3.2 Blog Discussion: Aircraft Systems and Flight
Cabin Pressurization System
Planes may love to go high, but the
human body does not. Our bodies need oxygen, and while in flight many planes
are equipped with a cabin pressurization system to meet that need. As aircraft
travel higher in altitude the air becomes less dense (FAA, 2016). This decrease
in density results in less oxygen available, and can lead to hypoxic effects like
confusion, inability to concentrate, and unconsciousness (Skybrary, 2019). To prevent
this situation, aircraft that operate frequently above 10,000’ utilize a cabin
pressurization system.
One of my favorite Instructor Pilots always said, when it comes to cabin pressurization “we mindlessly flood the cabin, and brilliantly meter the rest”. On a Beechcraft King Air 200, the cabin vessel is a sealed system with bleed air routed from the last stage of the compressor (CAE, 2017).
As this major consumer of bleed air
mindlessly floods the cabin during a climb in altitude, it begins to build a
pressure differential in comparison to the less dense air outside, effectively
keeping the occupants at a lower altitude and protecting against the effects of
hypoxia (FAA, 2016). From this point the air is “brilliantly metered” out with
the cabin controller which is essentially an altimeter for pilots to set the
cabin altitude, which then works with the outflow valve to allow air to escape
as metered by the cabin controller (FAA, 2016).
Cabin Pressurization Diagram (Photo Credit CAE 2017)
If this outflow valve were to fail a redundancy is in place known as the safety valve, which once the max cabin pressure differential becomes exceeded the safety valve will allow air to escape the cabin preventing over pressurization (FAA, 2016).
There a
couple ways failure of the cabin pressurization system can occur in flight: a
break in the pressure vessel (i.e. the cabin door opening in flight), a loss of
bleed air being supplied (i.e. engine failure, which then no longer is providing
air to the pressurization), or the most common is that the pressurization system
never gets turned on in the first place. If a loss of pressurization due to the
pressure vessel integrity were to occur, it is imperative the occupants of the
aircraft don oxygen masks quickly, as the time of useful consciousness varies
with altitude (Skybrary, 2019). At 35,000’ there is 30-60 seconds, at 25,000’ there
is 3-5 minutes, and at 15,000’ there is 30 min or more; it is important to note
time of useful consciousness does not mean time until unconsciousness, but the
point at which an individual is no longer capable of normal corrective actions (Skybrary,
2017). Many companies offer hypobaric chamber training to simulate these rapid
decompression events. Once the plane is below 10,000’ pressurization is no
longer critical, the critical part is to get the aircraft to a safe altitude without
the crewmembers becoming inoperative.
An example
of the pressurization system never getting turned and the effects of
unpressurized flight at critical altitudes sadly occurred on Helio Airways
Flight 522 in 2005 (Skybrary, 2017). When intercepted at 34,000’ by two military
aircraft flight 522 was observed to have incapacitated pilots (Skybrary, 2017).
The flight soon after impacted terrain and all 121 souls on board were lost (Skybrary,
2017). Through investigation it was found the pressurization panel was never
set to auto, the cabin was never pressurized, and hypoxia set in (Skybrary, 2017).
The most
effective mitigation strategy for this event is adherence to the flight
checklist items. As a result of this accident Boeing made changes to the flight
crew procedures modifying normal checklists (Skybrary, 2017).
References
Administration, F. A. (2016). Pilot’s
Handbook of Aeronautical Knowledge: FAA-H-8083-25B (ASA FAA Handbook Series)
(2016th ed.). Aviation Supplies & Academics, Inc.
B733, en-route,
northwest of Athens Greece, 2005 - SKYbrary Aviation Safety. (2017).
SkyBrary.Aero.
https://skybrary.aero/index.php/B733,_en-route,_northwest_of_Athens_Greece,_2005
C-12U1 Aircraft
Systems Handout (Version 1.0 ed.). (2017). C.A.E.
Hypoxia - SKYbrary
Aviation Safety. (2019). SkyBrary.Aero. ahttps://skybrary.aero/index.php/Hypoxia
Time of Useful
Consciousness - SKYbrary Aviation Safety. (2017). SkyBrary.Aero.
https://skybrary.aero/index.php/Time_of_Useful_Consciousness
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