Pressurization Flashcards
What is the pressure differential measured in?
Pounds per square inch
When does NO pressure differential exist?
When cabin altitude and airplane altitude are the same
When is pressure differential a positive number?
When cabin pressure is greater than airplane altitude
When is cabin pressure a negative number?
If cabin pressure is less than that of the outside ambient air
What is maximum differential pressure defined as?
A measure of the highest positive differential pressure the airplane structure can safely withstand
What is the differential pressure that’s maintained?
+5.0 +- 0.1 PSI
Is the airplane structure designed to withstand a negative differential?
No it is not
What is used to pressurize the vessel?
The bleed air from the compressor section of each engine
What controls the flow of the bleed air to be mixed with ambient air for pressurization?
The fuel control unit (FCU)
After the bleed air is mixed with the ambient air, where does it flow to next?
To the bleed air shutoff valve, which is controlled by the BLEED AIR VALVES switch in the cockpit
What occurs when the BLEED AIR VAVLE switch is in the OFF position?
The solenoid is closed and no bleed air can enter the flow control unit or the cabin
What occurs when the BLEED AIR VALVE switch is ON?
The solenoid is electrically held open and the mixture flows through the valve to keep the flow control solenoid open
With the bleed air valve switch on, what would happen to the solenoid if there were an electrical failure?
The solenoid would close and no more bleed air would enter the pressure vessel and the cabin pressure would leak out - since it is electrically controlled
After the air passes through the bleed air valve, where does it go?
Into the environmental bleed air duct and then that air is mixed with the recirculated cabin air in the mixing plenum, then the air flows into the cabin at the floor registers.
After the pressurized air is into the cabin, where does it go?
It flows out of the pressure vessel through the outflow valve.
At what PSI is air delivered from both FCUs into the pressure vessel?
Approximately 14 pounds per minute
What can control the pressure within the cabin and the rate of cabin pressure changes?
Pneumatic modulation of the outflow valve
A safety valve is mounted adjacent to the outflow valve. It has 3 functions:
- Provide pressure relief in the event of malfunction of normal outflow valve
- Allow depressurization of the pressure vessel whenever cabin pressure switch is moved into the dump position
- Keeps the pressure vessel unpressurized while the airplane is on the ground, with the left landing gear safety switch compressed
What’s the purpose of the negative-pressure relief that’s incorporated into the outflow and safety valves?
It prevents outside atmospheric pressure from exceeding cabin pressure by more than 0.1 PSI during rapid descents, even if bleed air flow increases
What happens on liftoff with the bleed air valve OPEN?
The landing gear safety switch is no longer compressed, allowing the safety valve to close and then the ambient air shutoff solenoid valve in the left FCU opens, then 6 seconds later…. the valve in the right FCU opens
By opening the volume of airflow into the pressure vessel in stages, excessive pressure bumps during takeoff are avoided
When selecting a rate for the cabin pressure to climb at, what would be the normal setting?
9 o’clock to 12 o’clock
How many selectors does the cabin pressure switch have?
3 - DUMP, PRESS, TEST
When the cabin pressure switch is in the DUMP position, what happens?
The safety valve is held open so the cabin will depressurize and/or remain unpressurized
When the cabin pressure switch is in the PRESS position, what is happening?
The safety valve is normally closed in flight and the outflow valve is controlled by the pressurization controller so that the cabin will pressurize
When the cabin pressure switch is in the TEST position, what’s happening?
The safety valve is held closed, bypassing the landing gear safety switch, to facilitate testing of the pressurization system on the ground
How can you test the pressurization system?
With the bleed air valves open, adjust cabin altitude selector knob so that CABIN ALT dial indicates an altitude 500’ below field pressure altitude. Rotate the selector knob to place the index between 9 o’clock and 12 o’clock positions. Move both condition levers to hi-idle. Hold cabin pressurization switch to TEST. Check cabin climb indicator for a descent indication. Release pressurization switch to PRESS position when pressurizing is confirmed and move both condition levers to their original position.
For in flight operations, if the cabin pressure altitude reaches a value of over 10,000’, what will happen?
A pressure sensing switch will close - causing the red ALTITUDE WARN annunciation light to illuminate
During flight, if the flight plan calls for an altitude change of 1,000’ or more, reselect the new altitude plus ______ on the CABIN ALT dial
500’
During descent and in preparation for landing, set the cabin altitude selector to indicate a cabin altitude of approximately ______ above field elevation
500’
