22 Induction & Cooling Systems Practice Questions & Answers

1. 1. In a normally aspirated horizontally opposed engine, what directly controls the manifold pressure that exists downstream of the throttle valve?

   • A.The setting of the mixture control, which fixes the fuel-air ratio supplied to the manifold
   • B.The propeller governor setting, which holds crankshaft speed and with it the manifold pressure
   • C.The position of the carburetor heat valve, which selects the source of the induction system air
   • D.The position of the throttle valve, which limits how much air can enter the intake manifold✓ Answer

   Induction air travels from the ram air scoop through the filter and the fuel metering device to the throttle. The throttle plate restricts that flow, so the pressure in the manifold behind it, read in inches of mercury, rises and falls with throttle position, which is why manifold pressure is used as a measure of power output.

   Source: FAA Aviation Maintenance Technician Handbook — Powerplant (FAA-H-8083-32), induction systems chapter: throttle position and manifold pressureReport a problem with this question

2. 2. An engine runs rough at idle but smooths out at higher power, and a leaking intake pipe is suspected at one cylinder. Why does such a leak affect idling most?

   • A.Manifold pressure is lowest at idle, so air is drawn through the leak and leans that cylinder✓ Answer
   • B.Exhaust back pressure is greatest at idle, so exhaust gas enters the leak and dilutes the charge
   • C.Intake valve timing is latest at idle, so the leaking air cannot reach the cylinder until cruise
   • D.Fuel discharge is greatest at idle, so the leak adds fuel to that cylinder and fouls its plugs

   With the throttle nearly closed, manifold pressure is far below atmospheric, so the pressure difference across a leak downstream of the throttle is at its greatest and unmetered air is pulled in. That extra air leans only the affected cylinder, which fires unevenly and produces the rough idle; opening the throttle raises manifold pressure and reduces the leakage effect.

   Source: FAA Aviation Maintenance Technician Handbook — Powerplant (FAA-H-8083-32), induction systems chapter: induction leaks and rough idlingReport a problem with this question

3. 3. An engine is operated repeatedly from a dusty unpaved field with its induction air filter missing. What is the principal damage this causes?

   • A.Manifold pressure falls at every power setting, so the engine cannot make rated takeoff power
   • B.Fuel vaporizes more completely, leaning the mixture and overheating the cylinder heads in cruise
   • C.Abrasive dust reaches the cylinders, scoring the walls and rings and contaminating the oil✓ Answer
   • D.Induction air temperature climbs steeply, pushing the engine toward detonation at high power

   The filter exists to keep abrasive airborne dirt out of the cylinders and off the metering elements. Dust that gets past it scores cylinder walls and piston rings, works into the oil and from there into bearings, gears and oil passages, so the engine wears out long before its normal service life.

   Source: FAA Aviation Maintenance Technician Handbook — Powerplant (FAA-H-8083-32), induction systems chapter: purpose of induction air filteringReport a problem with this question

4. 4. A float-type carburetor engine is running at a fixed throttle setting with no induction ice present. What happens when full carburetor heat is applied?

   • A.Power decreases and the mixture becomes leaner, since hot air burns the fuel more completely
   • B.Power decreases and the mixture becomes richer, since the same fuel enters air of lower density✓ Answer
   • C.Power stays the same and the mixture becomes leaner, since the metering system corrects for density
   • D.Power increases and the mixture becomes richer, since warm air vaporizes the fuel more readily

   Heated air is less dense, so the weight of the charge drawn into the cylinders drops and volumetric efficiency and power fall with it. The metering system still delivers about the same weight of fuel into that lighter charge, so the mixture becomes richer; the higher charge temperature also moves the engine toward detonation, which is why heat is not used at high power.

   Source: FAA Aviation Maintenance Technician Handbook — Powerplant (FAA-H-8083-32), induction systems chapter: effect of carburetor heat on charge density and mixtureReport a problem with this question

5. 5. Why is the air supplied to the carburetor in the HOT position of the carburetor heat control normally unfiltered?

   • A.It is bled from the compressor discharge, where any filter would be burned by the charge temperature
   • B.It is taken from the ram air scoop, and the filter is removed from the air box before every flight
   • C.It is heated by an electric element after the filter, and that heat would destroy the media
   • D.It is taken from inside the engine compartment through a shroud around the exhaust, bypassing the filter✓ Answer

   In the hot position the valve closes off the filtered ram air inlet and opens a duct to warm air collected from a shroud around the exhaust manifold or muffler, and that duct picks the air up inside the cowling downstream of the filter. Because the heated air is unfiltered, ground use is limited to what is needed for smooth running.

   Source: FAA Aviation Maintenance Technician Handbook — Powerplant (FAA-H-8083-32), induction systems chapter: carburetor heat system constructionReport a problem with this question

6. 6. Under which condition is throttle ice most likely to form in a carburetor induction system?

   • A.At idle with carburetor heat applied, where warm moist air condenses on the cold throttle valve
   • B.At part throttle, where the nearly closed butterfly speeds up the air and cools it below freezing✓ Answer
   • C.At wide open throttle, where the greatest mass of moist air passes across the throttle valve
   • D.At any throttle setting whenever the outside air temperature is below freezing in visible moisture

   Throttle ice forms on the throttle valve and the walls near it because a partly closed butterfly forces the air through a restricted opening. The air accelerates, its static pressure and temperature drop, and moisture freezes on the metal; at wide open throttle the restriction and the temperature drop are small, so this form of ice is a part-throttle problem.

   Source: FAA Aviation Maintenance Technician Handbook — Powerplant (FAA-H-8083-32), induction systems chapter: throttle iceReport a problem with this question

7. 7. Why can ice form inside a carburetor when the outside air temperature is well above freezing?

   • A.Exhaust gas leaking from the heat shroud condenses and freezes against the cold venturi wall
   • B.Ram air compressed in the intake scoop expands past the filter and cools below the freezing point
   • C.Fuel vaporizing at the discharge nozzle absorbs heat and can chill the charge below freezing✓ Answer
   • D.Oil circulating around the induction riser chills the charge before it reaches the intake port

   Fuel evaporation ice is a refrigeration effect: liquid fuel vaporizing in the airstream takes its heat of vaporization from the charge, and the handbook notes a drop on the order of fifty degrees Fahrenheit as fuel partly vaporizes in air entering at one hundred degrees. Moisture in humid air then freezes at the discharge nozzle and venturi even on a warm day.

   Source: FAA Aviation Maintenance Technician Handbook — Powerplant (FAA-H-8083-32), induction systems chapter: fuel evaporation iceReport a problem with this question

8. 8. An engine loses power and runs rough in flight, and no ice is visible on the airframe. Which set of indications points to induction system icing?

   • A.Loss of manifold pressure with a fixed-pitch propeller, and loss of rpm with a constant-speed propeller
   • B.Loss of rpm with a fixed-pitch propeller, and loss of manifold pressure with a constant-speed unit✓ Answer
   • C.A rise in rpm with a fixed-pitch propeller, and a rise in manifold pressure with a constant-speed unit
   • D.Loss of oil pressure with either propeller, followed by a rise in every cylinder head temperature

   Ice restricts the induction passage and reduces the weight of charge reaching the cylinders. With a fixed-pitch propeller the lost power shows up directly as falling rpm, while a constant-speed propeller holds rpm by changing blade angle, so the loss appears instead as a steadily falling manifold pressure.

   Source: FAA Aviation Maintenance Technician Handbook — Powerplant (FAA-H-8083-32), induction systems chapter: indications of induction system icingReport a problem with this question

9. 9. On a fuel-injected reciprocating engine, how does the alternate air system normally respond when the induction air filter becomes blocked with ice?

   • A.The carburetor heat control must be pulled to route the air through the exhaust heater shroud
   • B.A spring-loaded door is pulled open by induction suction, admitting unfiltered engine compartment air✓ Answer
   • C.A thermostatic valve opens at a preset temperature and admits filtered air from a second ram scoop
   • D.A relief valve dumps deck pressure overboard until the restriction across the filter clears itself

   The alternate air door is normally held closed by a spring, and when the normal inlet or filter becomes restricted the suction of the engine pulls it open so the engine can keep breathing warm unfiltered air from inside the cowling. A fuel-injected engine has no carburetor venturi to ice, so it needs alternate air but not carburetor heat.

   Source: FAA Aviation Maintenance Technician Handbook — Powerplant (FAA-H-8083-32), induction systems chapter: alternate air sourceReport a problem with this question

10. 10. What distinguishes an internally driven supercharger from an externally driven one?

    • A.The internally driven unit compresses the mixture and is turned by the flow of engine exhaust gas
    • B.The internally driven unit compresses the fuel-air mixture and is gear driven from the crankshaft✓ Answer
    • C.The internally driven unit compresses air ahead of the carburetor and is turned by an electric motor
    • D.The internally driven unit compresses air only and is driven by exhaust gas through a turbine wheel

    An internally driven supercharger sits downstream of the fuel metering device, so it compresses the air-fuel mixture, and it is driven mechanically through a gear train from the crankshaft. An externally driven unit, the turbocharger, compresses air only before fuel is added and takes its power from exhaust gas energy.

    Source: FAA Aviation Maintenance Technician Handbook — Powerplant (FAA-H-8083-32), induction systems chapter: internally and externally driven superchargersReport a problem with this question

11. 11. A boosting installation restores sea level manifold pressure as the aircraft climbs but never exceeds 30 in Hg. How is this system correctly described?

    • A.A ground-boosted engine, because rated power is produced at sea level pressure values
    • B.A normalizer, because it only restores the sea level pressure lost as altitude increases✓ Answer
    • C.A density controller, because charge density rather than pressure is what is held constant
    • D.A true supercharger, because any boost above the surrounding ambient pressure meets the definition

    The handbook draws the line at 30 in Hg: a system that merely brings manifold pressure back up to about sea level value as altitude increases is a normalizer, while a true, ground-boosted supercharger raises manifold pressure above 30 in Hg, commonly to about 40 in Hg. The distinction is about the pressure produced, not about how the compressor is driven.

    Source: FAA Aviation Maintenance Technician Handbook — Powerplant (FAA-H-8083-32), induction systems chapter: normalizing versus true superchargingReport a problem with this question

12. 12. What is the result when the waste gate of a turbocharged reciprocating engine moves to the fully closed position?

    • A.All of the exhaust gas is routed through the turbine wheel and the turbocharger makes maximum boost✓ Answer
    • B.All exhaust gas is dumped overboard through the tailpipe and the turbocharger produces no boost
    • C.Exhaust gas is split between the turbine and the tailpipe, holding deck pressure at a set value
    • D.Exhaust gas passes through the intercooler first, so boost develops above critical altitude

    The waste gate is a butterfly valve in a duct that lets exhaust gas bypass the turbine. Closing it blocks the bypass, so all of the exhaust must go through the turbine wheel and the compressor delivers its maximum output; opening it fully sends the gas overboard and the turbocharger does no work, which is why the engine is started with the waste gate open.

    Source: FAA Aviation Maintenance Technician Handbook — Powerplant (FAA-H-8083-32), induction systems chapter: waste gate operationReport a problem with this question

13. 13. On a test flight a turbocharged engine cannot hold rated manifold pressure as it climbs, and the waste gate is found not to travel fully closed. What does this condition mean?

    • A.The engine will overboost at low altitude, because the turbine receives too much exhaust gas flow
    • B.The engine loses power only during ground running, because the waste gate is closed at sea level
    • C.The engine cannot reach its critical altitude, because exhaust gas keeps bypassing the turbine✓ Answer
    • D.The engine will surge at every altitude, because deck pressure and manifold pressure become equal

    Critical altitude is the highest altitude at which a specified manifold pressure can still be maintained at a given rpm, and it is reached when the waste gate has closed completely. If the waste gate cannot close, part of the exhaust always bypasses the turbine, the compressor never reaches full output, and rated power at altitude is unobtainable.

    Source: FAA Aviation Maintenance Technician Handbook — Powerplant (FAA-H-8083-32), induction systems chapter: critical altitude and waste gate travelReport a problem with this question

14. 14. Which statement correctly describes when a density controller and a differential pressure controller act on the waste gate?

    • A.The density controller acts at part throttle, and the differential pressure controller at full throttle
    • B.Both act only at part throttle, one sensing deck pressure and the other sensing ambient air pressure
    • C.The density controller acts at full throttle, the differential pressure controller at part throttle✓ Answer
    • D.Both act only at the full throttle position, one sensing charge temperature and the other back pressure

    The density controller limits manifold pressure below critical altitude and bleeds oil from the waste gate actuator only at the full throttle position, using a sealed bellows so that charge density stays constant as temperature changes. The differential pressure controller takes over at every waste gate position other than full open, that is at part throttle, and its main purpose is to reduce bootstrapping.

    Source: FAA Aviation Maintenance Technician Handbook — Powerplant (FAA-H-8083-32), induction systems chapter: density and differential pressure controllersReport a problem with this question

15. 15. A pilot reports that manifold pressure drifts up and down slightly at a fixed throttle setting while the waste gate is closed. How should the mechanic characterize this?

    • A.Detonation, an abnormal combustion event that calls for cooling of the induction charge
    • B.Overboost, an exceedance of the engine limit that calls for an inspection before further flight
    • C.Bootstrapping, a self-correcting drift in manifold pressure that does not damage the engine✓ Answer
    • D.Compressor surge, a stall of the turbine wheel that calls for its replacement before flight

    Bootstrapping is a self-perpetuating cycle in which a small change in rpm or exhaust temperature changes turbine speed, which changes manifold pressure, which changes them again while the waste gate is closed. It is a transient nuisance rather than a defect and does not harm the engine, unlike overboost, where manifold pressure actually exceeds the engine limit and a pressure relief valve is provided to protect against it.

    Source: FAA Aviation Maintenance Technician Handbook — Powerplant (FAA-H-8083-32), induction systems chapter: bootstrapping and overboostReport a problem with this question

16. 16. Where is an intercooler installed in a turbocharged induction system, and what does it accomplish?

    • A.Between the compressor outlet and the throttle, cooling the charge and raising its density✓ Answer
    • B.Between the throttle and the intake ports, warming the charge so that fuel vaporizes evenly
    • C.Between the exhaust manifold and the turbine inlet, cooling the gas ahead of the waste gate
    • D.Between the air filter and the compressor inlet, preheating the air so the inlet cannot ice

    Compressing air heats it, and a turbocharger can raise induction air temperature by roughly a factor of five, so a full power takeoff on a hot day can deliver compressor discharge air far above the typical throttle inlet limit of about 230 to 300 degrees Fahrenheit. The intercooler is placed in the charge path after the compressor and before the throttle, where it lowers that temperature, moves the engine away from detonation and increases charge density.

    Source: FAA Aviation Maintenance Technician Handbook — Powerplant (FAA-H-8083-32), induction systems chapter: intercooling and induction air temperature limitsReport a problem with this question

17. 17. What is the effect of opening the cowl flaps, and how are they normally set during extended ground running?

    • A.Opening enlarges the cowl exit and increases the airflow past the fins; open them fully on the ground✓ Answer
    • B.Opening enlarges the cowl exit but reduces cooling airflow; leave them closed until airborne
    • C.Opening seals the cowl exit and forces air through the oil cooler; set them to trail in flight
    • D.Opening restricts the cowl exit and raises pressure inside the cowl; close them on the ground

    Pressure cooling depends on a pressure difference between the high pressure area above the cylinders and the low pressure cowl exit, and the cowl flaps control the size of that exit. Opening them increases the mass of air drawn down past the fins and lowers cylinder temperature at the cost of drag, so on the ground, where drag does not matter and airspeed cooling is poor, they are opened wide.

    Source: FAA Aviation Maintenance Technician Handbook — Powerplant (FAA-H-8083-32), engine cooling chapter: pressure cowling and cowl flapsReport a problem with this question

18. 18. One cylinder runs persistently hot, and the flexible baffle seal along that side of the engine is found hardened and curled away from the cowl panel. Why does this raise the head temperature?

    • A.Cooling air is forced through the fins so quickly that it cannot absorb heat from the head
    • B.Cooling air is deflected into the exhaust shroud and is heated before it reaches the cylinder
    • C.Cooling air escapes over the baffle instead of being forced down between the cylinder fins✓ Answer
    • D.Cooling air pressure below the engine rises above the pressure on top and reverses the flow

    The seal closes the gap between the baffle and the cowl panel so that the pressure chamber above the engine cannot leak. When it hardens, tears or curls away, air takes the easy path over the top of the baffle instead of being forced down through the fin passages, the pressure difference across that cylinder falls, and its head temperature climbs.

    Source: FAA Aviation Maintenance Technician Handbook — Powerplant (FAA-H-8083-32), engine cooling chapter: baffles, deflectors and cowl panel air sealsReport a problem with this question

19. 19. Several small cracks are found in the cooling fins of one cylinder head during a scheduled inspection. What is the correct action?

    • A.Remove the cylinder from the engine at once, because any crack in a cooling fin is disqualifying
    • B.Record the cracks and disregard them, because fin damage affects cooling but not airworthiness
    • C.File or stop-drill the cracks within the limits in the manufacturer's data and smooth the edges✓ Answer
    • D.Weld the cracked fins, restore them to their original contour and reinstall the cylinder as is

    Small fin cracks are not by themselves cause for removing a cylinder; they may be filed, blended or stop-drilled and the rough corners smoothed so that a new crack does not start there. What governs is the manufacturer's service data, because only a break large enough to remove a substantial amount of fin area creates a hot spot where almost no heat transfer occurs.

    Source: FAA Aviation Maintenance Technician Handbook — Powerplant (FAA-H-8083-32), engine cooling chapter: cylinder fin inspection and repair, with the manufacturer's overhaul data governing limitsReport a problem with this question

20. 20. One cylinder reads far hotter than the others in cruise, and the cowling, baffles and seals are all found correct. Which cause should the mechanic investigate next?

    • A.Cowl flaps rigged past their normal travel, which adds drag and reduces the cooling airflow
    • B.A rich idle mixture setting, which loads that cylinder with extra fuel during cruise flight
    • C.An induction leak at that cylinder's intake pipe, which leans its charge and raises its temperature✓ Answer
    • D.A partially blocked oil cooler, which raises oil temperature and every head temperature with it

    A single hot cylinder points to something local to that cylinder rather than to the cooling airflow, which has already been checked. A leak in that cylinder's intake pipe admits unmetered air, leans its charge, and a lean charge burns hotter and moves that cylinder toward detonation, so its head temperature runs high while the others stay normal.

    Source: FAA Aviation Maintenance Technician Handbook — Powerplant (FAA-H-8083-32), engine cooling chapter: causes of high cylinder head temperatureReport a problem with this question

21. 21. How does an augmentor system increase the flow of cooling air through the engine compartment?

    • A.Exhaust discharged into the augmentor tubes creates a low pressure that draws air past the cylinders✓ Answer
    • B.Propeller wash is ducted into the augmentor tubes and pressurizes the space above the cylinders
    • C.Cabin heat air is returned into the augmentor tubes and raises the pressure across the engine
    • D.A gear driven blower inside each augmentor tube forces additional ram air down between the fins

    Exhaust gas is fed into the inner augmentor tubes, where it mixes with air that has already passed over the engine and produces a fast, low pressure jet-like flow. That low pressure pulls additional cooling air through the compartment, so cooling does not depend entirely on propeller wash and remains effective at low airspeed and on the ground.

    Source: FAA Aviation Maintenance Technician Handbook — Powerplant (FAA-H-8083-32), engine cooling chapter: augmentor cooling systemsReport a problem with this question

22. 22. In a liquid-cooled light engine whose radiator sits below engine level, what do the expansion tank and its pressure cap do?

    • A.They separate the glycol from the water so that only water circulates through the heads
    • B.They store coolant under vacuum and dose it into the pump inlet as coolant is consumed
    • C.They hold circuit pressure steady by admitting engine compartment air through a return valve
    • D.They accept coolant expansion and vent the excess to an overflow bottle, drawing it back later✓ Answer

    In this arrangement a camshaft driven pump sends coolant from the radiator through the cylinder head jackets to an expansion tank mounted on top of the engine. As coolant heats and expands, the excess pressure valve in the cap lets it flow at atmospheric pressure into a transparent overflow bottle, and a return valve draws that coolant back into the circuit as the engine cools, so the circuit stays full.

    Source: FAA Aviation Maintenance Technician Handbook — Powerplant (FAA-H-8083-32), engine cooling chapter: liquid-cooled engine coolant circuit and expansion tankReport 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 →