13.11.2 Air Conditioning

Question 1

What are the four sources of air supplies for the air conditioning system on an aircraft?

Answer 1

Gas turbine compressors

Engine driven blowers

APU compressors

Ambient ram air inlets

Question 2

What is the first stage of cooling in an air conditioning system?

Answer 2

Primary heat exchanger

Question 3

What is the alternative name for the air cycle machine?

Answer 3

Cold air unit

Question 4

How can the mix valve be controlled in the event of failure of the temperature regulator?

Answer 4

Manually using the temperature selector switch

Question 5

What causes a signal to drive the mix valve to change positions when the temperature selector switch is in the ‘auto’ position?

Answer 5

Control bridge circuit with reference resistance from switch on one arm and cabin temperature sensor on the other arm

When unbalanced, a signal is developed

Question 6

What is the purpose of the anticipation and limit sensors in the ducting to the cabin?

Answer 6

Modulate any rapid changes as a result of unbalanced bridge

Prevents sudden blasts of hot or cold air

Question 7

What happens when the heat in the duct reaches 90°C?

Answer 7

Moves the mix valve to fully cold

Duct overheat light illuminates

Question 8

How can the system be returned to normal following a temperature of 90°C?

Answer 8

Pushing the reset button

Question 9

Under what condition does the other thermal switch protect the air conditioning duct from overheating?

Answer 9

Should control electrical power be lost

Question 10

In the air conditioning system, what does the 120°C switch operate?

Answer 10

Pack valve closing

Illuminating the pack trip off light

Question 11

Under what other conditions, other than the 120°C overheat switch being activated, will the pack valve close due to overheat?

Answer 11

If the air cycle machine compressor discharge overheat switch is actuated

or

If the air cycle machine turbine inlet overheat switch is actuated

Question 12

At what temperature will the air conditioning pack deliver air to the different aircraft zones?

Answer 12

Pack output temperature will be to the lowest zone temperature selected

Question 13

On long haul flights, what is the relative humidity kept at using humidifiers?

Answer 13

30%

Question 14

How does a humidifier operate?

Answer 14

Potable water jet impinges at right angles on the high pressure air jet

Water becomes atomised

Air becomes uniformly saturated

Question 15

Why is the recirculating air entering the mixing chamber made to swirl?

Answer 15

Ensures that the recirculated air and the fresh conditioned air are thoroughly mixed

Question 16

In the recirculation fan, what are the relative quantities of fresh air to recirculated cabin air?

Answer 16

60% fresh

40% recirculated cabin air

Question 17

What are the symptoms of ozone exposure?

Answer 17

Chest pain

Coughing

Shortness of breath

Fatigue

Headaches

Nasal congestion

Eye irritation

Question 18

How does an ozone converter work?

Answer 18

Separates ozone from the air and by catalytic action of the substance contained within the converter, converts ozone to oxygen

Question 19

What type of noble metal is used in a ozone converter?

Answer 19

Palladium

Question 20

What is the principle of vapour cycle cooling?

Answer 20

Latent heat

Question 21

What are the basic components of a basic vapour cycle cooling system?

Answer 21

Receiver dryer

Thermostatic expansion valve

Evaporator

Compressor

Condenser

Question 22

How is the amount of refrigerant put into a system determined?

Answer 22

By weight rather than volume

Question 23

How is refrigerant normally put into the system?

Answer 23

In its vapour form after the system has been evacuated and is still under vacuum

Question 24

What must be done when opening the refrigerant system?

Answer 24

All the refrigerant must be recovered using special equipment

Not permitted to release the refrigerant into the atmosphere

Question 25

What must be done if a refrigeration system has been opened?

Answer 25

It must be evacuated before it is recharged

Question 26

What properties must refrigerants used in aircraft installations have?

Answer 26

Non–flammable Non–explosive Non–toxic, both in pure state and when mixed in air The refrigerant must not react unfavourably with lubricating oils used within the system Ideally the lubricating oil should be miscible with the refrigerant Non–corrosive

Question 27

What refrigerant are aircraft systems being modified to use or designed for?

Answer 27

Refrigerant R–134a

Question 28

What are the three principles of air cycle cooling?

Answer 28

Surface heat exchange Expansion Energy conversion

Question 29

Where does the fuel in combustion heaters come from?

Answer 29

Aircraft fuel tanks

Question 30

What is the medium that is heated in a combustion heater?

Answer 30

Ram air

Question 31

Why is ram air kept at a positive pressure?

Answer 31

To prevent noxious gases from entering the cabin

Question 32

What is the purpose of the 'whirling' flame produced in the combustion chamber?

Answer 32

Ensures even burning of the fuel and heating of the chamber walls

Question 33

In a combustion heater, what is the purpose of the limit switch?

Answer 33

First line of defence against overheat Closes fuel valve, opens it again once the overheat condition is removed

Question 34

In a combustion heater, what is the purpose of the overheat switch and how does it differ from the limit switch?

Answer 34

If the temperature continues to rise past that which would normally operate the limit switch Shut the combustion heater down and cannot be restarted in flight

Question 35

What continues to operate in a combustion heater, even though the fuel valve is closed?

Answer 35

Spark plug and combustion blower

Question 36

What does CAP 747 GR11 require about combustion heaters?

Answer 36

Intervals not exceeding 500 operating hours, or 2 years, whichever is sooner, combustion heaters and their exhaust systems must be completely dismantled and inspected

Question 37

Which valve must open to allow engine bleed air into the air conditioning system?

Answer 37

Electro–pneumatic pack valves

Question 38

Which valves allow hot bleed air to bypass the cold air unit?

Answer 38

Mix valve 1.7°C valve

Question 39

What is the name of the system which allows passengers to be individually cooled?

Answer 39

Gasper outlets

Question 40

How is cooling achieved through the heat exchangers when the aircraft is on the ground?

Answer 40

A turbo fan induces air to flow across the heat exchangers

Question 41

How is the temperature in the duct between the compressor and the secondary heat exchanger controlled?

Answer 41

Ram air temperature sensor

Question 42

State the two ways the turbine of the ACM achieves cooling?

Answer 42

Pressure drop Heat energy converted to mechanical energy

Question 43

How is the water extractor prevented from freezing?

Answer 43

Temp sensor control 35°F (1.7°C) valve Valve will allow by–pass warm air past the ACM into the inlet of the water separator Prevents moisture freezing

Question 44

Which three sensors would trip the pack valve?

Answer 44

ACM compressor discharge overheat switch ACM turbine inlet overheat switch 120°C thermal switch

Question 45

What would cause the mix valve to motor to fully cold?

Answer 45

90°C duct overheat thermal switch

Question 46

What might cause the compressor discharge overheat sensor to operate?

Answer 46

A problem with the ram air inlet which has an effect on the primary heat exchanger

Question 47

What is the purpose of a three wheel air cycle machine?

Answer 47

Incorporates a fan which is used to induce airflow across the heat exchangers by its own action

Question 48

What two things would happen if the water extractor became blocked with ice?

Answer 48

Indicator piston moves from the white area into the red coloured area of the indicator cap Safety relief valve operates to ensure the flow of air into the cabin

Question 49

In a displacement blower system, which two components control the amount of air in the system?

Answer 49

Excess air is spilled off by opening or closing a spill valve Mass flow controller controls the spill valve

Question 50

What is the purpose of the choke valve in the displacement blower system?

Answer 50

Restricts the flow, thus producing a pressure rise to increase the air temperature

Question 51

What is latent heat?

Answer 51

Quantity of heat which causes a change in state of a substance without changing the temperature

Question 52

Which components control the amount of liquid refrigerant entering the evaporator?

Answer 52

Thermostatic expansion valve Temperature bulb

Question 53

What state is the refrigerant in when it enters the evaporator?

Answer 53

Mostly in liquid form

Question 54

Besides driving the refrigerant around the system, what other function does the compressor provide?

Answer 54

Lowers the pressure in the evaporator Supplies refrigerant to the condenser at a pressure and temperature at which it can be easily condensed back into liquid Raises the pressure of the refrigerant

Question 55

What state is the refrigerant in when it enters the condenser?

Answer 55

Hot gas

Question 56

What is the function of the receiver dryer?

Answer 56

Reservoir of the system

Question 57

Within the manifold set, what are the colours of the hoses for the high side, the low side and the centre hose?

Answer 57

High side – red Low side – blue Centre hose – yellow

Question 58

In a combustion heater, what would happen if the normal thermostat in the system failed?

Answer 58

There is a limit switch Closes fuel valve until overheat condition is corrected

Question 59

Why does the low side hose indicate a pressure below that of atmosphere?

Answer 59

It is the vacuum side