Pneumatics - Aircon and Pressurisation

Question 1

What cabin altitude are large jet transport aircraft usually pressurised to?

Answer 1

8000ft

Question 2

At what rate and quantity should air be provided to pax?

Answer 2

1lb per person per minute, at 18°C-24°C, with a minimum relative humidity of 30%

Question 3

Even though 60% humidity would be ideal, why do airlines rarely supply this?

Answer 3

Not practical to carry humidifiers and large quantities of water

Question 4

At what CABIN altitude will the cabin altitude warning horn sound?

Answer 4

10,000ft

Question 5

At what CABIN altitude will oxygen masks be automatically deployed?

Answer 5

14,000ft

Question 6

What is a typical maximum pressure differential?

Answer 6

8-9 psi. (typical at 8000ft cabin alt for 40,000ft actual alt).

Question 7

In regards to cabin pressurisation, what does AUTO mode require?

Answer 7

Intended cruising altitude, and destination elevation

Question 8

In regards to cabin pressurisation, what does STBY mode require?

Answer 8

Need to manually input cabin altitude in accordance with a placard, and rate of change also needs to be set.

Question 9

Why does a MANUAL mode of cabin pressurisation exist?

Answer 9

So that the inflow/outflow valves can be controlled in the event that AUTO and STBY modes are unserviceable.

Question 10

What is the purpose of the positive pressure relief valve?

Answer 10

Relieves pressure differential that could cause structural damage. Lets air OUT of the cabin

Question 11

What would happen if the pressure relief valve was not working?

Answer 11

Once the aircraft climbed to the selected altitude, any further climb would cause the cabin to climb at the same rate as the aircraft.

Question 12

What is the purpose of the negative pressure relief valve?

Answer 12

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.

Question 13

How many positive pressure relief valves are typically fitted?

Answer 13

2

Question 14

Are the outflow valves powered by AC or DC?

Answer 14

AC if in AUTO mode or MANUAL AC MODE, otherwise DC for STBY and MANUAL DC MODE

Question 15

Where does the air pressure data come from in AUTO mode?

Answer 15

Directly from the static ports to the pressure controller

Question 16

Where does the air pressure data come from in STBY mode?

Answer 16

ADC with barometric adjustment from the Captain’s altimiter.

Question 17

Where does the air pressure data come from in Manual mode?

Answer 17

ADC with barometric adjustment from the FO’s altimeter.

Question 18

What is the purpose of the air/ground logic system in regards to aircon?

Answer 18

Depressurises the aircraft when on the ground

Question 19

At what rate does pressurisation typically occur?

Answer 19

500ft/min during climb, and 350ft/min during descent

Question 20

Why does the aircraft landing elevation need to be set 200ft below the actual elevation?

Answer 20

To ensure a pressurised cabin during landing

Question 21

If using manual mode, would you use AC or DC?

Answer 21

AC mode is faster.

Question 22

The bleed air must be cooled before entering the cabin, what are the two main types of aircon cycles?

Answer 22

Vapour (refrigerant/freon) or air cyle.

Question 23

Describe a vapour cycle?

Answer 23

Not as common on larger aircraft. Consists of a compressor, evaporator, and condenser. Pretty much heat is exchanged with vapour that condenses.

Question 24

Describe the air cycle system

Answer 24

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

Question 25

Between the vapour cycle and air cycle system, which is more efficient?

Answer 25

The vapour cycle system

Question 26

What is the purpose of the air mix valves?

Answer 26

In an air cycle system to mix bleed air with RAM air

Question 27

What is one limitation of the air cycle system?

Answer 27

During ground ops or slow flight, additional bleed air is required from the engine.

Question 28

What can be done to reduce the load on the engines from bleed air operation?

Answer 28

Recirculate the air, but this reduces air quality.

Question 29

Under what conditions can the ram air valve be opened directly to supply air for the cabin?

Answer 29

In an unpressurised flight during an emergency (e.g. to get rid of smoke/toxic fumes)

Question 30

Are the aircon packs dependent?

Answer 30

No, and if one fails the other is capable of carrying the load

Question 31

What is a gasper fan?

Answer 31

Think of it as independent climate control. Each crew member and passenger can select their own temperature, and the system can be used continosusly.

Question 32

When will the ram air door be fully open?

Answer 32

During slow flight to maximise cooling

Question 33

What is the deflector door for?

Answer 33

Prevents slush and FOD entering the RAM air ducts. Operates automatically.