Cabin Pressurization

5 Answers

1. 
   

   
   

   Kent Shook on Nov 19, 2010

   Are you asking about cabin pressure in pressurized airplanes (like jets), or are you asking solely about the static system in an unpressurized airplane? Your subject line led me to believe the former, while most of the message content appears to be geared toward the latter – So I’m going to go with the latter.

   The pressure inside the airplane is always lower than the pressure outside when airborne due to airflow around the airplane. Unpressurized airplane cabins have enough ports to the outside that even when climbing, the interior pressure is lower than exterior pressure. So, the pressure inside the airplane isn’t going to be greater than the pressure outside.

   Yes, you can break the face of the VSI if there’s a static blockage and you don’t have an alternate static source that you can switch to.

   “If the pressure on the INSIDE of the plane is different from that on the outside (where the static port is located), how does cracking the VSI help the altimeter and airspeed indicator function again? Wouldn’t it get a more inaccurate reading?”

   If the static port is blocked, and you climb or descend, the altimeter will not move at all because the pressure in the static system will remain unchanged. While the pressure in the cabin is not the same as the pressure outside, it will move in a similar fashion and give you at least some indication of your altitude and airspeed. When you break the VSI, it introduces static pressure from the cabin back into the system.

   As for “why crack the VSI” – Which of the three pitot-static instruments would you want to be without? The altimeter is the most important, followed by the airspeed indicator, with the VSI coming in a distant third.

   For more information, see Chapter 7 of the Pilot’s Handbook of Aeronautical Knowledge at http://www.faa.gov/library/manuals/aviation/pilot_handbook/media/PHAK%20-%20Chapter%2007.pdf

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2. 
   

   
   

   Anonymous1 on Nov 19, 2010

   Kent:
   Thank you for your response.  However, I should have clarified.  I WAS asking about pressurized aircraft in the FIRST half of my question.
   On page 6-32 of the Pilot’s Handbook of Aeronautical Knowledge, it states “In a typical pressurization system, the cabin, flight compartment and baggage compartments are incorporated into a sealed unit capable of containing air under a pressure higher than the outside atmospheric pressure.”
   Therefore, I am assuming that in PRESSURIZED AIRCRAFT, the pressure is higher INSIDE the plane than on the outside because it is kept that way by design.  It’s not that additional pressure is pumped INTO the plane, it’s just “sealed” to contain higher pressure.  Is this statement accurate?  I know, however, that oxygen IS pumped into the plane due to the decrease of O2 at higher altitudes.
   In an UNPRESSURIZED AIRCRAFT, if there were no ports to the outside, pressure WOULD be higher inside the cabin, correct?  However, I am still am confused regarding one issue.  I would have thought the pressue on the inside of an unpressurized aircraft would be the same pressure than on the outside.
   For instance, I would assume that since pressure decreases with altitude, the pressure inside the plane would decrease at the same rate. Thus, equalizing the pressure.
   However, I am gathering that your point about increased airflow around the plane  would create a “vaccum” of low pressure immediately around/inside the plane.  This increased airflow is due to the fact that the plane is moving at a high rate of speed through the air…thus the air is exerting “more physical pressure” on the plane despite the atmospheric pressure being lower.

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3. 
   

   
   

   Flying Ace on Nov 19, 2010

   Also, by how many psi’s does pressure decrease per 1,000 feet?

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4. 
   

   
   

   Brian on Nov 19, 2010

   …Qt: Flying Ace…Also, by how many psi’s does pressure decrease per 1,000 feet?…

   Google: “inches mercury to psi”
    

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5. 
   

   
   

   Wesley Beard on Nov 20, 2010

   On a pressurized airplane
   When the aircraft doors are sealed shut, the pressurized cabin is fairly sealed from air leaking to the outside.  However, there is a published leak rate for every pressurized airplane should the pressurization fail.  On the Challenger 604 that I fly, it is around 1500 feet per minute.
    
   The way the pressurization system works on most, if not all, jets is like this.  The jet engines compress the outside air (in the compression stage).  There are bleed air lines that take a part of the pressurized air for the environmental system.  The other part goes to the third stage in the jet engine.  The bleed air is super hot around 700 degrees F due to amount of compression.  This air is routed to the air conditioning unit.  The ACU expands the super hot air (cooling it down) and routes it over a heat exchanger and then compresses the air again.  This time the air is around 380 degrees F.  This process is repeated once more and the end result is cooled bleed air around 37 degrees F.  This is obviously too cold for most people so, the designers take a little of that really hot (700 deg F) and mix it with the cooled air to get the correct temperature selectable by the first officer.
    
   This air is brought into the cockpit and the cabin to pressurize the airplane.  It is important to note that the air comes in at a constant rate no matter what happens.  There are outflow valves that regulate the cabin pressure.  These outflow valves will vary how wide open they are to keep a constant pressure inside the airplane.  It is important to note that most outflow valves holes are the size of softballs and air is always escaping through them to the outside.  Thus the air is constantly recirculated with fresh air.  The outflow valves have other functions to protect how high the cabin pressure of the airplane can rise or how low the cabin pressure can go.
    
   To answer your questions:  The pressure inside a pressurized airplane is always higher than outside pressure.  Pressurized, cool air is always coming into the cabin and the outflow valves are regulating the cabin altitudes.  Some airplane can have a pressure differential of 9 psi which is an airplane flying at 45,000 ft with a cabin pressure of 8,000 ft.
   As the airplane climbs, the outflow valves automatically adjust to keep the cabin pressure at a predetermined altitude or climb rate (usually 600 feet per minute) to a set altitude.  During the descent, the outflow valves vary to keep the cabin pressure at a set value again.
    
   The ratio of oxygen to other gases is the same at higher altitudes as lower altitudes and it is around 21% O2, 78% N2, 1% other gases.  What changes is the partial pressure of the oxygen.  Without going into human physiology, that lower partial pressure is unusable so we need to compress the air to force our bodies to use it.  We aren’t pumping in pure O2 into the cabin but simply compressing the outside air so our bodies can use it.
    
   Remember, we are talking about pressurized airplanes, if the static ports are clogged, breaking the VSI will immediately move the altimeter your cabin altitude and your airspeed indicator will show incorrect readings.  The reason to break the VSI, is that it is not required to be in the airplane per 91.205 day, night or IFR equipment.  The other five of the six pack are required in at least one kind of operation.

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