Engine Fuel and Fuel Metering Systems

Question 1

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

Answer 1

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.

Question 2

What does an air-fuel mixture ratio of 12:1 represent?

Answer 2

This represents an air-to-fuel ratio mixture of 12 pounds of air and 1 pound to fuel.

Question 3

What is the function of a float carburetor?

Answer 3

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

Question 4

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

Answer 4

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

Question 5

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

Answer 5

A malfunctioning accelerator pump.

Question 6

How does a pressure-type carburetor operate?

Answer 6

A pressure-type carburetor discharges fuel into the airstream at a pressure well above atmospheric. This results in better vaporization and permits the discharge of fuel into the airstream on the engines side of the throttle valve. With the discharge nozzle located at this point, the drop in temperature due to fuel vaporization takes place after the air has passed the throttle valve and at a point where engine heat tends to offset it. Thus, the danger of fuel vaporization icing is practically eliminated. The effects of rapid maneuvers and rough air on the pressure-type carburetors are negligible since its fuel chambers remain filled under all operating conditions.

Question 7

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

Answer 7

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

Question 8

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

Answer 8

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.

Question 9

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

Answer 9

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.

Question 10

What is a FADEC?

Answer 10

A full-authority digital electronic control (FADEC) is a high-precision digital electronic fuel control that functions during all engine operations. It includes the EEC and functions with the 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 EICAS.

Question 11

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

Answer 11

No, this is done by the FADEC.

Question 12

What are the basic principles of a hydromechanical fuel control?

Answer 12

A pure hydromechanical fuel control has no electronic interface assisting in computing or metering the fuel flow. It also is generally driven by the gas generator gear train of the engine to sense engine speed. Other mechanical engine parameters that are sensed are compressor discharge pressure, burner pressure, exhaust temperature, and inlet air temperature and pressure. Once the computing section determines the correct amount of fuel flow, the metering section through cams and servo valves delivers the fuel to the engine fuel system.

Question 13

What are the two basic types of fuel nozzles?

Answer 13

-Simplex
-Duplex

Question 14

Why does a duplex fuel nozzle usually require a dual fuel manifold?

Answer 14

The duplex nozzle usually requires a dual manifold and a pressurizing valve or flow divider for dividing primary and secondary (main)fuel flow.

Question 15

What is the advantage of duplex fuel nozzle?

Answer 15

The duplex nozzle offers a desirable spray pattern for combustion over a wide range of operating pressure.

Question 16

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

Answer 16

-Power lever angle
-Compressor inlet total temperature
-Compressor RPM
-Burner pressure

Question 17

What is meant by trimming a turbine engine?

Answer 17

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

Question 18

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

Answer 18

-Supervisory electronic engine control
-Full-authority digital electronic control

Question 19

What position should the flight deck fuel metering control (of fuel condition lever) be set to for starting a turbine engine?

Answer 19

The normal starting sequence is:
1. Rotate the compressor with the starter
2. Turn the ignition on
3. Open the engine fuel valve, either by moving the throttle to idle or by moving a fuel shutoff lever or turning a switch

Question 20

What specifications are used to trim a turbine engine?

Answer 20

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

Question 21

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

Answer 21

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

Question 22

When should a turbine engine be retrimmed?

Answer 22

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

Question 23

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

Answer 23

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

Question 24

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

Answer 24

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

Question 25

How can you tell when a fuel strainer for a turbine engine has been bypassed?

Answer 25

There is an indicator button that will pop out. This button is usually painted red for easy identification of a bypass condition.

Question 26

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

Answer 26

*Air-to-fuel
*Oil-to-fuel

Question 27

How are the fuel lines often protected in an engine compartment?

Answer 27

By wrapping the lines with a fire protection sleeve to protection sleeve to protect from fire and abrasion.

Question 28

What is the purpose of a lay line on a flexible fuel hose?

Answer 28

To monitor fuel hose twist. Hoses should be installed without twisting.

Question 29

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

Answer 29

Centrifugal pumps

Question 30

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

Answer 30

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

Question 31

What is meant by a compensated fuel pump?

Answer 31

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

Question 32

What are the two types or categories of engine driven fuel pumps?

Answer 32

-Nonconstant displacement
-Nonpositive displacement

Question 33

What are the three basic types of valves used in fuel systems?

Answer 33

-Cone
-Poppet
-Gate valves

Question 34

What are the three most common types of fuel filters?

Answer 34

-Micron
-Wafer screen
-Plaint screen mesh filter

Question 35

Why do some aircraft filters have a relief valve?

Answer 35

If the fuel screen, or filter, becomes clogged, the relief valve will allow fuel to continue to flow to the engine.

Question 36

Where are fuel drains located in an aircraft engine?

Answer 36

On a turbine engine, combustion chambers have a drain valve that drains fuel that accumulates in the combustion chamber after each shutdown, or any fuel that may have accumulated during a false start. Many reciprocating engines have drain valves in the intake manifold hat open when the engine is not running. These drains remove excess fuel if the engine is flooded from over-priming or an unsuccessful engine start.