13.11.3 Pressurisation

Question 1

What is anoxia?

Answer 1

Complete absence of oxygen in the blood

Question 2

What would be the common symptoms of anoxia at 5,000 to 8,000 feet?

Answer 2

Some loss of night vision

Question 3

What are the most common problems caused by barotrauma (as the aircraft climbs or descends, gases trapped within bodies expand or contract)?

Answer 3

Aerotitus (air trapped in the middle ear)

Air trapped in paranasal sinuses by a blocked eustachian tube or sinuses

Pain experienced in the gastrointestinal tract or even the teeth (barodontalgia)

Question 4

In pressurised passenger aircraft, what is the acceptable rate of change of altitude in the cabin?

Answer 4

300 feet per minute

Question 5

What is the cabin altitude limit for use in pressurised aircraft and why?

Answer 5

8,000 feet

Vast majority of people suffer no discomfort due to anoxia at altitudes up to this height

Question 6

How is control of cabin pressure and rate of change of cabin pressure obtained?

Answer 6

Regulating the rate at which cabin air escapes from the cabin to atmosphere

Question 7

What is a dump valve?

Answer 7

Manually operated device by which the pressure differential can be rapidly reduced to zero

Question 8

What is positive differential pressure?

Answer 8

Condition in an aircraft where the pressure in the cabin is greater than local atmospheric pressure

Question 9

What is an alternative name for an outflow valve?

Answer 9

Discharge valve

Question 10

What are the typical maximum differential pressures for:

Jet transport aircraft

Turbo prop transport aircraft

Turbo–charged twin engine aircraft

Answer 10

Jet transport aircraft – 8–9.5 psi

Turbo prop transport aircraft – 5.5 psi

Turbo–charged twin engine aircraft – 4.5 psi

Question 11

What is negative differential pressure?

Answer 11

Cabin pressure is less than the local atmospheric pressure

Question 12

What is the normal maximum value of negative differential pressure of aircraft?

Answer 12

0.5 psi

Question 13

What is pressure altitude?

Answer 13

Altitude in the Standard Atmosphere corresponding to a particular pressure

Question 14

What is density altitude?

Answer 14

Altitude in the Standard Atmosphere corresponding to a particular value of air density

Question 15

How is the negative differential pressure controlled?

Answer 15

Inward relief valves

Question 16

How does the depressurising solenoid operate on the ground?

Answer 16

Solenoids are energised by weight on wheels switch

Bleed air through an ejector causes suction through depressurising solenoid from discharge valves, thus ensuring the discharge valves remain open

Question 17

What functions does a discharge valve fulfil?

Answer 17

Discharge valve
Safety valve
Inward relief valve
Ditching

Question 18

Explain the operation of the safety valve function of the pressurisation system?

Answer 18

Should cabin pressure become excessive due to failure of the pressure control equipment

Increased differential pressure across the pressure relief valve diaphragm opens the internal control valve

Air in the control chamber is allowed to bleed to atmosphere

Upper diaphragm moves as a result of this reduced control chamber pressure

Valve opens wide to allow the excess cabin pressure to be relieved to atmosphere

Question 19

What is the aircraft ideal rate of climb relative to cabin pressurisation?

Answer 19

Adjusted to arrive at the selected flight altitude slightly after the aircraft cabin reaches the height selected on the pressure controller

Question 20

What is isobaric control?

Answer 20

Cabin altitude reaches controller setting

Conditions are stabilised in the cabin with regard to pressure

Discharge valves will open and close to maintain the pressure to the equivalent controller setting

Question 21

Prior to descent, what is the cabin altitude selector set to and why?

Answer 21

Landing airfield height

Cabin pressure and atmospheric pressure will be equal on landing

Question 22

What three parameters is the pressure controller looking at and which is the overriding parameter?

Answer 22

Rate of change

Cabin altitude

Maximum differential pressure (most important parameter)

Question 23

What three instruments are normally used to monitor the cabin pressurisation system?

Answer 23

Cabin altimeter

Cabin altitude rate of change indicator

Differential pressure gauge

Question 24

What is the purpose of the cabin altitude warning light fitted to the instrument panel?

Answer 24

Indicate when cabin altitude reaches and exceeds 10,000 feet

Question 25

While in cruise, what change in aircraft height will cause no change to the corresponding cabin altitude?

Answer 25

Climb or descent of less than 500 feet

Question 26

Where pressurisation systems are tested, what must all persons on board be?

Answer 26

Medically fit

Free from colds and sinus troubles

Question 27

What pressure is the proof pressure test carried out at?

Answer 27

1.33 times the maximum operating differential pressure

Question 28

When might a proof test be required following major repairs or alterations to structural design?

Answer 28

Only if the CAA and manufacturer require it

Question 29

When ground testing a pressure cabin, what will the cabin rate of change indicator show as the differential pressure increases?

Answer 29

A rate of descent

Question 30

Why is it ideal when the aircraft rate of climb is adjusted to arrive at the selected flight altitude slightly after the aircraft cabin reaches its predetermined height?

Answer 30

If aircraft climbs too steeply, it will reach the maximum differential altitude before the cabin has a chance to reach its set altitude

In effect the cabin’s rate of climb will then have to match the aircraft’s rate of climb to keep within the safe envelope

Question 31

Why does the cabin altitude need to rise slower than the aircraft altitude?

Answer 31

Effect of rapid altitude changes on the human body is to cause physical pain and discomfort in the inner ear

Due to the pressure difference across the ear drum

Question 32

Which valve prevents differential pressures exceeding a safe maximum?

Answer 32

Outflow valve (in conjunction with the cabin pressure controller)

Cabin safety valve

Question 33

With an electronic pressurisation schedule, how much deviation in aircraft altitude would result in cabin altitude variation? How?

Answer 33

500 ft

Cabin altitude controller will compare the cabin altitude to that of the selected landing altitude