ENGINE FUEL SYSTEMS (Jepp/Prep/Crane)

What is the purpose of a duplex fuel nozzle?

A duplex fuel nozzle discharges two different spray patterns. The spray pattern is wider during startup and acceleration and narrows when engine speed increases above idle.

What is done in most aircraft fuel systems to prevent vapor lock?

Boost pumps in the fuel tank pressurize the fuel in the lines and force the fuel into the fuel metering system.

What is a FADEC?

A Full-Authority Digital Electronic Control (FADEC) is a high-precision digital electronic fuel control that functions during all engine operations. It includes the Electronic Engine Control (EEC) and functions with the Flight Management Computer (FMC) to measure the fuel to the nozzles in such a way that prevents overshooting of power changes and over-temperature conditions. FADEC furnishes information to the Engine indication and Crew Alerting System (EICAS).

What is meant by a duplex nozzle in a turbine engine fuel metering system?

A duplex fuel nozzle is one that has two fuel discharge passages. A flow divider sends fuel for low-pressure operation through one discharge passage, and when the engine demands more fuel, it is sprayed out through the second discharge passage. The spray pattern keeps the flame centered in the burner for all operating conditions.

What would likely cause a reciprocating engine equipped with a float carburetor to hesitate momentarily when the throttle is rapidly advanced from idle to full power?

A malfunctioning accelerator pump.

What is meant by a compensated relief valve in an engine-driven fuel pump?

A pressure relief valve that is acted upon by a diaphragm as well as a spring. Atmospheric pressure acting on the diaphragm varies the pump discharge pressure so that it will remain a given amount higher than the pressure of the air entering the carburetor.

What type of engine-driven fuel pump is widely used other than gear or piston pumps?

A rotary vane type.

How does an automatic mixture control (AMC) function?

A sealed brass bellows connected to the fuel metering system, expands and contracts with changes in pressure and temperature, adjusting the mixture accordingly.

How does the exhaust gas temperature of a reciprocating engine tell anything about the fuel-air mixture being burned by the engine?

A stoichiometric mixture (a mixture in which all of the constituents of the fuel-air mixture are burned) produces the highest exhaust gas temperature. The mixture is adjusted to get the highest EGT, and then it is enriched to place it on the rich side of stoichiometric.

What type of pump can vary the amount of fuel discharged regardless of speed?

A variable displacement pump.

What is meant by trimming a turbine engine?

Adjusting the fuel control for the proper specific gravity of the fuel and for the proper idle and full throttle RPM.

What are two types of fuel heaters used in a jet transport aircraft?

Air-to-fuel and oil-to-fuel heat exchangers.

What should be done to an aircraft reciprocating engine if it has been operated on turbine engine fuel?

All of the turbine fuel should be drained out and the system filled with the proper grade of aviation gasoline. The engine should be given a compression check, and all of the cylinders should be inspected with a borescope. The oil should be drained and the filters carefully examined. The engine should be given a complete run-up check.

What maintains the fuel pressure in a pressure-injection carburetor?

An engine-driven fuel pump.

When should a turbine engine be retrimmed?

Any time there is a decrease in engine thrust, and after any maintenance that the manufacturer specifies as requiring retrimming.

What are two locations water may be injected into a turbine engine?

At the compressor inlet and at the inlet to the diffuser section.

Where does an engine-driven fuel pump direct the excess fuel from its pressure relief valve?

Back to the inlet side of the pump.

What are some of the advantages of a full authority digital engine control (FADEC) over a hydromechanical fuel control?

Better fuel economy, improved starts, requires no engine trimming, provides engine limit protection, provides constant idle speeds regardless of atmospheric conditions or bleed air requirements, fully modulates the active clearance control, and allows more repeatable engine transients.

What are some of the sources used to heat the fuel in a turbine engine fuel system?

Bleed air and engine lubricating oil.

Why is an aircraft reciprocating engine killed by placing the mixture control in the Cutoff position, rather than by using the magneto switch?

By shutting off the fuel to the cylinders, the combustion chambers are left full of air with no fuel. This makes the engine less likely to fire if the propeller is pulled through with the ignition switch accidentally on.

What kind of fuel boost pumps are normally installed inside the fuel tanks?

Centrifugal pumps.

Name the two categories of turbine engine pumps.

Constant and variable displacement.

What category is the gear-type pump classified in?

Constant displacement.

What function does the mixture control perform?

Controls the fuel/air mixture.

What are the different engine indications used for trimming a turbine engine?

Either E.P.R. or r.p.m. is used depending on the particular engine.

What kind of boost pump is used in most aircraft fuel systems?

Electrically operated centrifugal pumps.

What engine parameters are sensed by the fuel control unit of a turbine engine?

Engine RPM, inlet air pressure, compressor discharge pressure, burner can pressure, and inlet air temperature.

Where are the fuel strainers normally located in an aircraft fuel system?

Finger strainers are located in the tank outlet, the main strainer, located at the lowest point in the fuel system, and strainers in the carburetor or fuel injection system.

What are the most common types of fuel metering systems used on small reciprocating engines?

Float-type carburetors, pressure-injection carburetors and direct fuel injection systems.

What is the purpose of the pressurizing and dump valve in the fuel system for a turbine engine?

For normal engine operation, the pressurizing and dump valve acts as a flow divider, directing the fuel into the main or pilot manifold so it will be discharged from the proper orifice in the duplex fuel nozzle. When the engine is shut down, the dump function of the valve dumps all of the fuel from the manifold.

What are the two basic types of turbine engine fuel control units?

Hydromechanical and electronic.

What are two basic types of fuel controls for aircraft turbine engines?

Hydropneumatic and electro-hydromechanical.

What is the purpose of a shear section of the driveshaft in a dual element constant displacement pump?

If one element seizes, a portion of the driveshaft seizes, allowing the other element to continue to operate.

Does the fuel-air mixture provided by a float carburetor become richer or leaner as the aircraft goes up in altitude?

If the mixture is not adjusted, it will become richer as the aircraft gains in altitude.

In which direction relative to the wind should a turbine-powered aircraft be positioned when the fuel control is being trimmed?

If the wind velocity is less than 10 miles per hour, it can be faced in any direction. If the wind velocity is between 10 and 25 MPH, it should be trimmed facing into the wind. The engine should not be trimmed when the wind velocity is more than 25 MPH.

Where should ambient temperature be measured when trimming a turbine engine?

In a shaded area as near the engine inlet as is practical.

In what position is the carburetor heat control placed when starting an aircraft engine?

In the Cold position.

In what position is the mixture control placed for starting an engine that is equipped with a pressure carburetor?

In the Idle Cutoff position. The engine is started with fuel from the primer system.

How does the automatic mixture control in a pressure carburetor keep the fuel-air mixture constant as the aircraft changes altitude?

It automatically, and progressively, bleeds the air between the two sides of the air diaphragm and decreases the air metering force. This leans the mixture as the aircraft goes up in altitude.

What is meant by a compensated fuel pump?

It is a fuel pump that senses the ambient air pressure and maintains the fuel pressure a specific amount above this air pressure.

What is a carburetor economizer system?

It is a power enrichment system that provides a richer mixture at high power settings, where the excess fuel aids in engine cooling. This system functions at settings above cruise power settings.

What is the purpose of a bypass valve in an engine fuel system micron filter?

It is a safety feature that allows fuel to flow to the engine if the filter becomes blocked.

Where is the fuel from a continuous-flow fuel injection system discharged?

It is discharged through injector nozzles screwed into the cylinder heads near the intake valves.

What are two purposes of the manifold valve (or flow divider) in the fuel injection system of a horizontally opposed aircraft engine?

It provides a constant discharge fuel pressure for idling, and it provides a positive shutoff for the fuel when the engine is shut down.

What function does the engine-driven pump on a turbine engine perform?

It provides a continuous supply of fuel at the proper pressure while the engine is running.

What is the purpose of an accelerating system?

It provides an immediate but brief increase in fuel flow in the venturi to enrich the mixture.

What is the purpose of a pressure relief valve in a constant displacement pump?

It returns excess fuel that is not required by the engine to the inlet side of the pump.

What are the two basic types of turbine engine fuel?

Jet A and A-1, which are a special type of kerosine-base fuel. This is similar to military JP-5. Jet B is a gasoline-base fuel similar to military JP-4.

When performing an external inspection of an engine-driven fuel pump, what should you look for?

Leaks and security of mounting.

What are some of the advantages of fuel injection systems over carburetor systems?

Less danger of induction icing, better acceleration, better fuel distribution, better fuel economy, and reduced overheating of individual cylinders.

Are the instruments installed in an aircraft suitable for use when trimming a turbine engine?

No, a special analyzer/trimmer such as a JetCal should be used.

Does a turbine engine controlled by a FADEC require manual trimming?

No, this is done by the FADEC.

What generally causes spark plug fouling?

Operating the engine with an excessively rich mixture at idle.

What are the engine variables detected by the fuel control unit?

Power lever position, engine r.p.m., compressor inlet temperature, compressor inlet pressure, compressor discharge pressure, and burner pressure.

What are two reasons Prist is used in turbine engine fuel?

Prist is an antifreeze agent that lowers the freezing point of any water that precipitates out of the fuel. It also acts as a biocidal agent that kills the microbial growth that forms scum in fuel tanks. This scum traps and holds water against the aluminum alloy in the fuel tanks and causes corrosion.

Why must the diaphragms of a pressure carburetor be soaked before the carburetor is ready for flight?

Soaking the diaphragms of a pressure carburetor restores them to the condition of flexibility that they had when the carburetor was calibrated.

What maintenance adjustments are normally allowed on an installed hydromechanical fuel control?

Specific gravity for fuel, idle r.p.m., and maximum r.p.m. (or maximum E.P.R.).

What are two types of electronic fuel controls for turbine engines?

Supervisory Electronic Engine Control and Full-Authority Digital Electronic Control.

What are the two types of fuel injection systems used on modern reciprocating aircraft engines?

Teledyne Continental (TCM) system and Precision Airmotive (Bendix) system.

Why do aircraft fuel metering systems for reciprocating engines have mixture controls?

The air density decreases as the aircraft ascends, causing the mixture to become richer. The mixture control allows the fuel to be decreased to maintain a fuel-air mixture ratio that produces the desired power.

What is the purpose of the bypass valve in an engine-driven fuel pump?

The bypass valve allows fuel to flow around the engine-driven pump for starting and for emergency operation if the engine-driven pump should fail.

What is a commonly used type of fuel boost pump?

The centrifugal type.

What specifications are used to trim a turbine engine?

The data plate specifications for the specific engine. This data was obtained when the engine was calibrated in the manufacturer's test cell.

What is the function of the derichment valve in a pressure carburetor used on an aircraft engine with an antidetonation injection system?

The derichment valve automatically closes to lean the fuel-air mixture when ADI fluid is flowing. As soon as the ADI fluid stops flowing, the derichment valve opens and the mixture returns to its rich condition.

What is the function of the economizer system in an aircraft engine carburetor?

The economizer (or power enrichment) system allows the engine to operate with an economically lean mixture for all conditions other than full power. When the throttle is opened for full power, the power enrichment system automatically enriches the mixture to remove some of the heat that is developed during this type of operation.

What are the two major components of a supervisory electronics engine control?

The electronic control unit (computer), and the hydromechanical fuel control.

What is used in the TCM fuel injection system to control the amount of fuel sent to the nozzles?

The engine speed determines the injector pump output pressure. The relief valve determines the fuel for low-speed operation, and the adjustable orifice determines the fuel for high-speed operation.

What happens if the manual mixture control on a pressure injection carburetor is moved to the idle cutoff position?

The engine stops.

What is the function of a float carburetor?

The float carburetor's function is to measure airflow through the engine induction system and measure out the appropriate amount of gasoline into the air for all engine operating perimeters and conditions. It must also provide the fuel in as complete a vaporized state as possible by the time ignition occurs in the engine cylinders.

What causes a vapor lock in an aircraft fuel system?

The fuel becomes hot enough that it boils. Vapors are released from the liquid fuel, and these vapors block the fuel lines so the liquid fuel cannot flow to the engine.

Where in a turbine engine fuel system is ice formation likely to occur and how is it prevented?

The fuel filter is most susceptible to ice formation, so a fuel heater is used that consists of a heat exchanger using either engine oil or bleed air to warm the fuel.

Why do some turbine engine fuel systems incorporate a fuel heater?

The fuel heater keeps the fuel warm enough that any water that precipitates out of the fuel will not freeze on the filters.

Where does the fuel metering system of a turbine engine discharge its fuel?

The fuel is discharged through spray nozzles into the combustion chambers.

What is controlled by the adjustment of the orifice in the fuel pump of a Teledyne-Continental fuel injection system?

The high unmetered fuel pressure.

When the mixture is placed in the idle-cutoff position, a slight rise in r.p.m. is noted prior to engine decelerating. What does this indicate?

The idle mixture is set correctly.

What is adjusted when the fuel control unit of a turbine engine is trimmed?

The idle speed and the maximum-thrust speed.

What is used to keep water, sediment, and foreign matter out of the carburetor?

The main fuel strainer.

What is used in the Precision Airmotive system to control the amount of fuel sent to the nozzles?

The mass of the air entering the engine acts on an air diaphragm that controls a servo ball valve in the line between the regulator and the flow divider.

Does the fuel-air mixture provided by a float carburetor become richer or leaner when carburetor heat is applied?

The mixture becomes richer when carburetor heat is applied.

What would happen to the fuel-air mixture ratio in a float carburetor if the main air bleed were to become plugged?

The mixture would become excessively rich.

What are the functions of the pressurizing and dump portions of a pressurizing and dump valve?

The pressurizing portion provides primary and secondary fuel flow to dual-line duplex fuel nozzles, and the dump portion allows fuel to drain from the manifolds after engine shutdown.

On which side of the firewall is the engine fuel shutoff valve located?

The shutoff valve must be on the side of the firewall away from the engine.

What two things are adjusted when adjusting the idling of a reciprocating engine?

The throttle stop, to get the proper idling RPM, and the idle mixture control, to get the smoothest operation.

What is the significance of the letters that are stamped on the flats of a fuel injection nozzle?

These letters designate the relative size of the orifice in the injector nozzle. The lower the letter in the alphabet, the smaller the amount of fuel the nozzle will flow.

What purpose do fuel selector valves serve?

They allow tank and engine selection, and provide a means for shutting off fuel flow.

What purpose do turbine engine fuel control units serve?

They automatically meter fuel to the engine.

What do turbine engine fuel spray nozzles do?

They inject fuel into the combustion area.

What function do fuel boost pumps perform?

They supply pressurized fuel to the fuel pump, which helps prevent vapor lock and cavitation.

Why do some aircraft fuel filters have a built-in relief valve?

This relief valve will open and allow unfiltered fuel to flow to the fuel control device if the filter should become plugged with ice or other contaminants.

Why should engine r.p.m. be accelerated periodically when making carburetor adjustments?

To clear the engine.

What are three purposes for the boost pumps in an aircraft fuel system?

To provide fuel pressure for starting the engine, to pressurize the fuel lines to prevent vapor lock, and to transfer fuel from one tank to another.

What causes vapor lock and why is it of concern?

Vapor lock is caused by insufficient fuel pressure, high fuel temperatures and excessive fuel turbulence, which may completely block any fuel flow resulting in engine failure.

How does the mixture control change the fuel-air mixture ratio on the Teledyne-Continental fuel injection system?

When the mixture control is in its Full Rich position, all fuel goes to the manifold valve. When it is in the Idle Cutoff position, all fuel is returned to the pump inlet. Intermediate positions vary the amount of fuel that goes to the engine.

When does the acceleration system operate on an aircraft carburetor?

When the throttle is suddenly opened.

What two things are adjusted when setting the idling conditions on a float carburetor?

a. Idling RPM by adjusting the throttle stop. b. Idling mixture by adjusting idle needle valve.

What parameters are normally sensed by a turbine-engine fuel control?

a. Power lever angle. b. Compressor inlet total temperature. c. Compressor RPM. d. Burner pressure.

What are three functions of the boost pump in the tank of a jet transport aircraft?

a. To pressurize the fuel in the line between the tank and the engine-driven pump. b. To transfer fuel from one tank to another to balance the fuel load. c. To pump fuel from the tank into the dump chute when fuel is being dumped.