What is the difference between an air-cycle and a vapor-cycle air conditioning system?

A vapor-cycle system uses a refrigerant that changes state, circulating it through a compressor, condenser, receiver-dryer, expansion valve, and evaporator; it can cool the cabin on the ground with the engines shut down. An air-cycle system cools engine bleed air directly using heat exchangers and an expansion turbine, with no refrigerant at all, and it needs a bleed air source to work. Turbine aircraft normally use air cycle.

Where does cabin pressurization air come from on a jet?

On a turbine-powered aircraft, pressurization air is compressor bleed air tapped from the engines, cooled through the air conditioning packs, and then delivered to the cabin. An auxiliary power unit or a ground air cart can supply the same air on the ground. Reciprocating-engine aircraft use a turbocharger, a supercharger, or a separate engine-driven compressor instead, because they have no bleed air source.

What does the receiver-dryer do in a vapor-cycle system?

The receiver-dryer sits between the condenser and the expansion valve. It acts as a reservoir of liquid refrigerant so the expansion valve always sees liquid, while its desiccant absorbs moisture and a filter traps contaminants. Moisture matters because water in the system can freeze at the expansion valve and block refrigerant flow. Many units include a sight glass for checking charge condition.

Why can't petroleum products touch an aircraft oxygen system?

Oil, grease, and other hydrocarbons can ignite violently in the presence of high-pressure oxygen, so your hands, your tools, and the fittings must be clean and free of any petroleum product before you work on an oxygen system. Only lubricants specifically approved for oxygen service may be used, and lines and fittings should be capped whenever the system is opened.

How is cabin differential pressure controlled?

The cabin pressure controller positions the outflow valve, which meters how fast air leaves the cabin. Inflow from the packs is roughly constant, so closing the outflow valve raises cabin pressure and opening it lowers cabin pressure. Safety and relief valves cap the maximum differential pressure the structure will see, and a negative-pressure relief valve keeps outside pressure from exceeding cabin pressure.

Environmental Systems — FAA A&P Test Questions (ACS AM.II.G)

Environmental systems is the Airframe subject area that covers how an aircraft keeps its cabin cool, warm, and breathable at altitude. On the written test you are asked to identify components and trace what happens to the air or the refrigerant as it moves through the system: which part compresses vapor, where heat is absorbed from cabin air, and what supplies pressurization air on a turbine aircraft. These are component-and-function questions rather than math problems, so knowing each unit's job, in order, is what earns the points.

What ACS AM.II.G covers

AM.II.G pulls from the air conditioning, pressurization, heating, and oxygen material in FAA-H-8083-31. The largest block of questions is vapor-cycle air conditioning. You need the four major components in order and what each one does to the refrigerant: the compressor takes low-pressure vapor and raises both its pressure and its temperature, the condenser rejects that heat and returns the vapor to a liquid, the receiver-dryer stores liquid refrigerant and removes moisture and contaminants, and the thermostatic expansion valve meters refrigerant into the evaporator while dropping its pressure. Heat is absorbed from cabin air at the evaporator. Questions are often written so that you have to name the component from a description of its job alone. Air-cycle, or bootstrap, systems are the second block. Bleed air passes through a primary heat exchanger, the air cycle machine compressor, a secondary heat exchanger, and then the expansion turbine, where the air gives up energy doing work and drops in temperature the most. Ram air is the cooling medium in both heat exchangers. Expect a question on where the largest temperature drop occurs and one on water separation downstream of the turbine. Pressurization ties the two together. On turbine aircraft the pressurization air is engine compressor bleed air, cabin pressure is regulated by the outflow valve, and cabin differential pressure is the difference between cabin pressure and the ambient pressure outside the aircraft. Safety valves and negative-pressure relief valves protect the structure. The remaining questions cover heating, including combustion heaters, exhaust heat exchangers, and the carbon monoxide risk from a cracked shroud, and oxygen systems: cylinder markings, aviator's breathing oxygen, chemical oxygen generators, and the rule that no petroleum product may contact an oxygen fitting.

Where this sits on the test

ACS AM.II.G is tested on the FAA Airframe written test, one of 1,748 ACS-tagged questions in the Airframe 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-8083-31

Three traps candidates fall into

Candidates swap the condenser and the evaporator. The evaporator sits in the cabin airflow and absorbs heat, which is why the air leaving it feels cold; the condenser sits in outside airflow and rejects heat. If you remember only that the refrigerant gets cold somewhere, the component questions will catch you.
In an air-cycle system, many candidates pick a heat exchanger as the point of greatest cooling because it is the part that sounds like a cooler. The heat exchangers only pre-cool the bleed air with ram air; the expansion turbine produces the largest temperature drop, because the air does work driving it.
Cabin differential pressure gets confused with cabin altitude. Differential pressure is a pressure difference, in psi, between the inside of the cabin and the ambient air outside, and it is limited by structural strength. Cabin altitude is the pressure altitude the cabin is being held at, and it is read in feet.

7 free sample questions from ACS AM.II.G

AM.II.GTap an answer
In a vapor-cycle air conditioning system, which component compresses the low-pressure refrigerant vapor and raises both its temperature and pressure?
AM.II.GTap an answer
In a vapor-cycle air conditioning system, the receiver-dryer is primarily used to perform which function?
AM.II.GTap an answer
In a vapor-cycle system, the thermostatic expansion valve controls the refrigerant by performing which action?
AM.II.GTap an answer
In a vapor-cycle system, where does the refrigerant absorb heat from the cabin air?
AM.II.GTap an answer
In an air-cycle (bootstrap) air conditioning system, the temperature of the air is lowered the most as it passes through which component?
AM.II.GTap an answer
In an air-cycle air conditioning system, the primary and secondary heat exchangers remove heat from the bleed air using which cooling medium?
AM.II.GTap an answer
In a turbine-powered aircraft, the air used for cabin pressurization is normally supplied from which source?

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.

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Frequently asked questions

What is the difference between an air-cycle and a vapor-cycle air conditioning system?: A vapor-cycle system uses a refrigerant that changes state, circulating it through a compressor, condenser, receiver-dryer, expansion valve, and evaporator; it can cool the cabin on the ground with the engines shut down. An air-cycle system cools engine bleed air directly using heat exchangers and an expansion turbine, with no refrigerant at all, and it needs a bleed air source to work. Turbine aircraft normally use air cycle.
Where does cabin pressurization air come from on a jet?: On a turbine-powered aircraft, pressurization air is compressor bleed air tapped from the engines, cooled through the air conditioning packs, and then delivered to the cabin. An auxiliary power unit or a ground air cart can supply the same air on the ground. Reciprocating-engine aircraft use a turbocharger, a supercharger, or a separate engine-driven compressor instead, because they have no bleed air source.
What does the receiver-dryer do in a vapor-cycle system?: The receiver-dryer sits between the condenser and the expansion valve. It acts as a reservoir of liquid refrigerant so the expansion valve always sees liquid, while its desiccant absorbs moisture and a filter traps contaminants. Moisture matters because water in the system can freeze at the expansion valve and block refrigerant flow. Many units include a sight glass for checking charge condition.
Why can't petroleum products touch an aircraft oxygen system?: Oil, grease, and other hydrocarbons can ignite violently in the presence of high-pressure oxygen, so your hands, your tools, and the fittings must be clean and free of any petroleum product before you work on an oxygen system. Only lubricants specifically approved for oxygen service may be used, and lines and fittings should be capped whenever the system is opened.
How is cabin differential pressure controlled?: The cabin pressure controller positions the outflow valve, which meters how fast air leaves the cabin. Inflow from the packs is roughly constant, so closing the outflow valve raises cabin pressure and opening it lowers cabin pressure. Safety and relief valves cap the maximum differential pressure the structure will see, and a negative-pressure relief valve keeps outside pressure from exceeding cabin pressure.

All 40 ACS areas

General

Airframe

Powerplant

Primary sources

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