Pneumatics - Aircon and Pressurisation Flashcards
What cabin altitude are large jet transport aircraft usually pressurised to?
8000ft
At what rate and quantity should air be provided to pax?
1lb per person per minute, at 18°C-24°C, with a minimum relative humidity of 30%
Even though 60% humidity would be ideal, why do airlines rarely supply this?
Not practical to carry humidifiers and large quantities of water
At what CABIN altitude will the cabin altitude warning horn sound?
10,000ft
At what CABIN altitude will oxygen masks be automatically deployed?
14,000ft
What is a typical maximum pressure differential?
8-9 psi. (typical at 8000ft cabin alt for 40,000ft actual alt).
In regards to cabin pressurisation, what does AUTO mode require?
Intended cruising altitude, and destination elevation
In regards to cabin pressurisation, what does STBY mode require?
Need to manually input cabin altitude in accordance with a placard, and rate of change also needs to be set.
Why does a MANUAL mode of cabin pressurisation exist?
So that the inflow/outflow valves can be controlled in the event that AUTO and STBY modes are unserviceable.
What is the purpose of the positive pressure relief valve?
Relieves pressure differential that could cause structural damage. Lets air OUT of the cabin
What would happen if the pressure relief valve was not working?
Once the aircraft climbed to the selected altitude, any further climb would cause the cabin to climb at the same rate as the aircraft.
What is the purpose of the negative pressure relief valve?
Allows air into the cabin so that the air outside is not at a higher pressure than inside the cabin by a value of 0.5 PSI. Would make the doors dangerous to open on the ground.
How many positive pressure relief valves are typically fitted?
2
Are the outflow valves powered by AC or DC?
AC if in AUTO mode or MANUAL AC MODE, otherwise DC for STBY and MANUAL DC MODE
Where does the air pressure data come from in AUTO mode?
Directly from the static ports to the pressure controller
Where does the air pressure data come from in STBY mode?
ADC with barometric adjustment from the Captain’s altimiter.
Where does the air pressure data come from in Manual mode?
ADC with barometric adjustment from the FO’s altimeter.
What is the purpose of the air/ground logic system in regards to aircon?
Depressurises the aircraft when on the ground
At what rate does pressurisation typically occur?
500ft/min during climb, and 350ft/min during descent
Why does the aircraft landing elevation need to be set 200ft below the actual elevation?
To ensure a pressurised cabin during landing
If using manual mode, would you use AC or DC?
AC mode is faster.
The bleed air must be cooled before entering the cabin, what are the two main types of aircon cycles?
Vapour (refrigerant/freon) or air cyle.
Describe a vapour cycle?
Not as common on larger aircraft. Consists of a compressor, evaporator, and condenser. Pretty much heat is exchanged with vapour that condenses.
Describe the air cycle system
Consists of bleed air, heat exchanger, turbines and compressors. Literally expands and contracts the air to cool it (expanded air is cooler), and combines this air with ram air
Between the vapour cycle and air cycle system, which is more efficient?
The vapour cycle system
What is the purpose of the air mix valves?
In an air cycle system to mix bleed air with RAM air
What is one limitation of the air cycle system?
During ground ops or slow flight, additional bleed air is required from the engine.
What can be done to reduce the load on the engines from bleed air operation?
Recirculate the air, but this reduces air quality.
Under what conditions can the ram air valve be opened directly to supply air for the cabin?
In an unpressurised flight during an emergency (e.g. to get rid of smoke/toxic fumes)
Are the aircon packs dependent?
No, and if one fails the other is capable of carrying the load
What is a gasper fan?
Think of it as independent climate control. Each crew member and passenger can select their own temperature, and the system can be used continosusly.
When will the ram air door be fully open?
During slow flight to maximise cooling
What is the deflector door for?
Prevents slush and FOD entering the RAM air ducts. Operates automatically.
