In a vapor-cycle air conditioning system, which component compresses the low-pressure refrigerant vapor and raises both its temperature and pressure?
ACS code: AM.II.G
Correct answer: The compressor
Rationale: In a vapor-cycle system the compressor draws in low-pressure, low-temperature refrigerant vapor from the evaporator and compresses it, raising both its pressure and temperature before it flows to the condenser. The condenser only removes heat to liquefy the refrigerant, and the evaporator absorbs cabin heat to boil the refrigerant into vapor. FAA-H-8083-31 describes the compressor as the heart of the vapor-cycle loop.
In a vapor-cycle air conditioning system, the receiver-dryer is primarily used to perform which function?
ACS code: AM.II.G
Correct answer: Store liquid refrigerant and remove moisture
Rationale: The receiver-dryer acts as a reservoir for liquid refrigerant leaving the condenser and contains a desiccant and filter that remove moisture and contaminants from the system. Compressing vapor is the compressor's job, and metering flow into the evaporator is the expansion valve's job. FAA-H-8083-31 lists moisture removal and storage as the receiver-dryer's roles.
In a vapor-cycle system, the thermostatic expansion valve controls the refrigerant by performing which action?
ACS code: AM.II.G
Correct answer: Metering high-pressure liquid into the evaporator
Rationale: The thermostatic expansion valve meters high-pressure liquid refrigerant from the receiver-dryer into the low-pressure evaporator, where it expands and boils to absorb heat. Condensing vapor to liquid is the condenser's function, and drawing vapor from the evaporator is the compressor's function. FAA-H-8083-31 identifies the expansion valve as the metering device dividing the high and low sides of the system.
In a vapor-cycle system, where does the refrigerant absorb heat from the cabin air?
ACS code: AM.II.G
Correct answer: In the evaporator
Rationale: Heat from the cabin air is absorbed in the evaporator, where low-pressure liquid refrigerant boils into vapor as cabin air is blown across the coils. The condenser rejects heat to outside air, and the receiver-dryer only stores liquid and removes moisture. FAA-H-8083-31 describes the evaporator as the point of cabin heat absorption in the vapor-cycle loop.
In an air-cycle (bootstrap) air conditioning system, the temperature of the air is lowered the most as it passes through which component?
ACS code: AM.II.G
Correct answer: The expansion turbine
Rationale: In an air-cycle machine, the greatest temperature drop occurs across the expansion turbine, where the air does work on the turbine wheel and loses energy as it expands, cooling well below ambient. Heat exchangers remove some heat but cannot cool below ambient, the compressor raises temperature, and the water separator only removes condensed moisture. FAA-H-8083-31 identifies the expansion turbine as the primary cooling device in air-cycle systems.
In an air-cycle air conditioning system, the primary and secondary heat exchangers remove heat from the bleed air using which cooling medium?
ACS code: AM.II.G
Correct answer: Ram air
Rationale: Air-cycle heat exchangers use ram air as the cooling medium, passing ambient outside air across the cores to carry away heat from the hot bleed air. Refrigerant is used only in vapor-cycle systems, and engine oil is not used to cool conditioning air. FAA-H-8083-31 describes ram air flowing through the heat exchangers to cool the bleed-air charge.
In a turbine-powered aircraft, the air used for cabin pressurization is normally supplied from which source?
ACS code: AM.II.G
Correct answer: Engine compressor bleed air
Rationale: Pressurization air on turbine aircraft is normally tapped as bleed air from the engine compressor section, which delivers a continuous supply of high-pressure air to the cabin. A vacuum pump removes air rather than supplying it, and a vapor-cycle compressor circulates refrigerant, not cabin air. FAA-H-8083-31 identifies compressor bleed air as the typical pressurization source.
Cabin differential pressure is best defined as the difference between which two pressures?
ACS code: AM.II.G
Correct answer: Cabin pressure and ambient outside pressure
Rationale: Cabin differential pressure is the difference between the pressure inside the cabin and the ambient atmospheric pressure outside the aircraft. The structure is designed to a maximum allowable differential, which the controller will not exceed. The other choices do not describe the differential the fuselage structure is limited by. FAA-H-8083-31 defines differential pressure in these terms.
When servicing aircraft oxygen systems, why must oil, grease, and other petroleum products be kept away from the system?
ACS code: AM.II.G
Correct answer: They can ignite or explode in the presence of oxygen
Rationale: Oil and grease can ignite spontaneously or explode when in contact with concentrated oxygen under pressure, creating a serious fire hazard. The hazard is combustion, not leakage or a purity-meter error. FAA-H-8083-31 stresses that oxygen-system components and tools must be kept free of any petroleum products.
In the International Standard Atmosphere, the tropopause is located at approximately
ACS code: AM.II.G
Correct answer: 36,000 ft
Rationale: Per FAA-H-8083-31 and the International Standard Atmosphere, the tropopause—where the temperature stops decreasing and steadies near minus 56.5 degrees C—lies at about 36,000 ft. Below it, temperature decreases with altitude; above it, in the stratosphere, temperature holds constant for a band.
The fuel drain of a combustion heater must be protected primarily against
ACS code: AM.II.G
Correct answer: ice buildup blocking the drain
Rationale: A combustion heater drain carries fuel along with water from condensation, and at the cold drain outlet this moisture can freeze. The drain must be protected against ice buildup because a blocked drain would allow fuel to accumulate, creating a serious fire hazard. Protection against ice formation keeps the drain clear and the heater installation safe.
The approximate minimum fresh-air mass flow supplied to a pressurized cabin is
ACS code: AM.II.G
Correct answer: 0.5 pound per minute per person.
Rationale: Per FAA-H-8083-31, cabin air-conditioning and pressurization systems provide a minimum fresh-air flow of about 0.5 pound per minute per occupant to control temperature, carbon dioxide, and odors. Figures of 1 or 10 pounds per minute per person are well above the recognized minimum requirement.
The function of the mass airflow control valve in a cabin air system is to
ACS code: AM.II.G
Correct answer: maintain a reasonable mass flow of air into the cabin regardless of aircraft altitude.
Rationale: Per FAA-H-8083-31, a mass airflow control valve regulates the supply so a reasonably constant mass of air enters the cabin regardless of altitude, even though ambient density falls as the aircraft climbs. Limiting system pressure or controlling outflow are the jobs of relief and outflow valves, not the mass-flow control valve.
The hottest source of air supplied for cabin pressurization and conditioning is
ACS code: AM.II.G
Correct answer: an engine compressor bleed.
Rationale: Per FAA-H-8083-31, air bled directly from a gas turbine engine compressor is the hottest source because compression raises its temperature; this is the standard high-temperature supply for pressurization and conditioning. Air from a gearbox-driven blower is comparatively cooler.
In most cabin pressurization systems, the amount of compressed air delivered to the cabin is
ACS code: AM.II.G
Correct answer: reasonably constant, irrespective of altitude.
Rationale: Pressurization systems are designed to deliver a reasonably constant mass of air to the cabin regardless of altitude, with cabin pressure managed by controlling the outflow rather than the inflow. The supply is not varied by a rate-of-change selector, which controls how fast cabin pressure changes, not the delivered mass flow.
In a bleed-air air-conditioning system, the warm air supply is provided by
ACS code: AM.II.G
Correct answer: the compressor of the gas turbine engine.
Rationale: In a bleed-air air-conditioning system the warm air is tapped from the gas turbine engine compressor, where compression has already heated it. Engine exhaust gas is not used as the supply air because it is contaminated and cannot be fed into the cabin.
Approximately how much fresh air is required for ventilation of the flight deck?
ACS code: AM.II.G
Correct answer: About 10 cubic feet per minute per crew member.
Rationale: The recognized fresh-air requirement for the flight deck is about 10 cubic feet per minute per crew member to maintain a comfortable, well-ventilated environment. The figure is a volume flow rate, not a fixed pound-per-minute mass, and it is set by design rather than by crew selection.
The function of an air mass flow control valve is to
ACS code: AM.II.G
Correct answer: maintain a reasonably constant air mass flow into the cabin at all altitudes.
Rationale: An air mass flow control valve regulates the inflow so a reasonably constant mass of air is delivered to the cabin at all altitudes despite the fall in ambient air density with height. Controlling outflow from the cabin and limiting differential pressure are handled by the outflow valve and relief valve, respectively.
The function of a spill valve in a cabin air supply system is to control
ACS code: AM.II.G
Correct answer: the air supply mass flow to the cabin.
Rationale: Spill valves dump excess delivered air overboard so the air supply to the cabin is held to the required mass flow, particularly at low altitude when engine-driven blowers deliver more than is needed. They do not set the pressurization rate or the cabin differential, which are functions of the outflow valve and pressure controller.
The purpose of a mass flow controller is to
ACS code: AM.II.G
Correct answer: ensure that a constant mass of air is delivered to the cabin at all times.
Rationale: A mass flow controller meters the supply so a constant mass of air reaches the cabin at all times, compensating for the reduced air density at altitude. Selecting and holding cabin altitude is the role of the cabin pressure controller and outflow valve, not the mass flow controller.
The purpose of a spill valve in a cabin air supply system is
ACS code: AM.II.G
Correct answer: to spill overboard the excess air delivered at sea level and lower altitudes.
Rationale: At sea level and low altitude, engine-driven blowers deliver more air than the cabin needs; the spill valve dumps this excess overboard so a controlled mass flow is maintained. It is not a pressure-relief device for the supply ducting, nor does it provide cabin heating.
When engine-driven blowers supply the cabin, the mass flow delivered to the cabin is controlled by
ACS code: AM.II.G
Correct answer: spill valves that dump excess air overboard.
Rationale: With engine-driven blowers the delivered air increases with engine speed, so the mass flow reaching the cabin is regulated by spill valves that dump surplus air overboard. Relying on engine speed alone would not give a controlled flow, which is exactly why spill valves are fitted.
A spill valve opens to
ACS code: AM.II.G
Correct answer: control the supply air flow delivered to the cabin.
Rationale: A spill valve opens to discharge excess supply air overboard, thereby controlling the mass flow delivered to the cabin, especially when blowers over-supply at low altitude. Limiting cabin pressure differential is the relief valve's task, and outflow from the cabin is managed by the outflow valve.
In an air-conditioning system, a pack (flow control) valve controls the
ACS code: AM.II.G
Correct answer: flow of bleed air into the pack.
Rationale: A pack (flow control) valve is a pneumatic shutoff and modulating valve that meters the mass of bleed air entering the air-conditioning pack. It regulates air flow into the pack; the conditioned air temperature is set downstream by the air cycle machine and trim air, not by the pack valve itself.
In large air-conditioning systems, the main distribution fan is operated to
ACS code: AM.II.G
Correct answer: distribute the conditioned air and maintain positive duct pressure.
Rationale: In large systems the main distribution fan moves conditioned air through the ducting to the cabin zones and, by keeping the ducts charged, maintains a positive duct pressure that prevents backflow and ensures even delivery. Both functions are served, so the single-purpose answers are incomplete.
Cockpit ventilation requirements are normally specified as
ACS code: AM.II.G
Correct answer: about 10 cubic feet per crew member per minute.
Rationale: Airworthiness requirements specify cockpit ventilation in terms of volumetric fresh-air supply, the accepted figure being about 10 cubic feet per crew member per minute. A liter-based figure does not match the customary regulatory unit, and a crew-selected value is not a defined minimum.
In a large aircraft air-conditioning system, the cabin temperature control
ACS code: AM.II.G
Correct answer: is selectable for each zone individually from the flight station.
Rationale: Large aircraft use zoned temperature control so the flight deck, forward cabin, and aft cabin can each be set independently from the flight station, with trim air or mixing trimming each zone. A single master switch would prevent independent zone control, and temperature is not set by modulating the pack flow valve.
An advantage of an air cycle air-conditioning system over a vapor cycle system in aircraft is that
ACS code: AM.II.G
Correct answer: both cabin cooling and pressurization air are obtained from one system.
Rationale: An air cycle (open) system uses engine bleed air, so the same conditioned air both cools the cabin and supplies the pressurization air. A vapor cycle system only transfers heat and provides no pressurizing air, which is the key advantage of the air cycle machine.
What is the maximum allowable concentration (by volume) of carbon monoxide in an occupied aircraft compartment?
ACS code: AM.II.G
Correct answer: 0.005 percent (50 ppm).
Rationale: Airworthiness limits set the maximum allowable carbon monoxide concentration in an occupied compartment at one part in 20,000, i.e., 0.005 percent by volume, or 50 parts per million. The higher figures exceed the accepted safe exposure limit.
Bleed air tapped from an intermediate-pressure compressor stage first enters which component?
ACS code: AM.II.G
Correct answer: The check (non-return) valve.
Rationale: Bleed air tapped from the intermediate-pressure compressor first passes through a check (non-return) valve that prevents reverse flow back into the engine, for example during shutdown or when higher-pressure air is selected. Only after this does the air continue toward the precooler and the rest of the system.
In a vapor cycle air-conditioning system, the sensing bulb of the thermostatic expansion valve is located just before the
ACS code: AM.II.G
Correct answer: compressor inlet.
Rationale: In a vapor cycle system the thermostatic expansion valve's sensing bulb is clamped to the evaporator outlet line, just before the compressor inlet, so it senses the superheat of the suction gas returning to the compressor. This lets the valve meter refrigerant to keep the evaporator fully fed but not overfed.
A refrigerant that changes phase to absorb and reject heat is used in which type of aircraft air-conditioning system?
ACS code: AM.II.G
Correct answer: The vapor cycle system.
Rationale: A refrigerant, a substance that changes phase to absorb and reject heat, is used only in a vapor cycle system. An air cycle machine cools by expanding air across a turbine and uses no refrigerant, and a pneumatic pump simply moves air.
In a bootstrap air cycle air-conditioning pack, the water separator is located
ACS code: AM.II.G
Correct answer: downstream of the cooling turbine.
Rationale: In a bootstrap air cycle pack the cold air leaving the expansion (cooling) turbine is the coldest point, where moisture condenses out. The water separator is therefore placed downstream of the turbine to collect and drain that condensate before the air enters the cabin.
The defining function of an air conditioning (cooling) system is to
ACS code: AM.II.G
Correct answer: decrease the temperature of the supply air.
Rationale: An air conditioning cooling system's defining function is to remove heat and decrease the temperature of the supply air. Cabin heating is typically provided by the bleed-air source itself, so the conditioning system's role is to cool.
In a bootstrap air cycle machine, the cooling turbine
ACS code: AM.II.G
Correct answer: drives a compressor that increases the air temperature and pressure.
Rationale: In a bootstrap air cycle machine the cooling turbine extracts energy from the air and uses it to drive the secondary compressor on the common shaft. That compression raises the air's pressure and temperature, after which it is cooled in the heat exchanger before expanding back across the turbine.
Air exiting the compressor side of an air cycle machine (ACM) will have
ACS code: AM.II.G
Correct answer: increased pressure and increased temperature.
Rationale: The compressor side of an ACM raises the pressure of the air, and compression always produces a corresponding rise in temperature. The heated, compressed air is then routed to a heat exchanger for cooling before expansion across the turbine.
An air-to-air heat exchanger in an air conditioning pack is provided to
ACS code: AM.II.G
Correct answer: reduce the supply air temperature.
Rationale: An air-to-air heat exchanger uses cool ambient ram air as the heat sink to remove heat from the hot compressed bleed air, lowering the supply air temperature. It serves as the pack's primary and secondary cooler, not as a heater or an emergency ram supply.
Temperature control of cabin air is achieved by
ACS code: AM.II.G
Correct answer: regulating the amount of warm air bypassing the cooling system.
Rationale: Cabin temperature is trimmed by a temperature control valve that bypasses a proportion of warm air around the cooling pack and mixes it with the cold pack-discharge air. Varying that bypass sets the delivery temperature without altering the cooling system itself.
In a pressurized aircraft using engine bleed air, temperature conditioning of the cabin supply is achieved mainly by
ACS code: AM.II.G
Correct answer: extracting heat from the pressurizing air.
Rationale: Bleed air used for pressurization arrives very hot from the engine compressor, so temperature conditioning is achieved mainly by extracting that excess heat in the air conditioning pack before delivery. Heating is rarely needed because the source air is already hot.
When cabin pressurization air is bled from the engine compressor, an internal engine oil leak will
ACS code: AM.II.G
Correct answer: contaminate the cabin air supply.
Rationale: Because cabin pressurization air is bled from the engine compressor, an internal engine oil leak can pass into the bleed-air path and contaminate the cabin air supply, producing fumes and odors. There is no inherent feature in this basic arrangement that prevents oil from reaching the bleed air.
Heating for a pressurized cabin is normally obtained from
ACS code: AM.II.G
Correct answer: air supply heated by the pressurization process.
Rationale: Cabin heating is provided by the bleed/pressurizing air, which is heated naturally by compression in the engine compressor and the ACM compressor. Warm air is blended with cooled air to set cabin temperature, so no large electrical heater is normally required.
A cold air unit produces its drop in temperature by
ACS code: AM.II.G
Correct answer: expanding hot air across a turbine that drives a compressor.
Rationale: The cold air unit produces its temperature drop by expanding hot air across a turbine that simultaneously drives the bootstrap compressor. The air gives up energy both in expansion and in the work done turning the compressor, so it leaves the turbine cold.
When a shroud or 'muff' is built around the engine exhaust pipe and ventilation air is directed through it to pick up heat, the device is classified as a(n)
ACS code: AM.II.G
Correct answer: exhaust heater.
Rationale: A muff is a shroud built around the engine exhaust system through which ventilation air is passed to absorb heat from the hot exhaust pipe; this arrangement is an exhaust heater. A combustion heater burns its own fuel, which is a different device.
In an air-cycle (cold-air) environmental system, the heat exchanger that cools the bleed-air charge rejects its heat to
ACS code: AM.II.G
Correct answer: ram air drawn from outside ambient conditions.
Rationale: In an air-cycle system the heat exchanger acts as a radiator, transferring heat from the hot compressed bleed air to cool ambient ram air drawn from outside the aircraft. Using engine bleed air or cabin supply air would add heat rather than remove it, so ram air is the cooling medium.
In an aircraft environmental control system, the term "conditioned air" refers to air that has had its
ACS code: AM.II.G
Correct answer: temperature and pressure adjusted to cabin requirements.
Rationale: Conditioned air is air whose temperature and pressure have been adjusted to meet cabin requirements, with its moisture and cleanliness also controlled. Removing moisture alone or adding oxygen does not, by itself, define conditioned air.
On many turbine aircraft, an air-conditioning pack may be automatically inhibited (its flow reduced or shut off) by
ACS code: AM.II.G
Correct answer: thrust-lever (throttle) position switches at high power settings.
Rationale: The air-conditioning pack draws engine bleed air, which reduces available thrust. To preserve engine power during takeoff, pack operation is often inhibited by thrust-lever (throttle) position switches that close or limit the pack valve. Flap and landing-gear switches control other systems, not pack inhibition.
When performing a ground pressurization (leak) test on a cabin, the proper source of test air is
ACS code: AM.II.G
Correct answer: a ground air-supply cart delivering clean, regulated air.
Rationale: A ground pressurization or leak test should use a dedicated ground air supply that delivers clean, dry, filtered air at a controlled, regulated pressure. Running the aircraft engines would introduce uncontrolled, contaminated air and risk over-pressurizing the structure.
In a bootstrap air-cycle air-conditioning system, the source of the compressed supply air is
ACS code: AM.II.G
Correct answer: compressor bleed air from the turbine engine.
Rationale: A bootstrap air-cycle system is supplied with hot, high-pressure air bled from the turbine engine's compressor. Ram air is low pressure and serves only as the heat-exchanger cooling medium, not as the supply source.
The temperature inside a transport-category cabin is normally maintained within a comfort range of approximately
ACS code: AM.II.G
Correct answer: 65 degrees F to 75 degrees F (18 to 24 degrees C).
Rationale: Cabin temperature is normally held within a passenger-comfort band of roughly 65 to 75 degrees Fahrenheit (about 18 to 24 degrees Celsius). The lower band would be uncomfortably cool for occupants.
In a bootstrap air-cycle cooling system, the bleed-air supply is processed in which order?
ACS code: AM.II.G
Correct answer: Compressed, then cooled in a heat exchanger, then expanded across the cooling turbine.
Rationale: In a bootstrap cycle the bleed air is first further compressed by the bootstrap compressor, then cooled in a heat exchanger, and finally expanded across the cooling (expansion) turbine, where it drops sharply in temperature. Expanding the air before compressing it would defeat the bootstrap principle.