Tap an answer to reveal the correct choice and its worked rationale. Every question carries its ACS code and FAA handbook reference, so a miss tells you exactly which subject area to go and read.
In a four-stroke-cycle reciprocating engine, how many times does the crankshaft rotate to complete one full cycle of operation in a single cylinder?
ACS code: AM.III.A
Correct answer: Two complete revolutions (720 degrees).
Rationale: The four-stroke Otto cycle consists of the intake, compression, power, and exhaust strokes. Each stroke moves the piston the length of the cylinder, which corresponds to one-half crankshaft revolution. Four strokes therefore require two full crankshaft revolutions, or 720 degrees, to complete one cycle in a given cylinder.
In a gas turbine engine, which section is responsible for raising the pressure of the incoming air before it reaches the combustion section?
ACS code: AM.III.B
Correct answer: The compressor section
Rationale: In a gas turbine engine the compressor section draws in air and increases its pressure, delivering high-pressure air to the combustion section where fuel is added and burned. The turbine section extracts energy from the expanding gases to drive the compressor, and the exhaust section directs spent gases overboard. Only the compressor raises the pressure of the incoming air, which is why it is sometimes described as the heart of the engine's airflow path.
While performing a differential compression test, air is heard escaping from the engine exhaust outlet. This indicates a leaking
ACS code: AM.III.C
Correct answer: exhaust valve
Rationale: Air heard at the exhaust outlet means it is leaking past the exhaust valve. Air at the carburetor/intake indicates an intake valve leak, and air at the crankcase breather indicates worn or broken piston rings. Listening for the escape point localizes the leak during inspection.
A thermocouple-type cylinder head temperature (CHT) indicating system generates a voltage based on what principle?
ACS code: AM.III.D
Correct answer: The temperature difference between two junctions of dissimilar metals
Rationale: A thermocouple consists of two wires of dissimilar metals (commonly iron and constantan, or chromel and alumel) joined at a hot junction. When the hot junction is heated relative to the cold (reference) junction, a small voltage proportional to the temperature difference is produced. The handbook explains that CHT thermocouple systems rely on this thermoelectric (Seebeck) effect, not on resistance change (which describes a resistance bulb) or liquid expansion (which describes a vapor/liquid bourdon system).
A complete engine fire protection system on a turbine-powered aircraft consists of which two basic subsystems?
ACS code: AM.III.E
Correct answer: A fire detection system and a fire extinguishing system
Rationale: A fire protection system is divided into two functional parts: a detection system that warns the crew of a fire or overheat condition, and an extinguishing system that delivers an agent to put the fire out. The other choices name individual components but not the two-part division of the overall protection system.
In a high-tension magneto ignition system, the primary winding circuit current is interrupted by the action of the:
ACS code: AM.III.F
Correct answer: breaker points opening
Rationale: In a high-tension magneto, primary current flows while the breaker (contact) points are closed, building a magnetic field. When the points open, the primary circuit is interrupted, the field collapses rapidly, and the changing flux induces the high voltage in the secondary winding that fires the plug. The distributor only routes the high voltage to the correct cylinder; the secondary does not interrupt primary current.
What is the primary purpose of the oil system in a reciprocating aircraft engine?
ACS code: AM.III.G
Correct answer: To reduce friction between moving parts by lubricating bearing surfaces
Rationale: The handbook lists several functions of engine oil, but the principal one is lubrication—forming an oil film between moving parts to reduce friction and wear. Oil also cools, cleans, seals, and cushions, but lubrication is the primary purpose. Oil does not raise combustion temperature and is not the source of landing-gear hydraulic power.
In a reciprocating engine magneto ignition system, what is the primary function of the breaker points?
ACS code: AM.III.H
Correct answer: To interrupt current flow in the primary coil and induce high voltage
Rationale: The breaker points are in series with the primary winding. When they open, they interrupt the primary current; the collapsing magnetic field induces a high voltage in the secondary winding that fires the spark plug. Distribution is the distributor's job, and a magneto produces AC by rotation rather than rectifying it.
What is the primary purpose of the idle cutoff (ICO) position on the mixture control of a float-type carburetor?
ACS code: AM.III.I
Correct answer: To shut off fuel flow to the engine for normal shutdown
Rationale: Stopping a reciprocating engine by moving the mixture to idle cutoff cuts off fuel flow entirely, clearing the cylinders and induction system of fuel and reducing the chance of an inadvertent start (kickback). The handbook recommends idle cutoff over closing the throttle because the cylinders are left free of raw fuel, preventing fouling and fire hazards. ICO does not enrich the mixture (that is the full-rich position) and has no effect on venturi airflow.
What is the primary purpose of the carburetor heat system on a reciprocating engine induction system?
ACS code: AM.III.J
Correct answer: To prevent or eliminate ice forming in the carburetor venturi and throttle area
Rationale: Carburetor heat routes air that has been warmed by passing over an exhaust-heated muff into the carburetor. The fuel pressure drop and vaporization at the venturi cool the air and can drop it below freezing even on warm days, forming ice that restricts airflow. Heated air melts existing ice and prevents its formation. Because warm air is less dense, applying carb heat actually reduces power output, so it is not used to increase power.
In a typical gas turbine engine, approximately what percentage of the total airflow through the engine is used as primary air for combustion?
ACS code: AM.III.K
Correct answer: 25 percent
Rationale: Only about 25 percent of the compressor discharge air is used as primary air to support combustion at the desired fuel-air ratio. The remaining 75 percent is secondary air, used to cool the liner and dilute the hot gases before they reach the turbine. This is why the overall engine air-fuel ratio is far leaner than the ideal 15:1 burning ratio.
Why is carbon monoxide contamination of cabin air a concern with exhaust-shroud heating systems?
ACS code: AM.III.L
Correct answer: A crack in the exhaust can allow gases to enter the heated cabin air
Rationale: Because the cabin air is heated by passing through a shroud in direct contact with the hot exhaust, a crack or leak in the muffler or stack lets exhaust gas, which contains carbon monoxide, mix into the air being ducted to the cabin. This is why exhaust integrity is critical on shroud-heated aircraft. Carbon monoxide is a product of combustion in the engine, not created by the heat valve position or by the shroud altering oxygen.
On a propeller, the blade angle is best defined as the angle between the
ACS code: AM.III.M
Correct answer: blade chord line and the plane of rotation
Rationale: Blade angle is the angle formed between the chord line of a propeller blade section and the plane of rotation of the propeller. It is a geometric, built-in or set value. This is distinct from the angle of attack, which is measured between the chord line and the relative wind and changes with airspeed and rpm. Defining blade angle relative to the plane of rotation is fundamental to understanding pitch and pitch distribution.
What is the correct order of the four strokes in the Otto-cycle reciprocating engine?
ACS code: AM.III.A
Correct answer: Intake, compression, power, exhaust.
Rationale: The Otto cycle proceeds in a fixed sequence: the intake stroke draws in the fuel/air charge, the compression stroke compresses it, the power stroke delivers usable work after ignition, and the exhaust stroke expels the burned gases. This intake-compression-power-exhaust order is fundamental to four-stroke engine operation.
Which type of compressor uses a series of rotating and stationary blade rows to compress air in stages along the engine axis?
ACS code: AM.III.B
Correct answer: Axial-flow compressor
Rationale: An axial-flow compressor moves air parallel to the engine's axis through alternating rows of rotating blades (rotors) and stationary blades (stators), with each rotor-stator pair forming a stage that adds a small pressure rise. A centrifugal compressor instead slings air radially outward with an impeller. Reverse-flow describes a combustor arrangement, not a compressor type.
Local heat discoloration and small cracks found on turbine engine combustion liners and nozzle vanes are generally the result of
ACS code: AM.III.C
Correct answer: high operating temperatures and thermal stress
Rationale: Combustion liners and turbine nozzle vanes operate in the hottest gas path. Repeated heating and cooling produces thermal stress that leads to localized discoloration, warping, and cracking. FOD typically dents or nicks compressor blades, and fuel-borne moisture is not the usual cause of hot-section cracking.
A typical engine oil pressure gauge of the bourdon-tube type indicates pressure by which mechanical action?
ACS code: AM.III.D
Correct answer: A curved tube tends to straighten as internal pressure increases
Rationale: A bourdon-tube gauge uses a sealed, curved metal tube that tends to straighten when pressurized internally; this motion is transmitted through a gear-and-sector mechanism to the pointer. The handbook describes the bourdon tube as the classic mechanical pressure-sensing element. A diaphragm/resistor describes an electrical transmitter, and a bimetallic strip is a temperature-sensing device, not a pressure element.
Which type of fire detector senses fire by responding to the rate at which a monitored area's temperature increases?
ACS code: AM.III.E
Correct answer: Rate-of-rise (thermal switch) detector
Rationale: A rate-of-rise detector uses thermocouples or bimetallic elements arranged so that a rapid temperature increase, faster than normal engine warm-up, triggers the warning. Optical detectors sense radiant energy, and CO detectors sense a gas, not a rate of temperature change.
What is the function of the capacitor (condenser) connected across the breaker points in a magneto?
ACS code: AM.III.F
Correct answer: Reduce arcing at the points and speed collapse of the primary field
Rationale: The capacitor is connected in parallel with the breaker points. As the points open, the capacitor momentarily absorbs the primary current, which minimizes arcing and pitting of the points and causes the primary magnetic field to collapse more rapidly. A faster field collapse produces a higher induced secondary voltage. It does not increase primary current or directly step up secondary voltage by transformer action.
Besides lubrication, which additional function does engine oil perform in a reciprocating engine?
ACS code: AM.III.G
Correct answer: It carries heat away from internal engine parts
Rationale: In addition to reducing friction, oil cools the engine by absorbing heat from pistons, bearings, and other internal parts and transferring it to the oil cooler. Oil also seals, cleans, and cushions, but it has no role in fuel delivery to the carburetor or in producing ignition spark.
What is the purpose of the capacitor (condenser) connected across the breaker points in a magneto?
ACS code: AM.III.H
Correct answer: To absorb the primary current surge and reduce arcing at the points
Rationale: The capacitor is wired in parallel with the breaker points. As the points open it momentarily accepts the primary current, speeding the collapse of the magnetic field and minimizing arcing that would otherwise burn the contact surfaces. Voltage step-up is done by the secondary winding, not the capacitor.
In a float-type carburetor, the venturi is used primarily to
ACS code: AM.III.I
Correct answer: create a pressure drop that draws fuel from the discharge nozzle
Rationale: As air accelerates through the restricted throat of the venturi, its velocity increases and its static pressure decreases (Bernoulli's principle). This low pressure at the venturi throat, compared to the higher pressure acting on the fuel in the bowl, draws fuel out of the discharge nozzle. The venturi does not itself vaporize fuel or control float level; the float and needle valve maintain bowl level.
When carburetor heat is applied on an engine running with no carburetor ice present, what is the normal effect on engine RPM?
ACS code: AM.III.J
Correct answer: RPM decreases because the warmer, less dense air reduces power
Rationale: Heated induction air is less dense than ambient air, so the engine takes in less mass of air per cycle, enriching the mixture and reducing power. With no ice present, the result is a drop in RPM. If ice had been present, RPM would first drop, then rise as the ice melted and airflow was restored, which is how a mechanic confirms icing.
Secondary air passing through and around the combustion liner cools the combustion gases to approximately what temperature before they enter the turbine?
ACS code: AM.III.K
Correct answer: 1,500 °F
Rationale: Combustion can produce flame temperatures near 3,500 °F, which would destroy the turbine. Secondary (cooling) air dilutes and films the gases, reducing turbine inlet temperature to roughly 1,500 °F so the turbine nozzle and blades survive. The 3,500 °F figure is the undiluted combustion temperature, not the turbine inlet temperature.
In a turbine engine, the convergent exhaust nozzle is used primarily to:
ACS code: AM.III.L
Correct answer: Increase the velocity of the exhaust gases as they leave the engine
Rationale: A convergent exhaust nozzle has a decreasing cross-sectional area, which accelerates the subsonic exhaust gas and increases its velocity as it exits, converting pressure energy into kinetic energy to produce thrust. A divergent shape would be used to recover pressure or to handle supersonic flow; the convergent nozzle is not a cooling device.
Propeller blades are usually given a twist so that the blade angle is
ACS code: AM.III.M
Correct answer: greater near the hub than at the tip
Rationale: Because a propeller is a rotating airfoil, sections near the tip travel through a much greater distance per revolution and therefore move faster than sections near the hub. To produce a more uniform angle of attack and thrust along the blade, the blade is twisted so the blade angle is highest near the hub (slower-moving root) and decreases toward the tip (faster-moving). This blade twist, called pitch distribution, compensates for the difference in section speeds.
The compression ratio of a reciprocating engine is defined as the ratio of the cylinder volume with the piston at:
ACS code: AM.III.A
Correct answer: Bottom center compared to the volume at top center.
Rationale: Compression ratio is the total cylinder volume (piston at bottom center) divided by the clearance volume (piston at top center). It expresses how much the fuel/air charge is compressed during the compression stroke. A higher compression ratio generally increases power and efficiency but raises the tendency toward detonation.
Which combustion chamber design consists of individual cans, each with its own liner and outer case, arranged around the engine?
ACS code: AM.III.B
Correct answer: Can (multiple-can) type
Rationale: The can-type combustor uses several separate cylindrical chambers, each with an individual perforated liner inside its own outer housing, spaced around the engine. The annular type uses a single continuous combustion chamber surrounding the engine axis. The reverse-flow turbine choice is not a combustor classification, so the multiple individual chambers describe the can type.
A defect described as a sharp, V-shaped depression in the edge of a turbine compressor blade is properly called a
ACS code: AM.III.C
Correct answer: nick
Rationale: Standardized inspection terminology defines a nick as a sharp, V-shaped depression in an edge or surface, typically caused by foreign objects. A score is a deeper scratch from a harder material, and a pit is a small, rounded cavity often from corrosion. Using correct terms ensures accurate defect reporting.
On a turbine engine, exhaust gas temperature (EGT) is most commonly sensed using which type of device?
ACS code: AM.III.D
Correct answer: Several thermocouples wired in parallel
Rationale: Turbine EGT systems use multiple thermocouples positioned around the gas path and connected in parallel so the indication represents an average exhaust gas temperature. The handbook explains that averaging several probes reduces error from local hot spots. A bourdon tube senses pressure, and a bimetallic spring is used in simple direct-reading thermometers, not turbine EGT.
What is the principal advantage of a continuous-loop fire detection system over a series of individual spot detectors?
ACS code: AM.III.E
Correct answer: It provides more complete coverage of the fire hazard area
Rationale: A continuous loop runs as an unbroken sensing element around the entire hazard zone, so a fire anywhere along its length is detected, giving better area coverage than discrete spot units. Loop systems still require power and, like any system, can give false warnings, so those statements are incorrect.
When an aircraft magneto switch is placed in the OFF position, the primary circuit is:
ACS code: AM.III.F
Correct answer: grounded so the magneto cannot produce a spark
Rationale: The ignition switch is wired through the P-lead to ground the magneto primary circuit. With the switch OFF, the primary is grounded, so the field cannot collapse to induce high voltage and the magneto cannot fire. This is why a broken or disconnected P-lead leaves a magneto 'hot,' and a propeller must always be treated as if the ignition is on. The switch does not open the primary or connect it to the battery.
In a dry-sump oil system, where is the main supply of engine oil stored?
ACS code: AM.III.G
Correct answer: In a separate external oil tank
Rationale: A dry-sump system stores the bulk of its oil in a separate tank mounted away from the engine, and a scavenge pump returns oil from the engine sump to that tank. A wet-sump system, by contrast, keeps the oil supply in the crankcase sump. The oil cooler is a heat exchanger, not a storage reservoir.
Why is the rotating magnet in a high-tension magneto positioned for maximum voltage when the points open?
ACS code: AM.III.H
Correct answer: Because the rate of magnetic flux change in the coil is then greatest
Rationale: Maximum voltage is induced when the rate of change of magnetic flux is greatest, which occurs just past the magnet's neutral position. The points are timed to open at this point (the E-gap), so the primary collapse produces the strongest secondary voltage. Distributor and capacitor positions do not set the induced voltage peak.
What is the function of the economizer (power enrichment) system in a carburetor?
ACS code: AM.III.I
Correct answer: To supply additional fuel at high power settings
Rationale: The economizer (also called the power enrichment system) provides a richer mixture only at high power output settings, where the additional fuel is needed for cooling and to develop maximum power. At cruise and lower settings the system is closed so the engine runs on the leaner, more economical main metering mixture. It is not a leaning device and does not act at idle.
What is the function of the induction air filter on a reciprocating engine?
ACS code: AM.III.J
Correct answer: To remove dust and foreign particles from the air entering the engine
Rationale: The induction air filter cleans incoming air to prevent abrasive dust and debris from entering the cylinders, where it would accelerate wear on rings, cylinder walls, and valves. Metering of air is done by the throttle, and preheating is the job of the carburetor heat or alternate air system, not the filter.
Which engine area is generally considered the hottest and therefore receives the largest share of cooling air?
ACS code: AM.III.K
Correct answer: Turbine section
Rationale: The turbine section is exposed to the highest gas temperatures in the engine because it sits directly behind the combustion section. It therefore receives large amounts of relatively cool secondary air directed at the nozzle, disks, and blades. The compressor and inlet operate at far lower temperatures and need much less cooling.
What is the primary purpose of the exhaust cone (tail cone) located at the rear of a turbine engine exhaust section?
ACS code: AM.III.L
Correct answer: To smooth the gas flow and reduce turbulence at the turbine exit
Rationale: The exhaust cone, supported by struts in the exhaust collector, fills the center of the gas path behind the turbine to convert turbulent flow into a smoother, more uniform stream and to prevent the gases from flowing across the rear face of the turbine disk. Adding swirl is undesirable in the exhaust, and the cone does not ignite fuel.
Geometric pitch of a propeller is defined as the
ACS code: AM.III.M
Correct answer: distance the propeller would advance in one revolution with no slippage
Rationale: Geometric pitch is the theoretical distance a propeller would move forward in one revolution if it were advancing through a solid medium with no slippage, based on the blade angle. Effective pitch is the actual distance it advances through the air, which is less because air is not solid. The difference between geometric and effective pitch is propeller slip. Distinguishing these three terms is core propeller theory.
Detonation in a reciprocating engine is best described as:
ACS code: AM.III.A
Correct answer: An uncontrolled, explosive burning of the fuel/air charge after normal ignition.
Rationale: Detonation is the spontaneous, almost instantaneous combustion of the remaining unburned charge after the normal flame front has been started by the spark plug. The resulting sudden rise in pressure and temperature can damage pistons, rings, and cylinders. It is distinct from preignition, which occurs before the spark fires.