressure may cause the fuel to vaporise, leading to vapour locks and engine malfunction.
What is the purpose of collector tanks?
Each group of fuel tanks feeds a collector tank, which feeds an engine.
The collector tank is always full. Ensures an uninterrupted supply of fuel to the engine during turbulence.
What is the purpose of a fuel cross-feed?
Allows fuel to be taken from one fuel system/group of tanks to feed another engine (i.e. the collector tank of the other engine is blocked).
One fuel tank is now feeding both engines.
I.e. feed any engine from any tank/feed more than one engine from one fuel tank
What is fuel transfer?
Fuel Transfer Valve allows fuel to be transferred from one tank to another.
Why would fuel jettison be used?
To dump fuel to get below the maximum landing weight.
Max take-off weight is greater than max landing weight.
What is the purpose of the Low Pressure Pumps (aka booster pumps)?
Provide backing pressure to the high pressure pumps in the engine system.
What is the role of Jet Pumps (aka Injector Pumps)?
Feed fuel from the storage tanks to the collector tanks.
Use the venturi principle, with fuel bled from the LP pumps providing the motive force.
What is the purpose of a Recuperator Tank?
If the LP pump pressure drops due to surging during a manoeuvre, fuel from the recuperator line is forced into the system.
Fuel flow stops flowing due to negative g event.
As the tank rotates, the flip flop valve opens and allows in fuel from the ‘lower’ pipe but prevents air entering te system via a ‘top’ valve.
When the aircraft returns to normal, the tank refills via gravity.
Provides a supply of fuel to the engines during the manoeuvre to prevent engine cut-out.
What parameters can be displayed on fuel instrumentation?
Quantity
Pressure + Temperature
De-totalizer (shows fuel flow + how much is remaining)
What happens to AVTUR as it starts to freeze?
WAXY crystals begin to form in the fuel
What are the 8 components that can form part of the airframe LP fuel system?
Fuel Tanks
Fuel Pumps
LP cocks
Transfer valves
Crossfeed valves
Jet pumps
Dump valves
Recuperator tank
What is the definition of flashpoint?
The lowest temperature at which fuel begins to form a combustible vapour.
What is the purpose of the LP cock?
Prevents fuel getting to the engine (if switched off…)
Stops flow into the HP fuel system (i.e. it is the gateway between the HP and LP fuel system)?
Note - Be able to label the fuel tank/flow diagrams in the manual with fuel tanks, different pumps/valves, collector tanks, etc.
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What fuel is used on piston engines and what does LL stand for?
AVGAS
LL = Low Lead
