Airfly From United Kingdom, joined Mar 2005, 322 posts, RR: 2 Posted (4 years 4 months 2 weeks 13 hours ago) and read 2316 times:
Hello guys,
I love stuff about Air pressure and others...
But the question is: how is the pressure controlled, Where is it controlled in the cockpit?
As I know cabin pressure is controlled in a high dense above 10.000ft But is there any minor adjustments before take off or anything related to the climbing rate?
What would be the exact measurement at about 30,000ft? about 9psi?
Is Hypoxia in a highly range as the airplane descends from a high cruising alt? or would it be most common on a normal cruising rate?
777WT From United States of America, joined Jan 2005, 861 posts, RR: 5 Reply 1, posted (4 years 4 months 2 weeks 13 hours ago) and read 2310 times:
Cabin pressure is controlled by a outflow valve in the back of the aircraft.
It dumps excessive pressure in what it was set for.
And the pressure is a constant supply of bleed air from the main engines.
There's isobaric, density and ambient settings for cabin rate of climb.
Cabin pressure is set no higher than 6,000 to 8,000 ft above sea level.
When take off is started and the ground switch opens, the cabin pressure dump valve closes and the pressurization is about to begin but will not really be up to it's set pressure until it reaches near 10,000 ft which most of the pressurization will start to take place.
Once the plane lands, the ground switch closes and the cabin pressure is released at landing.
Each pressurization bleed connection from the engines to the a/c packs has check valves in it. So when engine failure occurs, the check valve closes and prevents cabin pressure from coming out the opposite way that it should when the engine is running. At the same time the outflow valve will close to keep cabin pressure steady as possible. But like all aircrafts, they have a limit on the cabin pressure leak.
There are tests carried out to pressurize the aircraft on the ground to check for leaks by connecting a pressurized source of air and set the aircraft up into flight mode that it thinks it's airborne while it's on the ground.
Hypoxia is a problem above 10,000 ft and up no matter climbing or diving or crusing.
The first thing to do in cabin pressure loss is don the oxygen mask and drive quickly to 10,000 ft and all will be fine.
For example, in a 747, it's banked 40-50 degrees and the nose is let to dive and the PIC rides the redline down to 10,000 ft then level off.
Bio15 From Colombia, joined Mar 2001, 1052 posts, RR: 9 Reply 2, posted (4 years 4 months 2 weeks 12 hours ago) and read 2301 times:
Pressurization is obtained by pumping air into the cabin, and it's amount is determined by how fast this air is let out, via outflow valves. This air will generally be extracted from the engines at some stage where fuel has not been added (so it's clean air) but at some point when the air is highly compressed and thus hot. Before being pumped into the cabin it has to be cooled, so it goes through the air conditioning units, usually located on the aircraft's belly. Once the air is at a proper temperature, it goes into the cabin.
If the outflow valve is fully open, there is no resistance to the air coming into the cabin, so there's no pressurization. As you close the valve it will be harder for the air to go out, so the pressure will increase in the cabin.
The cabin has to be pressurized to maintain the amount of oxigen in the cabin at aceptable levels, but keeping the cabin pressure at sea level during cruise altitude creates too much differential pressure and the fuselage could pop. So, during the climb the cabin has a pressurization climb rate, usually given in fpm, just as the vertical speed indicator. This is a different rate than the one the aircraft is climbing at, usually smaller. It represents what you would feel if climbimg at that rate in an unpressurized aircraft. Too high, and your ears will hurt.
The pressurization panel will allow the pilot to set a cabin climb or descent rate to keep things comfortable in the cabin while preventing the fuselage from popping (by increasing cabin altitude at a healthy rate). I'm not certaing about the differential pressure figures, but I guess it can go up to 12? Hope it helps
The pressurization panel is on the center right at the very bottom, you can see the climb rate, cabin alt, and diff. pressure gauges. My guess for maximum diff. pressure was a bit high! It shows 8 psi to be pretty high.
AirWillie6475 From , joined Dec 1969, posts, RR: Reply 3, posted (4 years 4 months 2 weeks 7 hours ago) and read 2243 times:
I'm no Airline Pilot but I have a few flight videos where they explain the pressure system on airplanes. Cabin altitude ranges from 4400 feet(27K feet actual A/C altitude) to 8000 feet(41K feet actual A/C altitude). I don't think any plane goes above 8000 feet as physically weaker people in planes would start to feel uncomfortable. The good news is that if you are in a Hypoxic situation at high altitudes, 13k feet and above, and you start to be unconscious it won't kill you untill you get to around 22000 feet.
Midnights From United States of America, joined Aug 2005, 62 posts, RR: 0 Reply 4, posted (4 years 4 months 2 weeks 6 hours ago) and read 2229 times:
We do checks of the pressurization system on the ground quite often on through trips. If the flight crew squaks a door squeal is the usual one. I have had an MD-80 up to 8 PSI differential pressure.It's always fun to dump the last 4 to 5 psi as fast as you can to fog the cabin. If you are quick enough you can get it to snow in the cabin....hurts like hell though.
Zeke From Hong Kong, joined Dec 2006, 4983 posts, RR: 65 Reply 5, posted (4 years 4 months 2 weeks 4 hours ago) and read 2217 times:
On airbuses its all automatic, the flight management guidance computers (FMGC) provide the cabin pressure controller with the cruise altitude and landing elevation, and qnh.
Pilots can regulate the cabin climb and desent rate in automatic mode through the FMGC (FMC) normally its left alone at 350 ft/min.
It does have a manual mode where the vales can be opened or closed, or the landing elevation set manually.
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HAWK21M From India, joined Jan 2001, 28091 posts, RR: 60 Reply 6, posted (4 years 4 months 2 weeks 3 hours ago) and read 2203 times:
B737 Pressurisation Wikipedia link
In short Air flows in to the Aircraft from the AC Packs & the Main outflow valve releases the Airflow from the Pressurised zone to determine the Cabin Alt by mantaining the constant Differential Pressure at that altitude.
regds
MEL
Woodreau From United States of America, joined Sep 2001, 775 posts, RR: 8 Reply 7, posted (4 years 4 months 1 week 6 days 19 hours ago) and read 2179 times:
Just to correct a minor statement Bio15 said...
The amount of oxygen in the air is constant whether at sea level or 50,000ft - oxygen comprises approx 20-21% of the air at all altitudes. It's just that above 14,000ft the partial pressure of oxygen is insufficent for the lungs to extract it from the air hence the need to pressurize the cabin at higher altitudes.
For the CRJ-200, the air for pressurization and air conditioning is extracted from the 10th stage high pressure compressor, it comes out of the engine at 370C and 170psi, and enters the 10th stage bleed air manifold. Bleed air entering the pressurization/air conditioning system is regulated by a PRSOV to 30-41psi. From there it goes into a precooler where the bleed air is cooled by heat exchanger (cooling from outside ram air) and then into the primary heat exchanger (more cooling from outside ram air). After being cooled it goes into a compressor where it is compressed (and picks up heat from the compression) and then it continues to the secondary heat exchanger where it is cooled again from the outside ram air. After the leaves the secondary heat exchanger it proceeds to the expansion turbine (which turns the compressor) where the pressure of the air drops from work being extracted out of it and most of the temperature drop occurs here. There is no refrigerant or freon used to cool the air - it's all PV=NRT from physics.
After this point it is mixed with uncooled bleed air to the appropriate temperature and it then goes to a moisture separator to remove excess moisture. Now the conditioned air goes into the distribution manifold for distribution into the cabin or cockpit. The cabin pressure is controlled by outflow valves (as explained in previous posts) which determines how fast the air is let out of the cabin.
Good judgement comes from experience. Experience comes from surviving bad judgement.
Bio15 From Colombia, joined Mar 2001, 1052 posts, RR: 9 Reply 8, posted (4 years 4 months 1 week 6 days 14 hours ago) and read 2143 times:
Quoting Woodreau (Reply 7): The amount of oxygen in the air is constant whether at sea level or 50,000ft - oxygen comprises approx 20-21% of the air at all altitudes.
You're correct when saying that the amount of oxygen in the air is constant. But the amount of oxygen in the cabin, as I mentioned, is actually less because it is a constant volume, and there is less air since it is expanded at such heigth. The air/oxygen proportion remains constant, but it occupies a larger volume, thus in constant volume lungs you will have less air and therefore less oxygen.
My mistake is in not mentioning that the pressure has to be high for another thing also, that is for there to be oxygen exchange within the lungs and bloodstream. Thanks for clearing it up.
HAWK21M From India, joined Jan 2001, 28091 posts, RR: 60 Reply 9, posted (4 years 4 months 1 week 6 days 3 hours ago) and read 2098 times:
Quoting Bio15 (Reply 8): You're correct when saying that the amount of oxygen in the air is constant. But the amount of oxygen in the cabin, as I mentioned, is actually less because it is a constant volume, and there is less air since it is expanded at such heigth. The air/oxygen proportion remains constant, but it occupies a larger volume, thus in constant volume lungs you will have less air and therefore less oxygen.
Im sure Recirculation Fans contribute to the ratio too.
regds
MEL
Bri2k1 From United States of America, joined Dec 2004, 898 posts, RR: 4 Reply 11, posted (4 years 4 months 1 week 4 days 16 hours ago) and read 2023 times:
One intersting point: The seemingly small number of 8 PSI is not insignificant. Consider the surface area of a large aircraft, in square inches. It's huge! Multiply that by 8 pounds, and you have an incredible force pushing against the plane. This is why cabin doors can't be opened in flight -- they must be pulled slightly inwards first, and even a small door at cruise altitude with pressurization requires literally thousands of pounds of force to move.
Obviously, the space shuttle does not pressurize the ambient air, because there isn't any. The crew compartments are kept at a constant pressure from liftoff. Airlocks permit transfer from the pressurized area to the space environment. Oxygen tanks store extra breathable "air," but the primary source of oxygen generation is through ionization powered by fuel cells. This is remarkable technology considering the space shuttles were initially designed over 40 years ago!
Widebody From Ireland, joined Aug 2000, 1128 posts, RR: 11 Reply 12, posted (4 years 4 months 1 week 4 days 11 hours ago) and read 1992 times:
The force is evident when you look at the amount of warping/deformation the structure sees when the aircraft is in-flight. Causes lots of hassle though for the design of some components/systems such as overhead bins, galleys, lavatories, air conditioning + ducts etc. The dimensions on the ground are not the same as in-flight!
Loggy From United Kingdom, joined Oct 2005, 15 posts, RR: 0 Reply 13, posted (4 years 4 months 1 week 3 days 22 hours ago) and read 1952 times:
Interesting, I always thought the reason you have to sit in a hot cabin till takeoff was because there wasn't enough electrical or mechanical power to switch on a conventionally powered AC system, I didn't realize aircraft AC is 'realtime' as it were, and it seems the air is also exchanged a few times with outside fresh air, I always imagined breathing recycled, re-conditioned air. How many times during a three hour flight would the air be changed ?.
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