11.4 AIR CON & CABIN PRESSURE👌🏼 Flashcards
Where is pressurised air received from?
- Engines
- APU
- Ground source unit
What stages is air bled off the compressor ?
- Stage 5 (considered low pressure) and supplied by ‘low stage bleed port’
- Stage 9 ( considered high pressure) and taken from ‘high stage bleed port’
Low pressure air is used when the aircraft is ?
- Take off, climb, and cruise conditions
High pressure bleed air is used at?
- Low engine RPM speeds
- During descent or when low pressure air supply is inadequate
When bleed air from an engine compressor is possibly contaminated what could it contain?
- The air could contain Oils or Fuels
Explain engine-driven compressors?
- Bleeding a engine is inefficient
- Power reduction in engines due to reduced air available for combustion
- reduce engine service life
Compressors are divided into 3 groups which are?
- Positive displacement compressors (Roots-type blower)
- Centrifugal compressors
- Turbo compressors
Explain Positive Displacement (Roots type) Blower…
- predetermined volume of air, compresses it, then delivers to cabin duct
- each rotation adds more air and pressure increases
- pressure builds up as the blower can deliver more air than the system can use
- rotated by gearbox (wears gearbox)
Explain centrifugal cabin compressor?
- fundamentally an air pump
- outside air, is let into supercharger, compressed by high speed impeller, then delivered to distribution system
- engine driven (mounted in engine nacelle)
What are turbo compressors?
- engine power required to compress air
- engine drives turbo compressor
- small amount of high pressure air is drawn from engine compressor stage (5th or 9th)
What is a cabin air compressor?
- electrically powered
- supply outside air to each a/c packs
- powered by 3-phase motor
Explain air supply from the APU?
- Engine starting + ground a/c require high volume and high pressure pneumatic air
- small turbine engine driving an electric generator used to supply bleed air
APU uses 3 methods of supplying air for a/c and cabin pressurisation ?
- Bleed air extracted from compressor section
- Bleed air extracted from separate load compressor driven by turbine power section
- Bleed air extraction from compressor driven from the 2 stage axial turbine
What flight level specifically is APU bleed extraction limited to ?
- approx 22000ft
What is a ground cart air supply?
- consists of small jet engine (diesel ran)
- supplies sufficient air volume for engine starting
- supports a/c pack operation
- conditioned air gets pumped straight into cabin
ACS systems can produce what temperature range?
- 21-27 degrees Celsius
What are the 2 types of ACS systems?
- Air cycle machine (large aircraft)
- vapour cycle machines (small, medium aircraft)
ACS systems perform what functions?
- ventilated air
- supply cool/heated air
- cabin pressurisation
- equipment cooling
Where is the a/c pack located?
- lower half of fuselage
Bleed air from the engines, why can it not be used right away?
- air is too hot
Hot pressurised air arriving from aircraft bleed system enters where first?
- Primary heat exchanger
When you compress air what happens to pressure and temperature?
- pressure and temperature both increases
What is the compressor section powered by?
- powered by turbine section of the ACM
Why does the air have to go through a secondary heat exchanger ?
- as when air is compressed, temp increases which means it requires cooling again
What does the water extractor duct do?
- It removes condensed moisture when cooled by secondary heat exchanger
- moisture is directed to a water collector
Why is cooling done at a minimum at cruising altitude?
- due to low ambient temperatures at that flight level (8000ft)
What does the pack/flow control valve do?
- regulates bleeding air into a/c system
- controlled by switch on a/c panel in flight deck
- most are electronically controlled pneumatic operated
- spring loaded to close valve
What does the pack/flow control valve do?
- regulates bleeding air into a/c system
- controlled by switch on a/c panel in flight deck
- most are electronically controlled pneumatic operated
- spring loaded to close valve
If the aircraft is not fully loaded, all of the condition air is not required (T or F) ?
True
What are primary and secondary heat exchangers?
- allow heat transfer between 2 fluids
- in this case air
How does the Primary and Secondary heat exchangers work?
- cold ram air passes over heat exchanger
- hot bleed air tubes are cooled be cold ram air
Why are spray nozzles fitted in the ram air duct?
- extracted water sprays into ram air duct to improve cooling efficiency
Briefly explain ACM?
- consists of centrifugal compressors and expansion turbine
- air delivered from primary heat exchanger to compressor (reheated), then flows through secondary heat exchanger for extra cooling
- cooled high pressure air flows into expansion turbine, where large percentage of remaining energy is used to drive the compressor. As air expands across turbine, large decrease in pressure and temp
What are the 2 types of ACMs used?
- oil lubricated
- air lubricated
2 functions of condenser/reheater ?
- cool air in ac pack to allow moisture to be collected in the water collector
- heat air in ac pack to add energy to air. Helps operate turbine in ACM
What is water separator used for?
- removes water from saturated air before entering aircraft cabin
What are trim air valves?
- mix with condition air
- provide accurate temperature controlled air to cabin and flight deck
Where is the ram air door/valve located ?
- ‘wing to body fairing’ underside of aircraft
When is maximum ram air available ?
- on ground and during take off
What is a vapour cycle machine ACM ?
- transfers heat from inside cabin to outside cabin (typically used in piston engines)
- It functions like a fridge
What are the components of a vapour cycle machine ?
- refrigerant - working fluid from liquid to gas repeated
- Compressor - pressurises (temp rise)
- Condensor -
- Expansion valve - high pressure liquid passes through expansion valve (decrease pressure, temp)
- Evaporator - absorb heat from surrounding air
What should not be mixed in a refrigerant?
- R12 and R134a should not be mixed
How can the desired temp air come into cabin?
- By mixing conditioned air with bleed air
What distribution air system is located overhead and adjustable for passengers?
- gasper air outlet
What fan is controlled from the flight deck that supplies a steady stream of gasper air ?
- Inline fan
Explain air from ground source?
- can be heated or cooled (cabin)
- check valve used to prevent ground source air flowing upstream into ac system (conditioned air straight into cabin)
Why is Humidity control an advantage?
- Improve sleep
- reduce jet lag
- hydrated
What is air recirculating?
- recirculating air to save on fuel
- recirculation fans are electric motor-driven
Usually how much cabin air is filtered and recirculated?
Up to 50%
Why is pressurisation required at high altitude ?
- Passenger and flight crew comfort
Cabin pressure systems are designed to produce conditions equivalent to those of approximately 8000ft or less (T or F)
- True
At cruise explain air pressure outside and inside the aircraft ?
- Inside pressure > Outside pressure
At gate, explain air pressure inside and outside the aircraft?
- Inside pressure = outside pressure
What happens to temperature in the troposphere?
- altitude increases = temperature decreases
Tropopause temperature?
- constant temperature
Altitude and temperature related to stratosphere?
- altitude increases = temperature increases
What does cabin altitude mean?
- cabin pressure in aircraft at specific altitude above sea level
What is cabin differential pressure?
- difference in air pressure inside and outside the cabin
Formula for cabin differential pressure?
- cabin pressure - ambient pressure
What is cabin altitude change also known as?
- cabin vertical speed
What are the unpressurised areas on an aircraft?
- Radome
- Nose and Main bay
- tail cone
Aircraft cabin pressurisation can be controlled by 3 modes which are?
- Isobaric mode
- differential mode
- manual mode (emergency)
Explain Isobaric mode?
- maintain cabin altitude at single pressure despite aircraft changing altitude
Explain differential mode?
- maintain cabin pressure to a constant pressure difference between inside and ambient air pressure
Explain differential mode?
- maintain cabin pressure to a constant pressure difference between inside and ambient air pressure
Relationship between cabin altitude and cabin pressure?
- increase in cabin altitude = decrease in cabin pressure
- decrease in cabin altitude = increase in cabin pressure
Relationship between outflow valve and pressure?
- closing valve reduces outflow and increases pressure
- opening valve increases outflow and decreases pressure
What is an outflow valve?
- opens, closes to maintain correct air pressure in cabin
- usually operated pneumatically and electrically
What are the 2 safety valves?
- Positive and Negative
Explain the positive safety valve?
- Due to excessive pressure on inside (about 8-10psi)
- Used to relieve pressure from inside cabin
- valve open outside
Explain the negative safety valve?
- opens if outside pressure of aircraft exceeds inside pressure by 0.5 to 1.0 psi
- spring loaded relief valve
-allows ambient air to enter cabin (opens inward)
An aircraft is designed for cabin pressure > ambient pressure (T or F) ?
- True
Explain dump valves?
- used to quickly remove air from cabin (usually emergencies)
What are the 3 flight deck indications to advise flight crew of any pressurisation defect?
- Cabin altimeter
- Cabin rate of climb or vertical speed indicator
- Cabin differential pressure indicator
What is a cabin pressure controller?
- control cabin air pressure
What are RED indicator lights?
- warning system (critical, it requires attention immediately)
What could occur if aircraft bleed air is leaking
- damage to aircraft wiring
- components to overheat
- damage to aircraft structures
- fire
What warning systems is incorporated into bleed air system to allow flight crew to be alerted?
- gauges
- overheat detectors
- bleed air monitoring systems
