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.