22 Fuel & Fuel Metering Systems Practice Questions & Answers

1. 1. A fuel/air mixture ratio of 12:1 in a reciprocating engine means that the charge entering the cylinder contains what?

   • A.12 pounds of air supplied for each pound of fuel burned✓ Answer
   • B.12 pounds of fuel supplied for each pound of air burned
   • C.12 volumes of air supplied for each volume of fuel burned
   • D.12 percent fuel by weight contained in the total charge

   Mixture ratios are always expressed by weight, never by volume: 12:1 means 12 pounds of air for every pound of fuel, which is also written as a fuel/air ratio of 0.083. A pound of fuel occupies far less space than a pound of air, so a comparison of volumes would tell the mechanic nothing useful about metering.

   Source: FAA Aviation Maintenance Technician Handbook — Powerplant, Engine Fuel and Fuel Metering Systems (fuel/air mixture ratios expressed by weight)Report a problem with this question

2. 2. During cruise the mixture is leaned into the best-economy range instead of the best-power range in order to do what?

   • A.produce the coolest cylinder head temperature at that power
   • B.produce the highest combustion temperature at that power
   • C.produce the greatest horsepower at that throttle opening
   • D.produce the lowest specific fuel consumption at that power✓ Answer

   The best-economy range, a fuel/air ratio of roughly 0.060 to 0.065, is leaner than the best-power range of about 0.0725 to 0.080 and gives the lowest specific fuel consumption, measured in pounds of fuel per hour per horsepower. Leaning from a rich setting actually raises combustion temperature, which peaks near the chemically correct 15:1 ratio, so leaning for economy does not cool the engine.

   Source: FAA Aviation Maintenance Technician Handbook — Powerplant, Engine Fuel and Fuel Metering Systems (best power and best economy mixtures, specific fuel consumption)Report a problem with this question

3. 3. A reciprocating engine operated at high power with an excessively lean mixture is most likely to do what?

   • A.run cool, misfire, and afterfire through the exhaust stacks
   • B.flood the cylinders, run rich, and lose power on acceleration
   • C.foul the spark plugs, run rough, and lose power at low speeds
   • D.overheat, lose power, and backfire through the induction system✓ Answer

   An excessively lean charge burns slowly, so it is still burning when the intake valve opens and it ignites the fresh mixture in the induction system, which is a backfire. The same lean charge raises cylinder temperatures, invites detonation, and reduces power, while an excessively rich mixture instead fouls plugs and can afterfire in the exhaust.

   Source: FAA Aviation Maintenance Technician Handbook — Powerplant, Engine Fuel and Fuel Metering Systems (effects of excessively lean and rich mixtures)Report a problem with this question

4. 4. A float-type carburetor must have a mixture control because, without one, what would happen as the airplane climbs?

   • A.the float chamber fuel level would rise during the climb
   • B.the venturi could not meter fuel above cruise altitude
   • C.the mixture would become too rich as altitude increased✓ Answer
   • D.the mixture would become too lean as altitude increased

   Air density falls as the airplane climbs, but the metering head acting on the fuel stays nearly the same, so about the same weight of fuel is delivered to a smaller weight of air and the mixture becomes progressively richer. The mixture control, whether a needle-type, back-suction, or rotating valve design, reduces fuel flow to correct this, and its idle cutoff position stops fuel flow entirely for shutdown.

   Source: FAA Aviation Maintenance Technician Handbook — Powerplant, Engine Fuel and Fuel Metering Systems (mixture control systems and idle cutoff)Report a problem with this question

5. 5. (1) A clogged main air bleed will cause the engine to run rich. (2) A clogged main metering jet will cause the engine to run lean. Regarding the above statements,

   • A.only statement #1 is true
   • B.both statements are true✓ Answer
   • C.neither statement is true
   • D.only statement #2 is true

   The main air bleed admits air into the fuel passage, emulsifies the fuel, and leans the mixture as airflow increases; if it plugs, that leaning action is lost and fuel flow increases, so the engine runs rich. The main metering jet is the orifice that limits fuel flow, so restricting it delivers less fuel and the engine runs lean. A blocked air passage richens and a blocked fuel passage leans.

   Source: FAA Aviation Maintenance Technician Handbook — Powerplant, Engine Fuel and Fuel Metering Systems (main metering system, air bleed and metering jet)Report a problem with this question

6. 6. With the throttle valve nearly closed at idle, fuel is discharged into the carburetor airstream from where?

   • A.from the main discharge nozzle in the throat of the venturi
   • B.from the idling jet at the edge of the throttle valve✓ Answer
   • C.from the accelerating pump nozzle above the venturi throat
   • D.from the economizer jet just downstream of the throttle valve

   At idle the airflow through the venturi is too small to create enough pressure drop to pull fuel from the main discharge nozzle, so fuel is drawn instead from the idling jet in the low-pressure region at the edge of the nearly closed throttle valve. The idle circuit has its own air bleed that emulsifies the fuel, and it stops discharging once the main nozzle begins to flow.

   Source: FAA Aviation Maintenance Technician Handbook — Powerplant, Engine Fuel and Fuel Metering Systems (float-type carburetor idling system)Report a problem with this question

7. 7. The accelerating system of a float-type carburetor is provided to prevent what condition?

   • A.a momentary lean mixture when the throttle is opened rapidly✓ Answer
   • B.a momentary lean mixture when the throttle is closed rapidly
   • C.a steady rich mixture when the engine is idled for long periods
   • D.a momentary rich mixture when the throttle is opened rapidly

   Airflow responds almost instantly when the throttle is opened, but the heavier fuel lags behind, so the charge momentarily leans out and the engine hesitates or stumbles. The accelerating pump, a diaphragm or piston unit actuated by throttle movement, discharges an extra quantity of fuel during rapid opening and restores the correct ratio until the main metering system catches up.

   Source: FAA Aviation Maintenance Technician Handbook — Powerplant, Engine Fuel and Fuel Metering Systems (accelerating system)Report a problem with this question

8. 8. At what point in the operating range does the economizer, or power enrichment, system of a float-type carburetor supply extra fuel?

   • A.at any power, whenever the mixture control is set to full rich
   • B.at idle only, with the throttle valve against its stop screw
   • C.at low power, below roughly 60 or 70 percent of rated power
   • D.at high power, above roughly 60 to 70 percent of rated power✓ Answer

   The economizer valve is held closed below about 60 to 70 percent of rated power so the engine cruises on the lean, economical schedule set by the main metering system. As the throttle approaches wide open the valve is opened, either by throttle linkage, by a manifold pressure bellows, or by a back-suction arrangement, adding fuel whose evaporation cools the cylinders and helps prevent detonation at high power.

   Source: FAA Aviation Maintenance Technician Handbook — Powerplant, Engine Fuel and Fuel Metering Systems (economizer or power enrichment system)Report a problem with this question

9. 9. Fuel drips from the discharge nozzle of a float-type carburetor after the engine has been shut down. What is the most probable cause?

   • A.the float needle valve and its seat are worn and not sealing✓ Answer
   • B.the throttle linkage is rigged past its full open stop pin
   • C.the main air bleed passage is partially obstructed by dirt
   • D.the idle mixture screw has been adjusted excessively rich

   The float chamber holds the fuel level slightly below the outlet of the discharge nozzle precisely so fuel cannot siphon out when the engine is stopped. If the needle valve and seat are worn or held off by debris, incoming fuel raises the level until it spills from the nozzle, and the correct corrective action is to replace the needle valve and seat rather than to adjust the mixture or the linkage.

   Source: FAA Aviation Maintenance Technician Handbook — Powerplant, Engine Fuel and Fuel Metering Systems (float chamber mechanism and carburetor troubleshooting)Report a problem with this question

10. 10. A float in a float-type carburetor is punctured and partly fills with fuel. What is the effect on carburetor operation?

    • A.the fuel level drops and the mixture then becomes richer
    • B.the fuel level rises and the mixture becomes leaner
    • C.the fuel level drops and the mixture becomes leaner
    • D.the fuel level rises and the mixture becomes richer✓ Answer

    A punctured float loses buoyancy and rides lower, so it holds the needle valve off its seat longer and the fuel level in the chamber settles higher than normal. The greater head at the discharge nozzle delivers more fuel for a given airflow, so the mixture runs rich, exactly as it does when the float level is set too high; a level set too low leans the mixture.

    Source: FAA Aviation Maintenance Technician Handbook — Powerplant, Engine Fuel and Fuel Metering Systems (float level and its effect on mixture)Report a problem with this question

11. 11. With the engine warmed to normal operating temperatures and idling, the mixture control is moved slowly toward idle cutoff and the tachometer shows a rise of about 25 rpm before the engine begins to die. The idle mixture is

    • A.too rich, and must be leaned at the mixture adjustment
    • B.too rich, and must be reset at the idle speed stop screw
    • C.correct as set, and no further adjustment is required✓ Answer
    • D.too lean, and must be enriched at the mixture adjustment

    A momentary rise of roughly 10 to 50 rpm as the mixture control is moved toward idle cutoff shows that the idle mixture is set correctly, because the small enrichment removed by leaning briefly improves combustion before fuel flow stops. A larger rise means the idle mixture is too rich and an immediate drop with no rise means it is too lean, and the check is valid only with the engine at normal oil and cylinder head temperature.

    Source: FAA Aviation Maintenance Technician Handbook — Powerplant, Engine Fuel and Fuel Metering Systems (idle mixture check and adjustment)Report a problem with this question

12. 12. The automatic mixture control of a pressure injection carburetor compensates for changes in what?

    • A.float chamber pressure resulting from changes in ram air pressure
    • B.venturi suction resulting from changes in throttle valve position
    • C.fuel density resulting from changes in fuel temperature and grade
    • D.air density resulting from changes in altitude and temperature✓ Answer

    The automatic mixture control uses a sealed, evacuated bellows exposed to carburetor inlet air, so it senses air density directly. As density falls with increasing altitude or rising temperature the bellows expands and reduces the metering force, cutting fuel flow to hold the ratio correct; it does not respond to float chamber pressure, since a pressure carburetor has no vented float chamber, nor to throttle-caused changes in venturi suction.

    Source: FAA Aviation Maintenance Technician Handbook — Powerplant, Engine Fuel and Fuel Metering Systems (pressure injection carburetor automatic mixture control)Report a problem with this question

13. 13. In a pressure injection carburetor, the air metering force that positions the fuel poppet valve is created by the difference between what two pressures?

    • A.atmospheric pressure and manifold pressure at the throttle body
    • B.float chamber pressure and discharge nozzle pressure at the jet
    • C.impact air pressure and boost venturi suction on the air diaphragm✓ Answer
    • D.metered fuel pressure and unmetered fuel pressure in the unit

    The regulator unit is divided into chambers by diaphragms, with impact air pressure in one air chamber and boost venturi suction in the next, and the difference between them acting across the air diaphragm is the air metering force. That force opens or closes the poppet valve to admit fuel in proportion to the airflow, which is why a pressure carburetor needs no vented float chamber and no nozzle suction pickup.

    Source: FAA Aviation Maintenance Technician Handbook — Powerplant, Engine Fuel and Fuel Metering Systems (pressure injection carburetor regulator unit and air metering force)Report a problem with this question

14. 14. In a continuous-flow fuel injection system, what does the flow divider do when the mixture control is placed in idle cutoff?

    • A.it vents each nozzle line to atmosphere through a bleed hole
    • B.it returns metered fuel to the airframe tank through a vent line
    • C.it holds residual pressure on the nozzles so they stay primed
    • D.it shuts off the fuel supply to the individual nozzle lines✓ Answer

    The flow divider keeps metered fuel under pressure, divides it evenly to the cylinders at idle, and its spring-loaded valve seats in idle cutoff so the individual nozzle lines are shut off, giving a positive fuel cutoff at shutdown. Above idle the divider valve is fully open and fuel distribution becomes a function of the discharge nozzles themselves.

    Source: FAA Aviation Maintenance Technician Handbook — Powerplant, Engine Fuel and Fuel Metering Systems (continuous-flow fuel injection flow divider)Report a problem with this question

15. 15. Several discharge nozzles of a continuous-flow fuel injection system become partly clogged. What does the cockpit fuel flow indication read?

    • A.higher than normal, because the gauge senses nozzle line pressure✓ Answer
    • B.lower than normal, because the gauge senses actual fuel flow
    • C.lower than normal, because the pump output has fallen off
    • D.normal, because the gauge senses only pump outlet pressure

    The gauge labeled fuel flow on a continuous-flow injection system is actually a pressure gauge that senses pressure at the flow divider or nozzle line and is calibrated in pounds per hour or gallons per hour. A restriction downstream raises that pressure, so the indication goes up while the fuel actually delivered goes down, and the reading is normally accompanied by rough running and uneven exhaust gas and cylinder head temperatures.

    Source: FAA Aviation Maintenance Technician Handbook — Powerplant, Engine Fuel and Fuel Metering Systems (fuel injection flow indication and nozzle troubleshooting)Report a problem with this question

16. 16. The engine-driven pump of a continuous-flow fuel injection system is a positive displacement type whose output rises with engine speed. Because of this, the system must also include what?

    • A.an air bleed in each nozzle to set the fuel pressure drop
    • B.a venturi in the throttle body to create the metering force
    • C.a recirculation path and a relief valve to control output✓ Answer
    • D.a vented float chamber and needle valve to set the fuel level

    A positive displacement pump delivers a fixed volume per revolution regardless of what the engine needs, so the excess must be handled by a recirculation passage with an adjustable orifice and by a relief valve that returns fuel to the inlet. Fuel entering the pump passes a swirl-well vapor separator that sends vapor back to the tank, and a check valve lets boost pump pressure bypass the engine-driven pump for starting.

    Source: FAA Aviation Maintenance Technician Handbook — Powerplant, Engine Fuel and Fuel Metering Systems (continuous-flow injection engine-driven pump and vapor separator)Report a problem with this question

17. 17. Why is a continuous-flow fuel injection system essentially free of the fuel vaporization icing that can affect a float-type carburetor?

    • A.the fuel is discharged as a vapor and absorbs no extra heat
    • B.the fuel is metered by pressure, so no venturi is used at all
    • C.the fuel is discharged at the intake ports, past the throttle valve✓ Answer
    • D.the fuel is preheated in the injector before it is metered

    In a carburetor the fuel vaporizes upstream in the venturi and throttle area, and the heat it absorbs can drop the local temperature enough to freeze moisture on the throttle valve and nozzle even on a warm day. Continuous-flow injection sprays fuel into the intake port at each cylinder head, downstream of the throttle valve, where engine heat offsets the temperature drop, which is why better distribution and freedom from vaporization icing are listed as advantages of injection.

    Source: FAA Aviation Maintenance Technician Handbook — Powerplant, Engine Fuel and Fuel Metering Systems (advantages of fuel injection over carburetion)Report a problem with this question

18. 18. A turbine engine fuel control schedules how fast fuel flow may increase when the power lever is advanced rapidly. What is it primarily preventing?

    • A.over-fueling, which can cause compressor stall and over-temperature✓ Answer
    • B.under-fueling, which can cause the oil temperature to rise sharply
    • C.under-fueling, which can cause the compressor to overspeed and surge
    • D.over-fueling, which can cause the fuel nozzles to carbonize and streak

    If fuel is added faster than the compressor can supply matching airflow, the burner runs very rich, which can produce compressor stall, turbine over-temperature, or a rich blowout of the flame. The acceleration and deceleration schedules keep fuel flow between those limits and the lean flameout limit, and the control does this by sensing engine speed, power lever angle, inlet air temperature and pressure, and burner or compressor discharge pressure.

    Source: FAA Aviation Maintenance Technician Handbook — Powerplant, Engine Fuel and Fuel Metering Systems (turbine fuel control acceleration and deceleration schedules)Report a problem with this question

19. 19. In a full authority digital engine control system, what normally happens if one channel of the two-channel electronic engine control fails?

    • A.the crew meters fuel manually with the power lever position
    • B.the second channel takes over and the engine keeps operating✓ Answer
    • C.the engine is limited to idle thrust until the unit is changed
    • D.the engine reverts to a hydromechanical backup fuel control

    An electronic engine control in a full authority system is built with two independent channels, one in command and one in standby, that continuously compare data through cross-talk logic, so a fault in the active channel transfers control to the other with no change in engine operation. A full authority system has no hydromechanical backup, which is what distinguishes it from a hydromechanical and electronic control where the electronic unit only trims a hydromechanical control.

    Source: FAA Aviation Maintenance Technician Handbook — Powerplant, Engine Fuel and Fuel Metering Systems (electronic engine controls and full authority digital engine control)Report a problem with this question

20. 20. In a turbine engine that uses duplex fuel nozzles, about 90 percent of the fuel at maximum flow passes through what?

    • A.the primary orifices, which flow throughout the whole range
    • B.the secondary orifices, opened by the fuel pressurizing valve✓ Answer
    • C.the primary orifices, once the drain valve has closed
    • D.the secondary orifices, only while the engine is starting

    A duplex nozzle has primary and secondary orifices fed through separate manifolds, and a pressurizing valve or flow divider decides which is used. All the fuel goes through the primary at starting and at low flows so it atomizes well at low pressure, and as flow increases the secondary opens and carries roughly 90 percent of the total at maximum flow.

    Source: FAA Aviation Maintenance Technician Handbook — Powerplant, Engine Fuel and Fuel Metering Systems (duplex fuel nozzles and fuel pressurizing valve)Report a problem with this question

21. 21. Why is a bypass valve required in a fuel system that uses a micron-type filter?

    • A.the element traps water that must be drained during flight
    • B.the element cannot withstand full pump discharge pressure
    • C.the element clogs readily and fuel flow must be maintained✓ Answer
    • D.the element must be back-flushed with fuel during operation

    The cellulose micron element removes very small particles, on the order of 10 to 25 microns, and also absorbs water, which makes it the most easily clogged of the common filter types. The bypass valve therefore lets unfiltered fuel continue to the engine rather than allowing fuel starvation, and a differential pressure switch normally lights a cockpit warning that bypass is impending.

    Source: FAA Aviation Maintenance Technician Handbook — Powerplant, Engine Fuel and Fuel Metering Systems (fuel filters, micron filter and bypass provisions)Report a problem with this question

22. 22. What is the purpose of the fuel heater in a turbine engine fuel system?

    • A.prevent vapor from forming in the high pressure lines
    • B.raise the fuel temperature to improve nozzle atomization
    • C.prevent ice from forming on the element of the main filter✓ Answer
    • D.vaporize the fuel before it reaches the metering unit

    Water held in the fuel freezes and collects on the filter element whenever tank fuel is at or below the freezing point, and a plugged filter can starve the engine or force the system into bypass. The fuel heater is a heat exchanger that uses engine bleed air or engine oil, is operated intermittently either manually or by a thermostat, and can also thaw ice that has already formed on the screen.

    Source: FAA Aviation Maintenance Technician Handbook — Powerplant, Engine Fuel and Fuel Metering Systems (fuel heater and fuel filter icing)Report a problem with this question

Practice questions based on the Aviation Mechanic Airman Certification Standards (FAA-S-ACS-1), 14 CFR parts 43, 65, and 91, and the FAA Aviation Maintenance Technician Handbook—Powerplant (FAA-H-8083-32). This site is not affiliated with or endorsed by the Federal Aviation Administration. This bank covers the POWERPLANT written test only; the General and Airframe written tests and the oral and practical tests are separate. Torque values, running clearances, compression limits, magneto timing angles, spark plug gaps, temperature and pressure limits, servicing quantities, and overhaul intervals always come from the manufacturer's current maintenance data, the type certificate data sheet, and the applicable airworthiness directives — never from a practice test. Confirm current eligibility and testing requirements with the FAA before you test. About the A&P mechanic certificate →