What are the main sections of a gas turbine engine?

Air flows through the inlet, the compressor, the combustion section, the turbine, and the exhaust. The compressor raises pressure, the combustor adds heat energy by burning fuel, and the turbine extracts enough energy to drive the compressor and the accessory gearbox. An accessory section carries the pumps and generators. Naming these in gas-path order answers a surprising number of test questions directly.

What is a hung start on a turbine engine?

A hung start is when the engine lights off normally but stops accelerating and stabilizes at an rpm below idle. Exhaust gas temperature may creep upward, because airflow through the engine is too low to cool it, so the start is aborted. Typical causes are a weak starter, low starting air pressure, or dropping the starter out of the sequence too early.

What is bypass ratio?

Bypass ratio is the mass of air that flows around the engine core through the fan duct divided by the mass of air that passes through the core itself. A high-bypass turbofan may move several times as much air around the core as through it, which produces high thrust with lower fuel consumption and much less noise at subsonic speeds.

What is the difference between a centrifugal and an axial-flow compressor?

A centrifugal compressor throws air outward from an impeller into a diffuser, giving a high pressure rise per stage in a short, rugged, stall-resistant package, but with a large frontal area. An axial-flow compressor moves air parallel to the shaft through many rotor and stator stages, giving a small frontal area and a much higher overall pressure ratio, at the cost of complexity and weight.

What does a free turbine do?

In a free-turbine turboprop or turboshaft engine, the power turbine rides on a separate shaft from the gas generator turbine and is driven only by the gas stream, with no mechanical connection to the compressor. That lets the propeller or rotor turn at a speed independent of the gas generator, and it allows the rotor to be held stationary while the gas generator runs.

What is the purpose of the turbine nozzle (turbine inlet guide vanes) located ahead of the first-stage turbine wheel?

To accelerate and direct gases onto the turbine blades at the correct angle

To extract shaft power from the exhaust gases

To compress the air before it enters the combustor

In a free-turbine (free-power-turbine) turboshaft or turboprop engine, the power turbine

has no mechanical connection to the compressor and is driven only by the gas stream.

is connected to the compressor through a clutch on the power output shaft.

is directly geared to the compressor through a freewheel unit.

A turbojet engine derives its thrust primarily by

the reaction to the rearward acceleration of the exhaust gases.

the impingement of the exhaust gases on the outside air.

the suction of air into the compressor inlet.

A gas turbine engine is divided into a cold section and a hot section. The hot section includes the

combustor, turbine, and exhaust sections.

combustor, diffuser, and exhaust sections.

inlet, compressor, and turbine sections.

Compared with a propeller, a turbojet engine produces thrust by giving a

large acceleration to a small mass of air.

small acceleration to a large mass of air.

large acceleration to a large mass of air.

According to Bernoulli's theorem, what remains constant at any point in the flow of a gas?

The total energy of the flow remains constant.

The static pressure and dynamic pressure remain equal.

The static pressure always remains below the dynamic pressure.

In a twin-spool gas turbine engine, the first-stage (high-pressure) turbine drives which compressor spool?

Both the N1 and N2 spools together.

In a gas turbine engine, the diffuser section is located between which two sections?

The compressor section and the burner section.

The burner section and the turbine section.

The turbine section and the exhaust section.

In a twin-spool gas turbine engine, what is the result if the low-pressure (LP) shaft shears?

Turbine runaway (overspeed) occurs.

A turbojet engine runs more smoothly than a reciprocating engine primarily because

it operates at a lower temperature

its lubrication system is more effective

The three main sections that form the gas generator of a gas turbine engine are the

compressor, combustion, and turbine

compressor, diffuser, and stator

turbine, combustion, and stator

When a given volume of air is compressed in a turbine engine compressor, its

temperature and heat content increase

temperature and heat content decrease

temperature and heat content remain unchanged

A high-bypass turbofan engine produces an

overall slower exhaust velocity and greater propulsive efficiency

overall faster exhaust velocity and greater fuel consumption

increased propulsive efficiency from a faster exhaust stream

A turboprop engine develops most of its thrust by the

reaction to the accelerated propeller slipstream

impingement of the slipstream on the outside air

reaction of the engine exhaust gases alone

Adiabatic compression is a process in which

there is no loss or gain of heat to the surroundings

heat is continuously removed to cool the gas

the temperature is held constant throughout

A gas turbine engine of modular construction is one in which

major components can be removed and replaced without disturbing the rest of the engine

all components must be replaced together as a single unit

the engine is assembled using a vertical build-up technique

On a multi-spool turbofan engine, the fan is driven by the

intermediate-pressure (IP) turbine

In a turbine engine, a divergent (increasing-area) duct causes subsonic airflow to

decrease in velocity and increase in pressure.

decrease in velocity while pressure remains constant.

decrease in pressure and increase in velocity.

In gas turbine engine theory, the Brayton cycle is

the continuous combustion cycle taking place in a gas turbine engine.

the intermittent cycling of an electrical de-ice system.

the constant-velocity combustion cycle in a reciprocating engine.

The purpose of the diffuser section in a turbine engine is to

increase the static pressure of the air.

increase the kinetic energy of the air.

induce a swirl in the air prior to combustion.

On a triple-spool turbine engine, the first stage of turbines drives the

intermediate-pressure (IP) compressor.

Ram effect in a turbine engine inlet is the

conversion of kinetic energy to pressure energy at the face of the compressor.

conversion of static pressure to kinetic energy at the face of the compressor.

increase of dynamic pressure measured at the face of the compressor.

Compared to a pure turbojet of equivalent thrust, the gas-generator core of a high-bypass turbofan has a

smaller compressor and a smaller combustion chamber.

larger compressor and a larger combustion chamber.

larger compressor and a smaller combustion chamber.

A waisted (reduced-diameter) accessory drive shaft is used primarily to

provide a shear point if the driven component becomes overloaded.

achieve dynamic balance of the rotating assembly.

reduce the overall weight of the accessory drive.

The basic principle of jet propulsion is best described by Newton's law that states

for every action there is an equal and opposite reaction.

propulsion results from the interaction of fluids and gases.

the calorific value of the fuel burned equals the energy of the aircraft.

In a gas turbine engine, the point of highest gas potential energy (pressure) is found

just before the combustion chamber.

immediately after the turbine section.

immediately after the combustion chamber.

Which statement is true regarding the relationship between thrust and RPM in a turbine engine?

At higher engine speeds, thrust increases rapidly with small increases in RPM.

Thrust per pound of air consumed is greater at low altitude than at high altitude.

At lower engine speeds, thrust increases rapidly with small increases in RPM.

Increasing ram effect with increased aircraft speed

increases thrust due to the increased mass airflow.

reduces thrust due to reduced turbine temperature.

reduces thrust due to reduced compressor efficiency.

Turbine Engines — FAA A&P Test Questions (ACS AM.III.B)

Turbine engines is one of the largest subject areas on the Powerplant written test, and the questions are mostly about what each section and component does to the air passing through it. You are asked which section raises pressure before combustion, how an axial-flow compressor is built, which combustor design uses individual cans, what the turbine nozzle does ahead of the first turbine wheel, what bypass air is, and how to name a starting malfunction from its symptoms. Trace the gas path in order and most of these answer themselves.

Written by the AMTprep editorial team · Published July 27, 2026 · Last reviewed October 21, 2026

Written against the FAA primary sources cited at the foot of this page.

What ACS AM.III.B covers

This area follows the turbine engine material in FAA-H-8083-32 and is organized around the gas path. Air enters the inlet, the compressor raises its pressure, the combustion section adds heat at roughly constant pressure, the turbine extracts work to drive the compressor and accessories, and the remaining energy leaves through the exhaust. Compressors come in centrifugal and axial-flow types. An axial compressor alternates rotating rotor blade rows with stationary stator vane rows, and one rotor row plus its stator row makes a stage. The stators diffuse the flow, converting velocity into pressure and straightening the air for the next rotor row.

Combustion sections appear in three arrangements: multiple can, can-annular, and annular. The turbine nozzle, also called the inlet guide vanes, forms convergent passages ahead of the first-stage turbine wheel that accelerate the gas and aim it onto the blades at the correct angle.

Engine types are the next block. A turbojet produces thrust by accelerating a relatively small mass of air to a high velocity. A turbofan adds a fan whose bypass air flows around the core, and in a high-bypass engine that fan produces most of the thrust. Turboprop and turboshaft engines convert most of the gas energy into shaft power, and in a free-turbine design the power turbine has no mechanical connection to the gas generator, so it is driven only by gas flow.

Operation and troubleshooting round out the area: hot starts, hung starts, and no-start conditions; compressor stall and surge; exhaust gas temperature and engine pressure ratio indications; foreign object damage; and blade repair and hot-section inspection limits.

Where this sits on the test

ACS AM.III.B is tested on the FAA Powerplant written test, one of 1,353 ACS-tagged questions in the Powerplant bank. Every question tagged to this area carries a worked rationale and its FAA handbook reference, so you can drill the code itself rather than the whole test.

FAA handbook references

FAA-H-8083-32

Three traps candidates fall into

Hot start and hung start get swapped. A hot start is a normal light-off with exhaust gas temperature climbing past the limit. A hung start also lights off normally, but the engine stalls at a low rpm and never accelerates to idle, which usually points at the starter or an inadequate starting air supply.
Candidates decide the turbine nozzle slows the gas down, because it sits ahead of a turbine that extracts energy. The nozzle vanes form convergent passages, so gas velocity increases and pressure drops through them. Their second job is aiming that flow onto the turbine blades at the correct angle.
The core is assumed to make the thrust on a high-bypass turbofan. Most of the thrust comes from the fan, which moves a large mass of bypass air at a modest velocity. That is also why a high-bypass engine is more efficient and far quieter than a straight turbojet at subsonic speeds.

50 free sample questions from ACS AM.III.B

AM.III.BTap an answer
In a gas turbine engine, which section is responsible for raising the pressure of the incoming air before it reaches the combustion section?
Show the answer and explanation
Answer: The compressor section
Why: 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.· FAA-H-8083-32 — Turbine Engines: compressor section
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Which type of compressor uses a series of rotating and stationary blade rows to compress air in stages along the engine axis?
Show the answer and explanation
Answer: Axial-flow compressor
Why: 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.· FAA-H-8083-32 — Turbine Engines: compressor types
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Which combustion chamber design consists of individual cans, each with its own liner and outer case, arranged around the engine?
Show the answer and explanation
Answer: Can (multiple-can) type
Why: 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.· FAA-H-8083-32 — Turbine Engines: combustion section
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What is the purpose of the turbine nozzle (turbine inlet guide vanes) located ahead of the first-stage turbine wheel?
Show the answer and explanation
Answer: To accelerate and direct gases onto the turbine blades at the correct angle
Why: The turbine nozzle, made up of stationary inlet guide vanes, forms convergent passages that accelerate the hot gases and turn them so they strike the turbine blades at the optimum angle for maximum energy transfer. Compression happens in the compressor, and extracting shaft power is the rotating turbine wheel's job, so the nozzle's role is to prepare and aim the gas flow.· FAA-H-8083-32 — Turbine Engines: turbine nozzle
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On a turbofan engine, the fan moves a large mass of air that bypasses the engine core. What is this airflow called?
Show the answer and explanation
Answer: Bypass air
Why: In a turbofan, the fan accelerates a large volume of air, much of which passes around the engine core through the bypass duct rather than through the combustion section; this is the bypass air and it produces a substantial portion of the engine's thrust. Bleed air is tapped from the compressor for other uses, and primary combustion air passes through the core, so the air going around the core is bypass air.· FAA-H-8083-32 — Turbine Engines: turbofan bypass
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A turbine engine fails to reach idle speed and stabilizes at a low rpm during start with normal lightoff. What starting malfunction does this describe?
Show the answer and explanation
Answer: Hung start
Why: A hung start occurs when the engine lights off normally but fails to accelerate to idle rpm, stabilizing at a lower-than-normal speed, typically due to insufficient starter assistance or inadequate power. A hot start involves exhaust temperature exceeding limits, and a wet (false) start is a failure to light off after fuel is introduced, so a normal lightoff that stalls below idle is a hung start.· FAA-H-8083-32 — Turbine Engines: starting malfunctions
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In a free-turbine (free-power-turbine) turboshaft or turboprop engine, the power turbine
Show the answer and explanation
Answer: has no mechanical connection to the compressor and is driven only by the gas stream.
Why: In a free-turbine engine, the power turbine is driven aerodynamically by the hot gas stream from the gas generator but has no mechanical shaft coupling to the compressor spool. It can therefore rotate independently of the gas generator, with no clutch or freewheel link between the two, per FAA-H-8083-32 turbine engine fundamentals.· FAA-H-8083-32 — Turbine Engines
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A turbojet engine derives its thrust primarily by
Show the answer and explanation
Answer: the reaction to the rearward acceleration of the exhaust gases.
Why: Jet thrust is a reaction force: accelerating the gas stream rearward produces an equal and opposite forward reaction on the engine, in accordance with Newton's third law. Thrust is not created by the gases striking the outside air, nor merely by drawing air into the compressor.· FAA-H-8083-32 — Turbine Engines
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A gas turbine engine is divided into a cold section and a hot section. The hot section includes the
Show the answer and explanation
Answer: combustor, turbine, and exhaust sections.
Why: The hot section is everything downstream of fuel ignition, so it comprises the combustor, turbine, and exhaust, where gas temperatures are highest. The inlet and compressor handle air before combustion and form the cold section.· FAA-H-8083-32 — Turbine Engines
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Compared with a propeller, a turbojet engine produces thrust by giving a
Show the answer and explanation
Answer: large acceleration to a small mass of air.
Why: A pure turbojet handles a comparatively small mass of air but accelerates it to a very high velocity, producing thrust from that large change in momentum. This is the opposite of a propeller, which gives a small acceleration to a large mass of air.· FAA-H-8083-32 — Turbine Engines
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On an axial-flow, dual-spool, forward-fan engine, the fan rotates at the same speed as the
Show the answer and explanation
Answer: low-pressure turbine.
Why: On a dual-spool front-fan engine the fan is mounted on the low-pressure (N1) spool, so it always rotates at the same speed as the low-pressure turbine that drives it. The high-pressure compressor belongs to the separate N2 spool and turns at a different speed.· FAA-H-8083-32 — Turbine Engines
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According to Bernoulli's theorem, what remains constant at any point in the flow of a gas?
Show the answer and explanation
Answer: The total energy of the flow remains constant.
Why: Bernoulli's theorem states that the total energy of a flow remains constant, so static and dynamic pressures trade off against one another. Where velocity rises the static pressure falls and vice versa, but their sum (total pressure) is unchanged; the static and dynamic pressures are not equal.· FAA-H-8083-32 — Turbine Engines
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As subsonic air flows through a convergent (reducing-area) duct, the velocity of the air does what?
Show the answer and explanation
Answer: Increases.
Why: For subsonic flow a convergent duct acts as a nozzle, so the velocity increases as the air speeds up through the narrowing section. By the continuity and Bernoulli relations the accompanying static pressure falls.· FAA-H-8083-32 — Turbine Engines
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In a twin-spool gas turbine engine, the first-stage (high-pressure) turbine drives which compressor spool?
Show the answer and explanation
Answer: The N2 (high-pressure) spool.
Why: In a twin-spool engine the first (high-pressure) turbine is on the same shaft as the high-pressure compressor, which is the N2 spool, so it drives N2. The low-pressure spool (N1) is driven by the later, low-pressure turbine stages.· FAA-H-8083-32 — Turbine Engines
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At what point in an axial-flow turbojet engine do the highest gas pressures occur?
Show the answer and explanation
Answer: At the compressor outlet.
Why: Pressure rises continuously through the compressor and peaks at its outlet, just before the diffuser and burner. In the combustor heat is added at roughly constant pressure with a slight loss, and pressure then falls across the turbine, so the compressor outlet sees the highest gas pressure.· FAA-H-8083-32 — Turbine Engines
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In a gas turbine engine, the diffuser section is located between which two sections?
Show the answer and explanation
Answer: The compressor section and the burner section.
Why: The diffuser sits between the compressor outlet and the combustion (burner) section, slowing the high-velocity compressor discharge and raising its static pressure before it enters the burner. This controlled, low-velocity flow is needed for stable combustion.· FAA-H-8083-32 — Turbine Engines
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In a twin-spool gas turbine engine, what is the result if the low-pressure (LP) shaft shears?
Show the answer and explanation
Answer: Turbine runaway (overspeed) occurs.
Why: If the LP shaft shears, the turbine loses its compressor load while still being driven by the gas stream, so it rapidly accelerates to a destructive overspeed known as turbine runaway. The compressor, no longer driven, slows down rather than overspeeding.· FAA-H-8083-32 — Turbine Engines
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What happens to the static pressure of supersonic air as it flows through a divergent (increasing-area) duct?
Show the answer and explanation
Answer: It decreases.
Why: For supersonic flow a divergent duct acts as a nozzle: the gas continues to accelerate, so its static pressure falls as it expands. This is the reverse of subsonic behavior, where a divergent duct would slow the flow and raise the pressure.· FAA-H-8083-32 — Turbine Engines
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In a dual-spool (twin-spool) turbine engine, the rotational speed of the low-pressure compressor and turbine is designated by which symbol?
Show the answer and explanation
Answer: N1
Why: In a dual-spool engine the low-pressure compressor and the low-pressure turbine are mechanically coupled as one rotor, whose speed is identified as N1. N alone denotes a generic spool speed, while N2 identifies the high-pressure rotor.· FAA-H-8083-32 — Turbine Engines
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A turbojet engine runs more smoothly than a reciprocating engine primarily because
Show the answer and explanation
Answer: it has no reciprocating parts
Why: A gas turbine produces power through continuous rotary motion and has no pistons changing direction, so it is free of the reciprocating inertia loads that cause a piston engine to vibrate. The smoother operation results from this rotating-only design rather than from lubrication or operating temperature.· FAA-H-8083-32 — Turbine Engines
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The three main sections that form the gas generator of a gas turbine engine are the
Show the answer and explanation
Answer: compressor, combustion, and turbine
Why: The core sections of any gas turbine are the compressor, the combustion (burner) section, and the turbine, which together make up the gas generator. A diffuser and stators are components located within these sections rather than main sections in their own right.· FAA-H-8083-32 — Turbine Engines
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When a given volume of air is compressed in a turbine engine compressor, its
Show the answer and explanation
Answer: temperature and heat content increase
Why: Compressing a gas does work on it and raises its internal energy, so its temperature and heat content increase. This is why compressor discharge air is hot; an isothermal process that gained no heat would require continuous heat removal, which does not occur in a turbine compressor.· FAA-H-8083-32 — Turbine Engines
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As subsonic air flows through a convergent nozzle, its static pressure
Show the answer and explanation
Answer: decreases
Why: In subsonic flow a convergent nozzle accelerates the air, and as velocity rises the static pressure falls in accordance with Bernoulli's principle. Energy is converted from pressure form into kinetic form through the contraction.· FAA-H-8083-32 — Turbine Engines
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A high-bypass turbofan engine produces an
Show the answer and explanation
Answer: overall slower exhaust velocity and greater propulsive efficiency
Why: A high-bypass engine moves a large mass of air at a relatively low jet velocity, and the slower overall exhaust velocity raises propulsive efficiency because less energy is wasted as residual jet kinetic energy. Both effects together describe a high-bypass design, so naming only the efficiency gain is incomplete.· FAA-H-8083-32 — Turbine Engines
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The thermodynamic cycle on which the gas turbine engine operates is the constant
Show the answer and explanation
Answer: pressure cycle
Why: The Brayton cycle, on which the gas turbine operates, adds heat to the working fluid at essentially constant pressure during combustion, so it is known as the constant-pressure cycle. This distinguishes it from the constant-volume (Otto) cycle of a reciprocating engine.· FAA-H-8083-32 — Turbine Engines
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As exhaust gas accelerates through a turbine engine's convergent exhaust nozzle toward choked (sonic) conditions, its velocity increases and its
Show the answer and explanation
Answer: density decreases
Why: As subsonic gas accelerates through a convergent nozzle, it expands: static pressure and temperature fall and density decreases as velocity rises, until the flow reaches sonic speed at the exit and the nozzle is choked. Once choked, exit velocity cannot rise further. Any additional upstream pressure instead raises the exit static pressure and density, producing pressure thrust.· FAA-H-8083-32 — Turbine Engines
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Under the standard atmosphere used for rating engine performance, the sea-level temperature is
Show the answer and explanation
Answer: 59 degrees F
Why: Under the standard atmosphere, sea-level temperature is 15 degrees C, which is equivalent to 59 degrees F. The values of 29 degrees C or 59 degrees C do not match this standard reference temperature.· FAA-H-8083-32 — Turbine Engines
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Under the standard atmosphere used for rating engine performance, the sea-level pressure is
Show the answer and explanation
Answer: 29.92 inches Hg
Why: Standard sea-level pressure in the standard atmosphere is 29.92 inches of mercury (1013.25 hPa). The other figures, 29.29 and 29.00 inches Hg, are simply incorrect transpositions of this value.· FAA-H-8083-32 — Turbine Engines
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A turboprop engine develops most of its thrust by the
Show the answer and explanation
Answer: reaction to the accelerated propeller slipstream
Why: A turboprop produces most of its thrust from the propeller, which accelerates a large mass of air rearward; the forward force is the reaction to that accelerated propeller slipstream. The thrust is not generated by the slipstream striking the surrounding air.· FAA-H-8083-32 — Turbine Engines
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Adiabatic compression is a process in which
Show the answer and explanation
Answer: there is no loss or gain of heat to the surroundings
Why: An adiabatic process is one in which no heat is transferred to or from the gas; any temperature rise during compression comes solely from the work done on the gas. This differs from an isothermal process, in which temperature is held constant by exchanging heat with the surroundings.· FAA-H-8083-32 — Turbine Engines
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A gas turbine engine of modular construction is one in which
Show the answer and explanation
Answer: major components can be removed and replaced without disturbing the rest of the engine
Why: Modular construction means the engine is built from self-contained modules so that a faulty major component can be removed and replaced individually without dismantling the whole engine. This reduces maintenance time and cost; it does not refer to any vertical assembly technique.· FAA-H-8083-32 — Turbine Engines
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On a multi-spool turbofan engine, the fan is driven by the
Show the answer and explanation
Answer: low-pressure (LP) turbine
Why: The fan is part of the low-pressure spool, so it is driven by the low-pressure (LP) turbine, which extracts energy from the gas stream downstream of the higher-pressure stages. The high-pressure and intermediate-pressure turbines drive the high-pressure and intermediate compressors, respectively, not the fan.· FAA-H-8083-32 — Turbine Engines
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Compared with operation at sea level, the thermal efficiency of a gas turbine engine at altitude
Show the answer and explanation
Answer: increases
Why: At altitude the air is much colder, which lowers specific fuel consumption and raises the engine's overall thermal efficiency for a given thrust. Although the absolute thrust produced drops with reduced air density, the lower intake temperature actually improves cycle efficiency, so it increases rather than decreases.· FAA-H-8083-32 — Turbine Engines
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The density of a gas may be correctly expressed as
Show the answer and explanation
Answer: weight divided by volume.
Why: Density is defined as mass (or weight) per unit volume, so for a gas it is expressed as weight divided by volume. Volume divided by weight is specific volume, and pressure divided by volume is not a density relationship.· FAA-H-8083-32 — Turbine Engines
AM.III.BTap an answer
In a turbine engine, a divergent (increasing-area) duct causes subsonic airflow to
Show the answer and explanation
Answer: decrease in velocity and increase in pressure.
Why: For subsonic flow, a divergent duct acts as a diffuser: the air slows down so its velocity decreases while static pressure rises. The kinetic energy lost in slowing the flow is recovered as pressure energy.· FAA-H-8083-32 — Turbine Engines
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In gas turbine engine theory, the Brayton cycle is
Show the answer and explanation
Answer: the continuous combustion cycle taking place in a gas turbine engine.
Why: The Brayton cycle describes the continuous, constant-pressure combustion process of a gas turbine, in which air is compressed, fuel is burned continuously, and the hot gas expands through the turbine and exhaust nozzle. It is not related to de-ice cycling and is not a constant-velocity process.· FAA-H-8083-32 — Turbine Engines
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The purpose of the diffuser section in a turbine engine is to
Show the answer and explanation
Answer: increase the static pressure of the air.
Why: The diffuser increases the flow area to slow the high-velocity compressor discharge air, converting its kinetic energy into a rise in static pressure before the air enters the combustor. It reduces, rather than increases, the kinetic energy of the air.· FAA-H-8083-32 — Turbine Engines
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On a triple-spool turbine engine, the first stage of turbines drives the
Show the answer and explanation
Answer: high-pressure (HP) compressor.
Why: On a triple-spool engine the first turbine stage sits directly behind the combustor where gas energy is greatest, and it drives the high-pressure (HP) compressor. The intermediate- and low-pressure compressors are driven by later, lower-pressure turbine stages.· FAA-H-8083-32 — Turbine Engines
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Ram effect in a turbine engine inlet is the
Show the answer and explanation
Answer: conversion of kinetic energy to pressure energy at the face of the compressor.
Why: Ram effect is the recovery of the aircraft's forward speed as a pressure rise. The inlet slows the incoming air, converting its kinetic energy into pressure energy at the compressor face, which raises engine pressure ratio and thrust at speed.· FAA-H-8083-32 — Turbine Engines
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Compared to a pure turbojet of equivalent thrust, the gas-generator core of a high-bypass turbofan has a
Show the answer and explanation
Answer: smaller compressor and a smaller combustion chamber.
Why: In a high-bypass turbofan the large fan produces most of the thrust, so the core only needs to drive that fan. For a given thrust the core compressor and combustion chamber are both smaller than those of a pure turbojet of equivalent thrust, which is why the core sits inside a much larger fan duct.· FAA-H-8083-32 — Turbine Engines
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A waisted (reduced-diameter) accessory drive shaft is used primarily to
Show the answer and explanation
Answer: provide a shear point if the driven component becomes overloaded.
Why: A waisted drive shaft has a deliberately reduced-diameter section that acts as a mechanical fuse, shearing at that point if the driven component seizes or is overloaded. This protects the rest of the drive train and accessory gearbox from damage.· FAA-H-8083-32 — Turbine Engines
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The basic principle of jet propulsion is best described by Newton's law that states
Show the answer and explanation
Answer: for every action there is an equal and opposite reaction.
Why: Jet propulsion is a direct application of Newton's third law: accelerating the exhaust gases rearward (the action) produces an equal and opposite forward reaction that drives the aircraft. It is not based on the fuel's heat value or on interaction between separate fluids.· FAA-H-8083-32 — Turbine Engines
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In a gas turbine engine, the point of highest gas potential energy (pressure) is found
Show the answer and explanation
Answer: just before the combustion chamber.
Why: Pressure is greatest at compressor discharge, just before the combustion chamber, and pressure represents stored potential energy. Once combustion adds heat the gas gains thermal and kinetic energy, but its pressure does not exceed the compressor delivery value.· FAA-H-8083-32 — Turbine Engines
AM.III.BTap an answer
Which statement is true regarding the relationship between thrust and RPM in a turbine engine?
Show the answer and explanation
Answer: At higher engine speeds, thrust increases rapidly with small increases in RPM.
Why: Thrust does not vary linearly with RPM; the thrust-versus-RPM curve is steep at the top end. Near maximum speed a small percentage change in RPM produces a large change in mass airflow and pressure rise, so thrust climbs rapidly. At low RPM the same RPM change yields only a small thrust change.· FAA-H-8083-32 — Turbine Engines
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At what point in a gas turbine engine are gas pressures the greatest?
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Answer: Compressor outlet.
Why: Pressure rises progressively through the compressor and reaches its peak at the compressor outlet, just before the combustion chamber. In the combustor heat is added at roughly constant pressure, and across the turbine and exhaust the pressure falls again, so the highest gas pressure is at compressor delivery.· FAA-H-8083-32 — Turbine Engines
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Increasing ram effect with increased aircraft speed
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Answer: increases thrust due to the increased mass airflow.
Why: As forward speed rises, ram effect raises the pressure and density of air entering the inlet, increasing the mass of air the engine can pass. Because thrust depends on mass airflow, this added airflow increases thrust, which is why ram recovery is valued at higher aircraft speeds.· FAA-H-8083-32 — Turbine Engines
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Which component of a gas turbine engine experiences the highest heat-to-metal contact?
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Answer: Turbine inlet (nozzle) guide vanes.
Why: The turbine inlet (first-stage nozzle) guide vanes sit immediately downstream of the combustion chamber and receive the gas stream at its peak temperature before any work has been extracted. They experience the highest heat-to-metal contact of any component and are typically the first to require cooling and inspection.· FAA-H-8083-32 — Turbine Engines
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Which compressor arrangement provides the greatest advantage for both starting flexibility and improved high-altitude performance?
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Answer: Split-spool, axial-flow compressor.
Why: A split (twin) spool axial-flow compressor lets the low- and high-pressure spools rotate at their own optimum speeds. Starting is easier because only the lighter high-pressure spool must be spun up, and the spools self-adjust to changing inlet conditions, giving better high-altitude performance than a single-spool design.· FAA-H-8083-32 — Turbine Engines
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Which of the following is the ultimate limiting factor of turbine engine operation?
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Answer: Turbine inlet temperature.
Why: Turbine inlet temperature is the ultimate limiting factor because the turbine blades and guide vanes operate closest to their metallurgical limits. Any increase in power requires more fuel and a hotter gas stream, so the maximum allowable turbine inlet temperature caps how much thrust the engine can safely produce.· FAA-H-8083-32 — Turbine Engines
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Compared with sea level, the scheduled idle RPM of a turbine engine at high altitude is
Show the answer and explanation
Answer: higher than at sea level.
Why: At altitude the air is much less dense, so to keep the engine above its self-sustaining speed and avoid flameout the fuel control schedules a higher idle RPM than at sea level. The thinner air requires more rotational speed to pass enough mass airflow to sustain stable combustion.· FAA-H-8083-32 — Turbine Engines

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Related subject areas

The areas that share the most ground with AM.III.B — study them alongside it.

Turbine Engine Air Systems (AM.III.K)
Engine Exhaust and Reverser Systems (AM.III.L)
Engine Lubrication Systems (AM.III.G)
Engine Inspection (AM.III.C)

Frequently asked questions

What are the main sections of a gas turbine engine?: Air flows through the inlet, the compressor, the combustion section, the turbine, and the exhaust. The compressor raises pressure, the combustor adds heat energy by burning fuel, and the turbine extracts enough energy to drive the compressor and the accessory gearbox. An accessory section carries the pumps and generators. Naming these in gas-path order answers a surprising number of test questions directly.
What is a hung start on a turbine engine?: A hung start is when the engine lights off normally but stops accelerating and stabilizes at an rpm below idle. Exhaust gas temperature may creep upward, because airflow through the engine is too low to cool it, so the start is aborted. Typical causes are a weak starter, low starting air pressure, or dropping the starter out of the sequence too early.
What is bypass ratio?: Bypass ratio is the mass of air that flows around the engine core through the fan duct divided by the mass of air that passes through the core itself. A high-bypass turbofan may move several times as much air around the core as through it, which produces high thrust with lower fuel consumption and much less noise at subsonic speeds.
What is the difference between a centrifugal and an axial-flow compressor?: A centrifugal compressor throws air outward from an impeller into a diffuser, giving a high pressure rise per stage in a short, rugged, stall-resistant package, but with a large frontal area. An axial-flow compressor moves air parallel to the shaft through many rotor and stator stages, giving a small frontal area and a much higher overall pressure ratio, at the cost of complexity and weight.
What does a free turbine do?: In a free-turbine turboprop or turboshaft engine, the power turbine rides on a separate shaft from the gas generator turbine and is driven only by the gas stream, with no mechanical connection to the compressor. That lets the propeller or rotor turn at a speed independent of the gas generator, and it allows the rotor to be held stationary while the gas generator runs.

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