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.
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.
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.
What is the purpose of the turbine nozzle (turbine inlet guide vanes) located ahead of the first-stage turbine wheel?
ACS code: AM.III.B
Correct answer: To accelerate and direct gases onto the turbine blades at the correct angle
Rationale: 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.
On a turbofan engine, the fan moves a large mass of air that bypasses the engine core. What is this airflow called?
ACS code: AM.III.B
Correct answer: Bypass air
Rationale: 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.
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?
ACS code: AM.III.B
Correct answer: Hung start
Rationale: 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.
In a free-turbine (free-power-turbine) turboshaft or turboprop engine, the power turbine
ACS code: AM.III.B
Correct answer: has no mechanical connection to the compressor and is driven only by the gas stream.
Rationale: 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.
A turbojet engine derives its thrust primarily by
ACS code: AM.III.B
Correct answer: the reaction to the rearward acceleration of the exhaust gases.
Rationale: 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.
A gas turbine engine is divided into a cold section and a hot section. The hot section includes the
ACS code: AM.III.B
Correct answer: combustor, turbine, and exhaust sections.
Rationale: 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.
Compared with a propeller, a turbojet engine produces thrust by giving a
ACS code: AM.III.B
Correct answer: large acceleration to a small mass of air.
Rationale: 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.
On an axial-flow, dual-spool, forward-fan engine, the fan rotates at the same speed as the
ACS code: AM.III.B
Correct answer: low-pressure turbine.
Rationale: 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.
According to Bernoulli's theorem, what remains constant at any point in the flow of a gas?
ACS code: AM.III.B
Correct answer: The total energy of the flow remains constant.
Rationale: 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.
As subsonic air flows through a convergent (reducing-area) duct, the velocity of the air does what?
ACS code: AM.III.B
Correct answer: Increases.
Rationale: 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.
In a twin-spool gas turbine engine, the first-stage (high-pressure) turbine drives which compressor spool?
ACS code: AM.III.B
Correct answer: The N2 (high-pressure) spool.
Rationale: 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.
At what point in an axial-flow turbojet engine do the highest gas pressures occur?
ACS code: AM.III.B
Correct answer: At the compressor outlet.
Rationale: 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.
In a gas turbine engine, the diffuser section is located between which two sections?
ACS code: AM.III.B
Correct answer: The compressor section and the burner section.
Rationale: 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.
In a twin-spool gas turbine engine, what is the result if the low-pressure (LP) shaft shears?
ACS code: AM.III.B
Correct answer: Turbine runaway (overspeed) occurs.
Rationale: 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.
What happens to the static pressure of supersonic air as it flows through a divergent (increasing-area) duct?
ACS code: AM.III.B
Correct answer: It decreases.
Rationale: 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.
In a dual-spool (twin-spool) turbine engine, the rotational speed of the low-pressure compressor and turbine is designated by which symbol?
ACS code: AM.III.B
Correct answer: N1
Rationale: 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.
A turbojet engine runs more smoothly than a reciprocating engine primarily because
ACS code: AM.III.B
Correct answer: it has no reciprocating parts
Rationale: 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.
The three main sections that form the gas generator of a gas turbine engine are the
ACS code: AM.III.B
Correct answer: compressor, combustion, and turbine
Rationale: 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.
When a given volume of air is compressed in a turbine engine compressor, its
ACS code: AM.III.B
Correct answer: temperature and heat content increase
Rationale: 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.
As subsonic air flows through a convergent nozzle, its static pressure
ACS code: AM.III.B
Correct answer: decreases
Rationale: 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.
A high-bypass turbofan engine produces an
ACS code: AM.III.B
Correct answer: overall slower exhaust velocity and greater propulsive efficiency
Rationale: 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.
The thermodynamic cycle on which the gas turbine engine operates is the constant
ACS code: AM.III.B
Correct answer: pressure cycle
Rationale: 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.
When a turbine engine nozzle becomes choked, the gas velocity increases and the gas
ACS code: AM.III.B
Correct answer: density decreases
Rationale: When a nozzle is choked the flow reaches sonic speed, and as it continues to expand and accelerate the gas density falls. Pressure also drops rather than rising, so a decrease in density is the correct accompaniment to the rising velocity.
Under the standard atmosphere used for rating engine performance, the sea-level temperature is
ACS code: AM.III.B
Correct answer: 59 degrees F
Rationale: 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.
Under the standard atmosphere used for rating engine performance, the sea-level pressure is
ACS code: AM.III.B
Correct answer: 29.92 inches Hg
Rationale: 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.
A turboprop engine develops most of its thrust by the
ACS code: AM.III.B
Correct answer: reaction to the accelerated propeller slipstream
Rationale: 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.
Adiabatic compression is a process in which
ACS code: AM.III.B
Correct answer: there is no loss or gain of heat to the surroundings
Rationale: 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.
A gas turbine engine of modular construction is one in which
ACS code: AM.III.B
Correct answer: major components can be removed and replaced without disturbing the rest of the engine
Rationale: 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.
On a multi-spool turbofan engine, the fan is driven by the
ACS code: AM.III.B
Correct answer: low-pressure (LP) turbine
Rationale: 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.
Compared with operation at sea level, the thermal efficiency of a gas turbine engine at altitude
ACS code: AM.III.B
Correct answer: increases
Rationale: 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.
The density of a gas may be correctly expressed as
ACS code: AM.III.B
Correct answer: weight divided by volume.
Rationale: 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.
In a turbine engine, a divergent (increasing-area) duct causes subsonic airflow to
ACS code: AM.III.B
Correct answer: decrease in velocity and increase in pressure.
Rationale: 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.
In gas turbine engine theory, the Brayton cycle is
ACS code: AM.III.B
Correct answer: the continuous combustion cycle taking place in a gas turbine engine.
Rationale: 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.
The purpose of the diffuser section in a turbine engine is to
ACS code: AM.III.B
Correct answer: increase the static pressure of the air.
Rationale: 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.
On a triple-spool turbine engine, the first stage of turbines drives the
ACS code: AM.III.B
Correct answer: high-pressure (HP) compressor.
Rationale: 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.
Ram effect in a turbine engine inlet is the
ACS code: AM.III.B
Correct answer: conversion of kinetic energy to pressure energy at the face of the compressor.
Rationale: 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.
Compared to a pure turbojet of equivalent thrust, the gas-generator core of a high-bypass turbofan has a
ACS code: AM.III.B
Correct answer: smaller compressor and a smaller combustion chamber.
Rationale: 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.
A waisted (reduced-diameter) accessory drive shaft is used primarily to
ACS code: AM.III.B
Correct answer: provide a shear point if the driven component becomes overloaded.
Rationale: 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.
The basic principle of jet propulsion is best described by Newton's law that states
ACS code: AM.III.B
Correct answer: for every action there is an equal and opposite reaction.
Rationale: 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.
In a gas turbine engine, the point of highest gas potential energy (pressure) is found
ACS code: AM.III.B
Correct answer: just before the combustion chamber.
Rationale: 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.
Which statement is true regarding the relationship between thrust and RPM in a turbine engine?
ACS code: AM.III.B
Correct answer: At higher engine speeds, thrust increases rapidly with small increases in RPM.
Rationale: 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.
At what point in a gas turbine engine are gas pressures the greatest?
ACS code: AM.III.B
Correct answer: Compressor outlet.
Rationale: 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.
Increasing ram effect with increased aircraft speed
ACS code: AM.III.B
Correct answer: increases thrust due to the increased mass airflow.
Rationale: 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.
Which component of a gas turbine engine experiences the highest heat-to-metal contact?
ACS code: AM.III.B
Correct answer: Turbine inlet (nozzle) guide vanes.
Rationale: 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.
Which compressor arrangement provides the greatest advantage for both starting flexibility and improved high-altitude performance?
ACS code: AM.III.B
Correct answer: Split-spool, axial-flow compressor.
Rationale: 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.
Which of the following is the ultimate limiting factor of turbine engine operation?
ACS code: AM.III.B
Correct answer: Turbine inlet temperature.
Rationale: 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.
Compared with sea level, the scheduled idle RPM of a turbine engine at high altitude is
ACS code: AM.III.B
Correct answer: higher than at sea level.
Rationale: 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.