13.11.3 Pressurisation Flashcards
What is anoxia?
Complete absence of oxygen in the blood
What would be the common symptoms of anoxia at 5,000 to 8,000 feet?
Some loss of night vision
What are the most common problems caused by barotrauma (as the aircraft climbs or descends, gases trapped within bodies expand or contract)?
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)
In pressurised passenger aircraft, what is the acceptable rate of change of altitude in the cabin?
300 feet per minute
What is the cabin altitude limit for use in pressurised aircraft and why?
8,000 feet
Vast majority of people suffer no discomfort due to anoxia at altitudes up to this height
How is control of cabin pressure and rate of change of cabin pressure obtained?
Regulating the rate at which cabin air escapes from the cabin to atmosphere
What is a dump valve?
Manually operated device by which the pressure differential can be rapidly reduced to zero
What is positive differential pressure?
Condition in an aircraft where the pressure in the cabin is greater than local atmospheric pressure
What is an alternative name for an outflow valve?
Discharge valve
What are the typical maximum differential pressures for:
Jet transport aircraft
Turbo prop transport aircraft
Turbo–charged twin engine aircraft
Jet transport aircraft – 8–9.5 psi
Turbo prop transport aircraft – 5.5 psi
Turbo–charged twin engine aircraft – 4.5 psi
What is negative differential pressure?
Cabin pressure is less than the local atmospheric pressure
What is the normal maximum value of negative differential pressure of aircraft?
0.5 psi
What is pressure altitude?
Altitude in the Standard Atmosphere corresponding to a particular pressure
What is density altitude?
Altitude in the Standard Atmosphere corresponding to a particular value of air density
How is the negative differential pressure controlled?
Inward relief valves
How does the depressurising solenoid operate on the ground?
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
What functions does a discharge valve fulfil?
Discharge valve
Safety valve
Inward relief valve
Ditching
Explain the operation of the safety valve function of the pressurisation system?
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
What is the aircraft ideal rate of climb relative to cabin pressurisation?
Adjusted to arrive at the selected flight altitude slightly after the aircraft cabin reaches the height selected on the pressure controller
What is isobaric control?
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
Prior to descent, what is the cabin altitude selector set to and why?
Landing airfield height
Cabin pressure and atmospheric pressure will be equal on landing
What three parameters is the pressure controller looking at and which is the overriding parameter?
Rate of change
Cabin altitude
Maximum differential pressure (most important parameter)
What three instruments are normally used to monitor the cabin pressurisation system?
Cabin altimeter
Cabin altitude rate of change indicator
Differential pressure gauge
What is the purpose of the cabin altitude warning light fitted to the instrument panel?
Indicate when cabin altitude reaches and exceeds 10,000 feet
While in cruise, what change in aircraft height will cause no change to the corresponding cabin altitude?
Climb or descent of less than 500 feet
Where pressurisation systems are tested, what must all persons on board be?
Medically fit
Free from colds and sinus troubles
What pressure is the proof pressure test carried out at?
1.33 times the maximum operating differential pressure
When might a proof test be required following major repairs or alterations to structural design?
Only if the CAA and manufacturer require it
When ground testing a pressure cabin, what will the cabin rate of change indicator show as the differential pressure increases?
A rate of descent
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?
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
Why does the cabin altitude need to rise slower than the aircraft altitude?
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
Which valve prevents differential pressures exceeding a safe maximum?
Outflow valve (in conjunction with the cabin pressure controller)
Cabin safety valve
With an electronic pressurisation schedule, how much deviation in aircraft altitude would result in cabin altitude variation? How?
500 ft
Cabin altitude controller will compare the cabin altitude to that of the selected landing altitude
