4 Flashcards
What are the 4 things air con systems control?
- Temperature
- Pressure
- Humidity
- Flow rate
Air from the 5th stage is considered what a supplied by what?
Low pressure and supplied by the low stage bleed port
Where is high pressure air taken from?
High stage bleed port
How many bleed ports are open at once?
One
How is air pressure adjusted?
Pressure Regulating Valve or Bleed Air Valve
These valves may also be used as what and how is it controlled?
Shut off, controlled by flight deck or automatically by pneumatic controller
What occurs when air is bled from the engine compressor?
Reduction in engine power
What are the 4 air con sources?
- Engine
- Compressor
- APU
-GPU
What are the 3 types of compressor?
- Positive displacement
- Centrifugal
- Turbo compressor
Why does pressure build up in a positive displacement blower?
This is because the blower can deliver more air than the system can use
Why is a positive displacement blower finned?
- Cooling
- Allow outside air through it
How does a centrifugal compressor work?
Atmospheric air comes into the impeller. The impeller condenses the air and distributes it.
When the engine is running at low RPM (less than idle) where is air taken from?
High Pressure port (HP port)
The supercharger has what mechanism to allow it adapt to fluctuations in engine RPM?
Variable ratio drive mechanism
How does the turbo compressor system work?
Bleed air drives a turbine, this directly drives a compressor attached along the same shaft as the turbine.
How is a turbo compressor beneficial to the engine?
Less power is taken from the engine
Where is a turbo compressor more common?
Smaller aircraft
Cabin Air Compressor is used for what purpose?
Apply outside air to each of the air con packs (Boeing thing)
When are APU’s designed to extract bleed air?
When the aircraft is on the ground
If an APU is designed for bleed air extraction in flight, what is the height limit for this?
22,000ft
APU systems use 3 methods for supplying air for air con and cabin pressure, what are they?
Bleed air extracted from:
- Compressor section
- Separate load compressor driven by the turbine power section
- Compressor driven from the two stage axial turbine
Does air from a ground cart supply pass through the AC packs?
No
What can cart air be used for?
Pneumatic system trouble shooting
Where is the air expanded?
In the turbine
The expansion of the air over the turbine creates what?
A temperature drop, primary method of supplying cool air to the aircraft
What does the ACS control?
Temperature and humidity
What does ACS stand for?
Air Cycle System
What are the two types of ACS?
- Air Cycle Machine
- Vapour cycle Machine
The majority of ACS systems are designed to do what?
- Supply ventilation air
- Supply heated or cooled air
- Cabin pressurisation
- Equipment cooling
Can bleed air be used for cabin air?
No it is too hot
What does the air con pack consist of?
- Heat exchangers
- Air cycle machines
Once air has passed through reheater for a second time it drives the turbine, what happens from here?
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
What valve regulates the amount of bleed air in the AC pack?
Pack or flow control valve
Bleed air is connected to what?
Common bleed air duct
What is the pack control valve connected to?
Common bleed air duct
What position is the pack control valve set to under abnormal conditions?
Closed position
The pack valve has a shut off function, what is this controlled by?
- Fire handle
- During engine start up
- Pack switch
What is the failsafe philosophy of the pack valve?
Open when electrical supply is broken
The manual override allows the valve to be fixed in what position?
Closed position
What is kept constant throughout the flight?
The volumetric flow
How can AC systems save energy?
Adjust the air flow to the level necessary for the amount of passengers on board
What type of heat exchangers are in an AC pack?
Air-to-air
What is used to induce adequate airflow when on the ground?
RAM impeller fan, driven by:
- Bleed air
- ACM drive shaft
- Electric motor
When does the air enter the secondary heat exchanger?
Prior to entering the compressor
What are the two types of ACM?
- Oil lubricated
- Air lubricated
What are the elements of an ACM?
- Turbine
- Compressor
- Sometimes an impeller fan
What is the boot strap system?
Most heat is removed by expansion.
The rest is removed by converting the heat energy to drive compressor
What are the two functions of the condenser/ reheater?
- Heating
- Cooling
How is water removed in the separator?
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
What is controlled by the temperature control valve?
- Anti ice of the ACM turbine
- Cabin air temp
Where is air added to trim the air temp values?
After the pack air outlet
What is the mix chamber/ manifold?
Collect the temp controlled air and distribute it to the flight compartments
How does RAM air enter?
RAM air door/valves
When are the RAM air doors closed?
During Take-off and landing roll
Vapour cycle machine air con systems are typically used where?
Piston aircraft
Why is the vapour cycle used on piston aircraft?
There is no bleed air
What are the 4 elements of the vapour cycle?
- Expansion
- Evaporation
- Compression
- Condensing
What does the blower do?
The blower pushes warm air over the evaporator, this cools the air down
Where does the evaporator get refrigerant from?
The expansion valve
What is the order of the refrigeration process?
- Compression
- Condensation
- Receiver dryer
- Expansion
- Evaporation
- Repeat
What is the receiver dryer?
The receiver dryer is a reservoir that stores refrigerant. When the air is hotter to begin with there is a need form more refrigerant.
What cools the gas in the condenser?
RAM air
Can a system that is designed for R12 refrigerant operate on R134A?
No
What is the refrigerant most commonly used now?
R134A
What is the trade name of refrigerant?
Freon
What are the two problems of water in the refrigerant?
- It can freeze in the pipe work
- Mixed with anti-freeze it forms an acid the degrades the material
What can been seen in the sight glass of the receiver dryer if there are low refrigerant levels or vapour?
Bubbles
Cool air is supplied to the cabin via what?
Ducts
What supplies the air to the overhead nozzles?
Gasper fan
What prevents the ground source air from travelling through the air con system?
Check valve
What air is mixed with the pack air in the mixer unit?
Recirculated air
How is used air drawn into the recirculating air tube?
Recirculating air fan
What is done to the recirculated air before it enters the mixer unit?
Filtered
Where does the exhaust air leave the cabin?
Return air grilles on the cabin floor
What is the max cabin altitude?
8000ft
What causes structural stresses inflight?
Pressure differential between cabin and outside
What areas of the aircraft are pressurised?
- Cockpit
- Cabin
- Forward and aft cargo bay
Aircraft cabin pressurisation can be controlled by what 3 different modes of operation?
- Isobaric
- Differential
- Manual
How does the isobaric mode work?
The pressure of the aircraft is kept at a constant altitude regardless of changing aircraft altitude
How do you find the differential pressure?
Cabin - Ambient
How is an air tight seal maintained?
Door seals, grommets, sealant
What is the cabin rate of climb?
Rate of change of cabin altitude
How does differential pressurisation mode work?
Maintains cabin pressure to a constant pressure difference between the cabin and ambient
What level is the constant differential kept at?
Below the max differential pressure
What does manual pressurisation mode allow?
Cabin isobaric control and rate of change control to be turned off
During an emergency with the pressurisation system on manual mode, what control remains on?
Max differential control
What problem may occur if the mode is left on isobaric?
Max differential pressure may be exceeded
What will occur if the max differential is exceeded while in isobaric mode?
Automatically switch to constant differential mode
What two things control the cabin altitude used?
- Max differential pressure
- Passenger comfort
How is air discharged from the aircraft?
Outflow valve
What does the outflow valve modulate?
Cabin air pressure
What position will the airflow valve be in if cabin altitude needs to increase?
Open
What additional modes do outflow valves with pneumatic and electrics have?
- Pneumatic standby
- Manual
In addition to the outflow valves, what other safety devices are required?
- Positive and negative safety valves
- Dump valves
- Pressurisation gauges
When will the positive safety valve open?
When the max differential pressure is exceeded
When will the negative pressure valve open?
When the outside pressure is greater than the pressure inside the cabin
0.5 to 1.0 psi
What is the purpose of the dump valve?
Rapidly remove air and air pressure from cabin
What are the 3 pressurisation gauges?
- Cabin altimeter
- Cabin rate of climb or V/S indicator
- Cabin differential pressure indicator
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?
Aircraft climb rate
Pressurisation checks should be done how often and check for what?
Periodically, check for leaks and see if pressure control components operate correct
What do leak detectors utilise to help find leaks?
Thermistors
What are two other methods of leak detection?
- Apply soapy solution
- Leak detection dyes
How should HP and LP systems be tested?
Separately
What is the purpose of the yellow hose?
Vacuum and replenish vapour cycle fluid
What controls the cabin pressure?
Cabin Pressure Controller
How many CPC’s are installed on an aircraft?
2, one is failsafe
Where are cabin pressure controllers located?
Usually out of sight near the flight deck, but inside the pressurised portion of the aircraft
What is the “excessive differential pressure relief valves” commonly referred to as?
Positive pressure relief valves
What is the most obvious sign of a leak in a vapour cycle air conditioning system?
Low refrigerant level
What computer follows the pressurisation profile for flight?
CPC
The majority of air conditioning pack operations are protected by?
By thermal switch monitoring
How are bleed air leaks detected?
Leak or heat sensors
If a leak is detected what will occur?
Flight crew alerted, then the pipe will be isolated
The lines on both sides of the receiver dryer should both be?
The same temperature
In the vapour cycle system, what should the low side lines be?
Cold
On hot humid days, why may the cooling output be comprised?
Large volume of water condensing on the evaporator
