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CAA Module 11 > Module 11.4 Air con & cabin pressure > Flashcards

Module 11.4 Air con & cabin pressure Flashcards

1
Q

What are the 4 things air con systems control?

A
  • Temperature
  • Pressure
  • Humidity
  • Flow rate
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2
Q

Air from the 5th stage is considered what a supplied by what?

A

Low pressure and supplied by the low stage bleed port

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3
Q

Where is high pressure air taken from?

A

High stage bleed port

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4
Q

How many bleed ports are open at once?

A

One

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5
Q

How is air pressure adjusted?

A

Pressure Regulating Valve or Bleed Air Valve

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6
Q

These valves may also be used as what and how is it controlled?

A

Shut off, controlled by flight deck or automatically by pneumatic controller

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7
Q

What occurs when air is bled from the engine compressor?

A

Reduction in engine power

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8
Q

What are the 4 air con sources?

A
  • Engine
  • Compressor
  • APU
    -GPU
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9
Q

What are the 3 types of compressor?

A
  • Positive displacement
  • Centrifugal
  • Turbo compressor
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10
Q

Why does pressure build up in a positive displacement blower?

A

This is because the blower can deliver more air than the system can use

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11
Q

Why is a positive displacement blower finned?

A
  • Cooling
  • Allow outside air through it
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12
Q

How does a centrifugal compressor work?

A

Atmospheric air comes into the impeller. The impeller condenses the air and distributes it.

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13
Q

When the engine is running at low RPM (less than idle) where is air taken from?

A

High Pressure port (HP port)

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14
Q

The supercharger has what mechanism to allow it adapt to fluctuations in engine RPM?

A

Variable ratio drive mechanism

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15
Q

How does the turbo compressor system work?

A

Bleed air drives a turbine, this directly drives a compressor attached along the same shaft as the turbine.

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16
Q

How is a turbo compressor beneficial to the engine?

A

Less power is taken from the engine

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17
Q

Where is a turbo compressor more common?

A

Smaller aircraft

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18
Q

Cabin Air Compressor is used for what purpose?

A

Apply outside air to each of the air con packs (Boeing thing)

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19
Q

When are APU’s designed to extract bleed air?

A

When the aircraft is on the ground

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20
Q

If an APU is designed for bleed air extraction in flight, what is the height limit for this?

A

22,000ft

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21
Q

APU systems use 3 methods for supplying air for air con and cabin pressure, what are they?

A

Bleed air extracted from:
- Compressor section
- Separate load compressor driven by the turbine power section
- Compressor driven from the two stage axial turbine

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22
Q

Does air from a ground cart supply pass through the AC packs?

A

No

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23
Q

What can cart air be used for?

A

Pneumatic system trouble shooting

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24
Q

Where is the air expanded?

A

In the turbine

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25
Q

The expansion of the air over the turbine creates what?

A

A temperature drop, primary method of supplying cool air to the aircraft

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26
Q

What does the ACS control?

A

Temperature and humidity

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27
Q

What does ACS stand for?

A

Air Cycle System

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28
Q

What are the two types of ACS?

A
  • Air Cycle Machine
  • Vapour cycle Machine
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29
Q

The majority of ACS systems are designed to do what?

A
  • Supply ventilation air
  • Supply heated or cooled air
  • Cabin pressurisation
  • Equipment cooling
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30
Q

Can bleed air be used for cabin air?

A

No it is too hot

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31
Q

What does the air con pack consist of?

A
  • Heat exchangers
  • Air cycle machines
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32
Q

Once air has passed through reheater for a second time it drives the turbine, what happens from here?

A

The air loses energy due to spinning the turbine. This air is pushed to the mixer unit where bleed air is mixed in to get the correct temperature for the cabin

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33
Q

What valve regulates the amount of bleed air in the AC pack?

A

Pack or flow control valve

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34
Q

Bleed air is connected to what?

A

Common bleed air duct

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35
Q

What is the pack control valve connected to?

A

Common bleed air duct

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36
Q

What position is the pack control valve set to under abnormal conditions?

A

Closed position

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37
Q

The pack valve has a shut off function, what is this controlled by?

A
  • Fire handle
  • During engine start up
  • Pack switch
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38
Q

What is the failsafe philosophy of the pack valve?

A

Open when electrical supply is broken

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39
Q

The manual override allows the valve to be fixed in what position?

A

Closed position

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40
Q

What is kept constant throughout the flight?

A

The volumetric flow

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41
Q

How can AC systems save energy?

A

Adjust the air flow to the level necessary for the amount of passengers on board

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42
Q

What type of heat exchangers are in an AC pack?

A

Air-to-air

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43
Q

What is used to induce adequate airflow when on the ground?

A

RAM impeller fan, driven by:
- Bleed air
- ACM drive shaft
- Electric motor

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44
Q

When does the air enter the secondary heat exchanger?

A

Prior to entering the compressor

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45
Q

What are the two types of ACM?

A
  • Oil lubricated
  • Air lubricated
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46
Q

What are the elements of an ACM?

A
  • Turbine
  • Compressor
  • Sometimes an impeller fan
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47
Q

What is the boot strap system?

A

Most heat is removed by expansion.

The rest is removed by converting the heat energy to drive compressor

48
Q

What are the two functions of the condenser/ reheater?

A
  • Heating
  • Cooling
49
Q

How is water removed in the separator?

A

The air is spiralled, this leads to the heavier air to separate from the lighter moisture. The heavier air moves to the outside of the separator and the moisture collects on the fibreglass sock

50
Q

What is controlled by the temperature control valve?

A
  • Anti ice of the ACM turbine
  • Cabin air temp
51
Q

Where is air added to trim the air temp values?

A

After the pack air outlet

52
Q

What is the mix chamber/ manifold?

A

Collect the temp controlled air and distribute it to the flight compartments

53
Q

How does RAM air enter?

A

RAM air door/valves

54
Q

When are the RAM air doors closed?

A

During Take-off and landing roll

55
Q

Vapour cycle machine air con systems are typically used where?

A

Piston aircraft

56
Q

Why is the vapour cycle used on piston aircraft?

A

There is no bleed air

57
Q

What are the 4 elements of the vapour cycle?

A
  • Expansion
  • Evaporation
  • Compression
  • Condensing
58
Q

What does the blower do?

A

The blower pushes warm air over the evaporator, this cools the air down

59
Q

Where does the evaporator get refrigerant from?

A

The expansion valve

60
Q

What is the order of the refrigeration process?

A
  1. Compression
  2. Condensation
  3. Receiver dryer
  4. Expansion
  5. Evaporation
  6. Repeat
61
Q

What is the receiver dryer?

A

The receiver dryer is a reservoir that stores refrigerant. When the air is hotter to begin with there is a need form more refrigerant.

62
Q

What cools the gas in the condenser?

A

RAM air

63
Q

Can a system that is designed for R12 refrigerant operate on R134A?

A

No

64
Q

What is the refrigerant most commonly used now?

A

R134A

65
Q

What is the trade name of refrigerant?

A

Freon

66
Q

What are the two problems of water in the refrigerant?

A
  • It can freeze in the pipe work
  • Mixed with anti-freeze it forms an acid the degrades the material
67
Q

What can been seen in the sight glass of the receiver dryer if there are low refrigerant levels or vapour?

A

Bubbles

68
Q

Cool air is supplied to the cabin via what?

A

Ducts

69
Q

What supplies the air to the overhead nozzles?

A

Gasper fan

70
Q

What prevents the ground source air from travelling through the air con system?

A

Check valve

71
Q

What air is mixed with the pack air in the mixer unit?

A

Recirculated air

72
Q

How is used air drawn into the recirculating air tube?

A

Recirculating air fan

73
Q

What is done to the recirculated air before it enters the mixer unit?

A

Filtered

74
Q

Where does the exhaust air leave the cabin?

A

Return air grilles on the cabin floor

75
Q

What is the max cabin altitude?

A

8000ft

76
Q

What causes structural stresses inflight?

A

Pressure differential between cabin and outside

77
Q

What areas of the aircraft are pressurised?

A
  • Cockpit
  • Cabin
  • Forward and aft cargo bay
78
Q

Aircraft cabin pressurisation can be controlled by what 3 different modes of operation?

A
  • Isobaric
  • Differential
  • Manual
79
Q

How does the isobaric mode work?

A

The pressure of the aircraft is kept at a constant altitude regardless of changing aircraft altitude

80
Q

How do you find the differential pressure?

A

Cabin - Ambient

81
Q

How is an air tight seal maintained?

A

Door seals, grommets, sealant

82
Q

What is the cabin rate of climb?

A

Rate of change of cabin altitude

83
Q

How does differential pressurisation mode work?

A

Maintains cabin pressure to a constant pressure difference between the cabin and ambient

84
Q

What level is the constant differential kept at?

A

Below the max differential pressure

85
Q

What does manual pressurisation mode allow?

A

Cabin isobaric control and rate of change control to be turned off

86
Q

During an emergency with the pressurisation system on manual mode, what control remains on?

A

Max differential control

87
Q

What problem may occur if the mode is left on isobaric?

A

Max differential pressure may be exceeded

88
Q

What will occur if the max differential is exceeded while in isobaric mode?

A

Automatically switch to constant differential mode

89
Q

What two things control the cabin altitude used?

A
  • Max differential pressure
  • Passenger comfort
90
Q

How is air discharged from the aircraft?

A

Outflow valve

91
Q

What does the outflow valve modulate?

A

Cabin air pressure

92
Q

What position will the airflow valve be in if cabin altitude needs to increase?

A

Open

93
Q

What additional modes do outflow valves with pneumatic and electrics have?

A
  • Pneumatic standby
  • Manual
94
Q

In addition to the outflow valves, what other safety devices are required?

A
  • Positive and negative safety valves
  • Dump valves
  • Pressurisation gauges
95
Q

When will the positive safety valve open?

A

When the max differential pressure is exceeded

96
Q

When will the negative pressure valve open?

A

When the outside pressure is greater than the pressure inside the cabin

0.5 to 1.0 psi

97
Q

What is the purpose of the dump valve?

A

Rapidly remove air and air pressure from cabin

98
Q

What are the 3 pressurisation gauges?

A
  • Cabin altimeter
  • Cabin rate of climb or V/S indicator
  • Cabin differential pressure indicator
99
Q

During take off and climb the outflow valves are positioned to control the rate of change of cabin altitude, what should the position be in proportion to?

A

Aircraft climb rate

100
Q

Pressurisation checks should be done how often and check for what?

A

Periodically, check for leaks and see if pressure control components operate correct

101
Q

What do leak detectors utilise to help find leaks?

A

Thermistors

102
Q

What are two other methods of leak detection?

A
  • Apply soapy solution
  • Leak detection dyes
103
Q

How should HP and LP systems be tested?

A

Separately

104
Q

What is the purpose of the yellow hose?

A

Vacuum and replenish vapour cycle fluid

105
Q

What controls the cabin pressure?

A

Cabin Pressure Controller

106
Q

How many CPC’s are installed on an aircraft?

A

2, one is failsafe

107
Q

Where are cabin pressure controllers located?

A

Usually out of sight near the flight deck, but inside the pressurised portion of the aircraft

108
Q

What is the “excessive differential pressure relief valves” commonly referred to as?

A

Positive pressure relief valves

109
Q

What is the most obvious sign of a leak in a vapour cycle air conditioning system?

A

Low refrigerant level

110
Q

What computer follows the pressurisation profile for flight?

A

CPC

111
Q

The majority of air conditioning pack operations are protected by?

A

By thermal switch monitoring

112
Q

How are bleed air leaks detected?

A

Leak or heat sensors

113
Q

If a leak is detected what will occur?

A

Flight crew alerted, then the pipe will be isolated

114
Q

The lines on both sides of the receiver dryer should both be?

A

The same temperature

115
Q

In the vapour cycle system, what should the low side lines be?

A

Cold

116
Q

On hot humid days, why may the cooling output be comprised?

A

Large volume of water condensing on the evaporator

CAA Module 11 (22 decks)

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