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.
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.
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.
The exhaust gas temperature (EGT) thermocouples in a turbine engine are generally located:
ACS code: AM.III.L
Correct answer: In the turbine or exhaust section downstream of the combustion area
Rationale: EGT (or turbine inlet/interstage temperature) thermocouples are positioned in the hot section, in or just aft of the turbine where gas temperatures can be measured to monitor engine operating limits. Placing them in the cold compressor inlet or in the fuel manifold would not provide a meaningful indication of turbine operating temperature.
Turbine engine exhaust noise suppressors reduce noise primarily by:
ACS code: AM.III.L
Correct answer: Increasing the gas perimeter to mix exhaust quickly with ambient air
Rationale: Jet exhaust noise is largely caused by the high-velocity exhaust shearing against still ambient air. Noise suppressors (such as multi-tube and corrugated/lobed designs) break the single large exhaust stream into many smaller streams, increasing the perimeter of contact so the high-velocity gas mixes more rapidly with surrounding air. This reduces the low-frequency noise. Suppressors do not reduce gas mass or rely on raising temperature.
A mechanical-blockage (clamshell) thrust reverser produces reverse thrust by:
ACS code: AM.III.L
Correct answer: Deploying doors into the gas stream to deflect it forward and outward
Rationale: A mechanical-blockage reverser uses movable clamshell-type doors that swing into the exhaust gas stream behind the engine, physically blocking the rearward flow and deflecting the gases forward and outward at an angle to create reverse thrust. Reverse thrust comes from redirecting the exhaust, not from bleeding compressor air or restricting the inlet.
Stainless steel and similar exhaust components should not be marked with an ordinary lead pencil because:
ACS code: AM.III.L
Correct answer: Lead-pencil or zinc/lead-based marks can cause cracking when heated
Rationale: Marking high-temperature exhaust alloys with a common lead pencil or with zinc- or lead-based materials can lead to intergranular embrittlement and cracking when the part reaches operating temperature, because these elements penetrate the hot metal. Approved layout dye or chalk should be used instead. The concern is metallurgical damage, not certificate validity or weight.
Of the following, which engine type is most likely to have an exhaust noise-suppression unit installed?
ACS code: AM.III.L
Correct answer: A turbojet.
Rationale: A turbojet exhausts a high-velocity jet, and the large shear between that jet and the surrounding air is the main noise source, so it most often needs an exhaust noise suppressor. Turboprops and turboshafts expel lower-velocity gas streams and are inherently quieter at the exhaust.
In a turbojet engine, the exhaust (propelling) nozzle is located in which section?
ACS code: AM.III.L
Correct answer: Exhaust
Rationale: The propelling nozzle forms the rear of the exhaust section, where the gases are accelerated to produce thrust. The turbine extracts shaft power upstream of it, and combustion occurs farther forward, so the nozzle is not located in those sections.
The reverse thrust developed by a typical thrust reverser at full reverse power is approximately
ACS code: AM.III.L
Correct answer: 50% of forward thrust.
Rationale: Thrust reversers redirect only part of the exhaust or fan flow and do so at an angle, so the rearward-acting force recovered is roughly half of the forward thrust the engine produces at the same setting. Full reverse power is therefore approximately 50% of forward thrust.
The function of the exhaust cone assembly of a turbine engine is to
ACS code: AM.III.L
Correct answer: straighten and collect the exhaust gases into a solid exhaust jet.
Rationale: The exhaust cone assembly straightens the swirling gas leaving the turbine and gathers it into a smooth, solid jet, while its struts also support the cone. This recovers residual swirl energy; it is not designed to add swirl or to act primarily as a noise suppressor.
What is the maximum practical angle through which the gas flow can be turned during thrust reversal?
ACS code: AM.III.L
Correct answer: About 135 degrees.
Rationale: In practice the gas flow in a thrust reverser cannot be turned a full 180 degrees because the cascade and deflector cannot reverse it completely; the practical limit is around 135 degrees. The remaining forward component is why reverse thrust is always less than full forward thrust.
To accelerate a gas from subsonic to supersonic velocity, the exhaust duct must be shaped
ACS code: AM.III.L
Correct answer: convergent first and then divergent along its length.
Rationale: To accelerate a gas from subsonic to supersonic speed the duct must first converge to reach Mach 1 at the throat and then diverge to continue accelerating the now-supersonic flow. This convergent-divergent (con-di) shape is the defining feature of a supersonic nozzle.
Untreated jet wake noise from a turbine engine exhaust is predominantly
ACS code: AM.III.L
Correct answer: low frequency at high decibel level.
Rationale: Untreated jet wake noise comes from the violent mixing of the high-velocity exhaust with the surrounding air, producing predominantly low-frequency sound at high sound-pressure levels. Suppressors break the wake into smaller streams to shift energy toward less troublesome higher frequencies.
Hot spots on the tail cone of a turbine engine are possible indicators of a malfunctioning fuel nozzle or
ACS code: AM.III.L
Correct answer: a faulty combustion chamber.
Rationale: A localized hot spot on the tail cone indicates an uneven temperature pattern in the gas, which points to a malfunctioning fuel nozzle or a faulty (cracked or burned) combustion chamber distorting the flame and gas distribution. An igniter or the tail cone position would not produce a localized hot streak.
Placing an exhaust cone aft of the turbine in a turbine engine causes the pressure in the first part of the exhaust duct to
ACS code: AM.III.L
Correct answer: increase as the gas velocity decreases.
Rationale: The exhaust cone makes the gas passage divergent in its first part, so the gas slows and, by Bernoulli's principle, its static pressure rises. Raising the static pressure ahead of the propelling nozzle helps the nozzle produce thrust efficiently.
A convergent-divergent exhaust nozzle is used on a high-pressure-ratio engine in order to
ACS code: AM.III.L
Correct answer: make maximum use of pressure thrust.
Rationale: A convergent-divergent nozzle allows the gas to keep expanding to near-ambient pressure in the divergent section, extracting the maximum pressure thrust from a high-pressure-ratio engine. It does not require the aircraft to be supersonic, and the thrust it produces is not a separate kinetic type.
The jet pipe (exhaust duct) of a gas turbine engine
ACS code: AM.III.L
Correct answer: protects the airframe from heat damage.
Rationale: The exhaust duct carries the hot gas to the propelling nozzle and is shielded so it protects the surrounding airframe structure from the high exhaust temperatures. The inner cone is part of the exhaust cone assembly and serves a different purpose (FAA-H-8083-32, Engine Exhaust and Reverser Systems).
For what purpose is the propelling (exhaust) nozzle of a gas turbine engine designed?
ACS code: AM.III.L
Correct answer: To increase the velocity and decrease the pressure of the exiting gas stream.
Rationale: A convergent propelling nozzle converts the gas stream's pressure energy into kinetic energy, so the gas leaves at increased velocity and reduced pressure, and this high exit velocity produces thrust. Pressure and velocity cannot both rise together in a simple converging nozzle (FAA-H-8083-32, Engine Exhaust and Reverser Systems).
If the exit area of an exhaust nozzle is too large, the result is
ACS code: AM.III.L
Correct answer: a lower exit velocity, causing a loss of thrust.
Rationale: If the nozzle exit area is too large the gas over-expands and leaves at a lower exit velocity. Since thrust depends on jet velocity, this produces a loss of thrust — a significant, not negligible, effect (FAA-H-8083-32, Engine Exhaust and Reverser Systems).
A choked exhaust nozzle
ACS code: AM.III.L
Correct answer: increases thrust.
Rationale: When a nozzle is choked the throat reaches Mach 1 and a residual pressure difference exists across the nozzle exit; this pressure thrust adds to the momentum thrust, so total thrust increases. A choked nozzle is a normal high-power condition, not a loss (FAA-H-8083-32, Engine Exhaust and Reverser Systems).
The exhaust section of a gas turbine engine is designed to
ACS code: AM.III.L
Correct answer: impart a high exit velocity to the exhaust gases.
Rationale: The exhaust (propelling nozzle) section converts the remaining pressure energy into kinetic energy, imparting a high exit velocity to the gas, which generates jet thrust. It is not designed merely to raise or lower temperature (FAA-H-8083-32, Engine Exhaust and Reverser Systems).
Reverse thrust can only be selected when the throttle is
ACS code: AM.III.L
Correct answer: in the closed (idle) position.
Rationale: For safety, reverse thrust can only be selected with the throttle closed (at idle) so the reverser deploys against low engine power before reverse thrust is increased. Deploying at high power risks reverser damage and uncontrolled forces (FAA-H-8083-32, Engine Exhaust and Reverser Systems).
A convergent-divergent exhaust nozzle is designed to
ACS code: AM.III.L
Correct answer: make maximum use of pressure thrust.
Rationale: A convergent-divergent nozzle lets the gas keep expanding toward ambient pressure in its divergent section, extracting maximum pressure thrust from a high pressure-ratio engine. It does not require the aircraft itself to be supersonic, and its thrust is not a separate kinetic type (FAA-H-8083-32, Engine Exhaust and Reverser Systems).
On forward-fan turbofan engines, thrust reversal is normally obtained by reversing the
ACS code: AM.III.L
Correct answer: cold (fan) stream.
Rationale: On forward-fan engines the bypass (cold) airflow is the dominant thrust producer, so reversers redirect the cold fan stream forward to generate reverse thrust. The hot core stream is usually left to exhaust normally because its contribution is comparatively small and its high temperature complicates reversal hardware (FAA-H-8083-32, Engine Exhaust and Reverser Systems).
Jet exhaust noise can be reduced by
ACS code: AM.III.L
Correct answer: increasing the rate at which the jet mixes with ambient air.
Rationale: Jet exhaust noise comes from the violent shearing between the high-velocity gas and the still ambient air. Promoting faster mixing of the jet with the surrounding air smooths this velocity gradient and lowers noise. Increasing jet velocity raises noise, since acoustic power increases steeply with exhaust speed (FAA-H-8083-32, Engine Exhaust and Reverser Systems).
Operating thrust reversers at low ground speeds can sometimes cause
ACS code: AM.III.L
Correct answer: foreign-object ingestion, hot-gas re-ingestion, and compressor stalls.
Rationale: At low ground speed the reversed efflux is no longer carried clear of the inlet, so it can recirculate into the engine. This causes hot gas re-ingestion (raising inlet temperature), stirs up sand and debris for foreign-object ingestion, and the disturbed, hot inlet air can trigger compressor stalls. All three effects can occur (FAA-H-8083-32, Engine Exhaust and Reverser Systems).
The purpose of the cascade vanes in a thrust reversing system is to
ACS code: AM.III.L
Correct answer: turn the fan and/or hot gases blocked from the exhaust nozzle to a forward direction.
Rationale: Once the blocker doors close off the normal rearward path, the diverted airflow must be turned forward to produce reverse thrust. The cascade vanes are the curved vane grid that redirects this blocked fan and/or hot gas flow forward and outward, creating the rearward-acting reverse-thrust component (FAA-H-8083-32, Engine Exhaust and Reverser Systems).
A simple convergent exhaust nozzle produces mainly
ACS code: AM.III.L
Correct answer: momentum thrust.
Rationale: A simple convergent nozzle accelerates the gas to produce thrust chiefly by the rate of change of momentum of the exhaust. Significant pressure thrust appears only when the nozzle is choked and a residual exit pressure exists; for a basic convergent nozzle the thrust is predominantly momentum thrust (FAA-H-8083-32, Engine Exhaust and Reverser Systems).
With the thrust reverser system deployed, the rearward thrust an engine can produce is
ACS code: AM.III.L
Correct answer: less than its forward thrust capability.
Rationale: Reverse thrust is always a fraction of forward thrust because the cascades and blocker doors cannot turn the efflux fully forward, and typically only the fan stream is reversed at reduced efficiency. Reverse thrust is on the order of half of forward thrust, so it is less than the forward capability (FAA-H-8083-32, Engine Exhaust and Reverser Systems).
Which statement is generally true regarding thrust reverser systems?
ACS code: AM.III.L
Correct answer: It is possible to move some aircraft backward on the ground using reverse thrust.
Rationale: Reverse thrust produces a genuine rearward force, and on some aircraft it is sufficient to overcome rolling resistance and move the aircraft backward (power-back). The practice is restricted because of re-ingestion and FOD risk, but it is physically possible (FAA-H-8083-32, Engine Exhaust and Reverser Systems).
What is the proper operating sequence when using thrust reversers to slow an aircraft after landing?
ACS code: AM.III.L
Correct answer: Retard thrust levers to ground idle, raise the thrust reverser levers as required, then retard the reverser levers to ground idle.
Rationale: After touchdown the engines must first be at idle before reverse is selected, so the thrust levers are brought to ground idle, the reverser levers are then raised to apply reverse power as required, and finally returned to ground idle before taxi speed. Selecting reverse from a high power setting is not permitted (FAA-H-8083-32, Engine Exhaust and Reverser Systems).
Most exhaust system failures result from thermal fatigue cracking at areas of stress concentration. This cracking is usually caused by
ACS code: AM.III.L
Correct answer: the high temperatures at which the exhaust system operates.
Rationale: The exhaust system runs continuously at very high gas temperatures and undergoes repeated thermal cycling. The resulting expansion and contraction sets up cyclic stresses at points of stress concentration, producing thermal-fatigue cracking. This normal high-temperature operating environment, not a manufacturing fault, is the usual cause (FAA-H-8083-32, Engine Exhaust and Reverser Systems).
Thrust reversal on a high-bypass turbofan engine is normally achieved using
ACS code: AM.III.L
Correct answer: blocker doors and cascade vanes.
Rationale: High-bypass engines reverse only the large cold fan stream, using blocker doors that close the bypass duct and cascade vanes that turn the flow forward. Clamshell and bucket (target) doors are associated with low-bypass or turbojet engines that reverse the hot exhaust stream (FAA-H-8083-32, Engine Exhaust and Reverser Systems).
If damage is found to thrust reverser cascade vanes and they must be replaced, the technician must
ACS code: AM.III.L
Correct answer: replace the vanes only with new ones of the correct part number for that position.
Rationale: Cascade vanes are designed with a specific turning angle and profile for that engine position and are not interchangeable; the discharge angle must be maintained. Replacements must be the correct approved part number for the position, since different-angle or non-matching vanes would alter the reverse-thrust pattern (FAA-H-8083-32, Engine Exhaust and Reverser Systems).
Lobe-type (corrugated) exhaust noise suppressors used in the hot exhaust stream of a turbine engine are made from
ACS code: AM.III.L
Correct answer: heat-resistant alloy.
Rationale: Lobe (corrugated) noise suppressors sit directly in the hot exhaust efflux and must withstand high gas temperatures while holding their shape, so they are made from heat-resistant alloy. Ordinary steel would not survive the thermal environment, and composite materials are unsuitable for the raw exhaust stream (FAA-H-8083-32, Exhaust Systems).
What flight-deck indication confirms to the crew that the thrust reversers have deployed?
ACS code: AM.III.L
Correct answer: A sequence of position indicator lights.
Rationale: Reverser deployment is monitored by position sensors that drive flight-deck indications, so the crew receives a sequence of lights (such as unlock and deployed indications) confirming the reverser state. Deceleration is felt rather than a positive indication, and a dedicated audible warning is not the normal deploy confirmation (FAA-H-8083-32, Thrust Reversers).
Through approximately what angle are the exhaust gases turned in a clamshell-type thrust reverser?
ACS code: AM.III.L
Correct answer: About 135 degrees.
Rationale: A clamshell (hot-stream) reverser does not turn the gas fully forward; the doors deflect the efflux through about 135 degrees, giving a forward-and-outward component that produces reverse thrust while allowing the flow to clear the airframe. A full 180-degree turn is not achieved with this design (FAA-H-8083-32, Thrust Reversers).
The purpose of a convergent propelling (exhaust) nozzle on a turbojet engine is to
ACS code: AM.III.L
Correct answer: increase the velocity of the gas to increase thrust.
Rationale: The propelling (exhaust) nozzle has a converging area that accelerates the gas as it leaves the engine. This rise in exhaust velocity increases the rate of change of momentum and therefore the thrust. It does not decelerate the flow, nor does it direct gas onto the turbines, which is the role of the turbine nozzle guide vanes (FAA-H-8083-32, Exhaust Nozzle).
If a thrust reverser is left deployed below the normal stow speed during the landing roll,
ACS code: AM.III.L
Correct answer: hot exhaust gases can be reingested into the engine inlet.
Rationale: If reverse is left deployed below the speed at which the forward-directed efflux is carried clear, the hot reversed gases recirculate forward and are drawn back into the inlet. This hot-gas reingestion raises inlet temperature and can cause a compressor stall, which is why reversers are stowed before the aircraft slows too far (FAA-H-8083-32, Thrust Reversers).
The physical size of a turbine engine's exhaust section is primarily dictated by the
ACS code: AM.III.L
Correct answer: size and installed location of the engine.
Rationale: The exhaust section must match the gas flow leaving the engine and route it past the surrounding structure, so its dimensions are governed by both engine size, which sets the gas mass flow, and engine location/installation, which dictates the available space and ducting. Neither factor alone fully determines it (FAA-H-8083-32, Exhaust Systems).
Acoustic linings made from composite materials are used in which section of a turbine engine?
ACS code: AM.III.L
Correct answer: The cold (fan and inlet) section only.
Rationale: Composite acoustic linings are fitted in the cold (fan, inlet, and bypass) sections, where temperatures are low enough for the bonded honeycomb materials to survive while absorbing fan noise. The hot section's high temperatures would damage composite linings, so they are not used there.
Fibrous metallic lining used for noise suppression is installed
ACS code: AM.III.L
Correct answer: in the hot (exhaust) area of the engine.
Rationale: Fibrous metallic noise-suppression lining is used in the hot areas of the engine, such as the exhaust region, because metallic fibers tolerate the high temperatures that would destroy composite linings. Composite honeycomb acoustic panels are reserved for the cooler fan and inlet sections.
Acoustic noise-suppression lining in the fan area of a turbofan is typically constructed from a
ACS code: AM.III.L
Correct answer: porous (perforated) facing over a honeycomb core with a backing sheet.
Rationale: Acoustic lining in the cooler fan area is built from a porous (perforated) facing over a honeycomb core with a solid backing sheet, forming resonator cells that absorb fan noise. Simple resin layers or felt-and-aluminum constructions do not provide this tuned sound-absorbing structure.
Acoustic blankets are installed on a turbine engine installation primarily to
ACS code: AM.III.L
Correct answer: reduce the noise levels radiating from the engine.
Rationale: Acoustic blankets are fitted specifically to absorb sound and reduce the noise levels radiating from the engine. They are not installed to improve thermal efficiency or to streamline the engine, which are unrelated functions.
A convergent propelling (exhaust) nozzle operating in a choked condition produces
ACS code: AM.III.L
Correct answer: Both pressure thrust and velocity (momentum) thrust
Rationale: Per FAA-H-8083-32, a propelling nozzle produces thrust from two components: the momentum (velocity) thrust of the accelerated gas leaving the nozzle, plus pressure thrust when the nozzle is choked and exit static pressure exceeds ambient. A choked propelling nozzle therefore delivers both pressure and velocity thrust, not one alone.
Reverse thrust can only be selected when the thrust lever (throttle) is
ACS code: AM.III.L
Correct answer: At the idle stop with forward power retarded
Rationale: Reverse thrust can only be selected with the thrust lever at the idle stop. This interlock prevents the reverser being deployed at high forward power, which could be hazardous; the lever must be brought to idle before the reverse mechanism can be engaged and reverse power then applied.
What indication does the flight crew receive that the thrust reversers have deployed?
ACS code: AM.III.L
Correct answer: A sequence of position-indicating lights
Rationale: Thrust-reverser deployment is confirmed to the crew by cockpit position indications, typically a sequence of lights showing the reverser unlocking, transiting, and reaching the deployed position. The crew rely on these positive position indications rather than on the physical sensation of deceleration or an audible warning.
The exhaust manifold of a reciprocating engine collects:
ACS code: AM.III.L
Correct answer: Exhaust gases from the individual cylinders and routes them to the exhaust pipe
Rationale: Per FAA-H-8083-32, the exhaust manifold gathers the burnt combustion gases leaving each cylinder's exhaust port and channels them into the exhaust pipe or tailpipe for discharge overboard. It handles hot gases only; coolant and scavenged oil are managed by separate systems.