39 Questions
What is the primary function of the shut-off valve?
To control the air flowing through to the flow control valve
What triggers the shut-off valve to open?
An electrical signal from the air conditioning control panel
What is the function of the metering valve ball?
To meter the air flow into the piston chamber
What is the purpose of the actuating piston?
To open the butterfly valve, allowing bleed air to flow through the system
What is the function of the spring load on the piston?
To calibrated the metering valve to bleed air into the piston chamber
What does the flow control valve maintain?
A pressure ratio between the bleed air pressure and the cabin air pressure
What is the purpose of the pressure ratio limiting unit?
To maintain a pressure ratio between the bleed air pressure and the cabin air pressure
What is the source of the bleed air that enters the air cycle system?
The aircraft's engines or auxiliary power unit
What are the three difficulties that have been solved with the advent of aircraft environmental systems?
Effects of altitude, low temperature, and low humidity
What is the primary function of air-conditioning in an aircraft?
To condition the air for human habitation
What is the result of a decrease in partial pressure of oxygen in the lungs?
Decreased oxygen absorption
What is the approximate oxygen pressure at sea level?
3 PSI
What is the term for the lack of sufficient oxygen reaching the cells in the body?
Hypoxia
What is the purpose of pressurisation in an aircraft?
To combat low oxygen pressure
What percentage of the total air pressure is exerted by oxygen at any given altitude?
One-fifth
What is the consequence of hypoxia?
Decreased oxygen in the blood
What is the primary function of the temperature selector in the cabin temperature control panel?
To select a desired cabin temperature
In automatic mode, what receives inputs from the temperature selector and temperature sensors?
The centralised environmental controller
What happens when the mode selector is set to manual?
The mixing valves are electrically driven by the switch position
What is the main cause of dehydration at high altitude?
Excess water loss through breathing
What type of switch is used in the manual mode?
Spring-loaded to the centre position
What is the ideal relative humidity for human comfort?
Between 30% and 70%
What is recommended when using manual temperature control?
Make small changes to the temperature setting
Why is it necessary to increase the moisture content of the conditioned air in an aircraft at high altitude?
To overcome physical discomfort arising from low humidity
What type of aircraft typically has a simplified control panel?
Fighter aircraft
What is the purpose of the indicators in the cabin temperature control system?
To advise the aircrew of system parameters
What is the purpose of a heat exchanger in a pressurisation system?
To cool the pressurisation air
What type of information may be displayed on the indicators in some larger aircraft?
The mixing valve positions and temperature gauges
What is the ideal temperature range for human comfort?
Between 20°C and 30°C
What is the effect of compression on air temperature?
It increases the temperature
What is the primary factor that determines the source of air to pressurise an aircraft?
Engine type
What happens when the mode selector is moved to 'manual'?
The centralised environmental controller is disabled
What is the primary function of the air distribution system?
To divide conditioned air to the airplane zones
What happens to the air in the cabin after it is released from ceiling vents?
It circulates and flows out floor-level vents
Where is the ducting typically located in the aircraft?
Below the cabin floor and behind walls and ceiling panels
What is the purpose of the air distribution system in relation to lavatories and galleys?
To remove offensive air from lavatories and galleys
What is the result of the air flowing through the cabin and baggage compartments?
The air flows out of the pressure vessel through the outflow valve
What should be done when using manual temperature control?
Make small changes to the temperature setting
What is the characteristic of the air flow in the cabin?
It is nearly imperceptible
Study Notes
Shut-off Valve
- The shut-off valve controls air flow to the flow control valve and its design varies depending on the type and application.
- When bleed air is required, an electrical signal is sent to the solenoid, which moves the metering valve ball, allowing bleed air to flow.
Flow Control Valve
- The flow control valve maintains a pressure ratio between bleed air pressure and cabin air pressure.
- It solves problems caused by altitude, low temperature, and low humidity.
Effects of Altitude
- Hypoxia occurs when the lungs do not absorb sufficient oxygen due to decreased oxygen pressure.
- Oxygen pressure is approximately 1/5 of total air pressure, and at sea level, it is sufficient to saturate the blood.
- At high altitude, oxygen pressure decreases, leading to hypoxia.
Air Conditioning
- Air conditioning conditions air for human habitation and combats low oxygen pressure.
- Conditioned air is treated and circulated throughout the flight deck and cabin.
Humidity and Comfort
- At high altitude, air is dry, and humidity is low, leading to dehydration.
- The comfort zone for humans is 20°C to 30°C with 30% to 70% relative humidity.
Pressurisation Systems
- Sources of pressurised air vary with engine type.
- Air is cooled using heat exchangers or cold ambient air.
Cabin Temperature Control
- The control panel varies depending on the aircraft type.
- In automatic mode, the system processes inputs from temperature sensors and sends drive signals to mixing valves.
- In manual mode, the system is controlled by the temperature selector switch.
Air Distribution System
- The system distributes conditioned air to airplane zones, reduces engine bleed requirements, and removes offensive air from lavatories and galleys.
- Air is ducted from the pressurisation source into and throughout the cabin.
Learn about the design and operation of shut-off valves in aviation technical systems, including their role in controlling air flow to flow control valves.
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