Mr Spaceman From Canada, joined Mar 2001, 2777 posts, RR: 18 Posted (10 years 11 months 1 week 3 days 13 hours ago) and read 2917 times:
Hi guys. After reading some recent posts and associated links, I've been wondering about some aspects of cabin pressurization.
With the understanding that an airliner's cabin is pressurized to around 8,000 ft msl while cruising enroute, my question is....When do the pilots pressurize the cabin?
Do they start to pressurize the cabin as soon as the doors are closed and the engines have been started, or perhaps while they are taxiing to the active runway?
Do the pilots wait untill they are climbing out and approaching 8,000 ft before they pressurize the cabin?
How long does it take to pressurize the cabin? (I suspect this might vary, do to different sizes).
The main reason why I'm asking this question, is because I was thinking that pressurizing the cabin to 8,000 ft while on the ground at an airport with an elevation that's close to sea level, would cause stress on the airframe. The outside air would be pushing in against the fuselage. However, this level of stress could be nothing compared to when the jet is at FL 410.
CV640 From United States of America, joined Aug 2000, 946 posts, RR: 6 Reply 1, posted (10 years 11 months 1 week 3 days 13 hours ago) and read 2773 times:
AT the airline I am at I fly the SF340. We normally open ethe bleed valves, which takes air from the turbines and is used to pressurize the aircraft, at 400 feet Above ground level. This can vary if there is an obstacle clearance or it is very cool, then we take off with them on and it starts pressurizing the second teh gear is up. Most aircraft have their pressurization systems connected to their gear. If their is weight on it, on the ground, the aircraft will not pressurize or if it is, if landed while pressuized, it will depressurize automatically. In the Saab if this did it happen it will take place over 60 seconds.
As for Cabin alititude that is more a function of cabin pressure differential. Most aircarft have a cabin differential Pressure limit, on the Saab it is 7.5 PSI, although the system is not supposed to allow more then 7.1 PSI differential.
Differntial Pressure is the difference between pressure outside and inside the cabin. An example of this if you are at 30,000 feet in an aircraft with a cabin differential pressure of 6.5, the cabin is at an altitude of 8,000 feet. So cabin altitude can vary from aircrfat to aircraft. we usually just set he landing airporst field elevation in the system and the rest is automatic. It is designed so that when we touch down the cabin should be at the same altitude as the airport. Whether it is 200 feet MSL or 9000 feet MSL.
Mr Spaceman From Canada, joined Mar 2001, 2777 posts, RR: 18 Reply 2, posted (10 years 11 months 1 week 3 days 12 hours ago) and read 2733 times:
Thanks for your reply CV640. That's great information. I keep on learning from you guys about all the different systems that are connected to the landing gear. It helps explain what all those accessories are used for, that you can see attached to the gear struts.
Would I be correct in thinking that most airline aircraft, from the Regionals to the Heavies, probably use the same procedure or one very similar, when it comes to pressurizing their cabins?
CV640 From United States of America, joined Aug 2000, 946 posts, RR: 6 Reply 3, posted (10 years 11 months 1 week 3 days 9 hours ago) and read 2707 times:
I have only flown a few pressurized aircraft but with talking with crewson the CRJ and other aircraft that seems to be the way it is done everywhere. Most aircraft have some sort of relief system in case they land presuurized, with out this you couldn't open the doors. Now that would be very embarassing.
767hvydoc From , joined Dec 1969, posts, RR: Reply 4, posted (10 years 11 months 1 week 3 days 8 hours ago) and read 2713 times:
Without getting too long winded, I have a few additions. Most Boeing Aircraft do takeoff & land UN-pressurized. This will allow the fuselage to flex somewhat during these stressful transition times... that is, when the weight of the aircraft goes from being on the gear to the wings and back. The stretched DC8's would do the same, but the reason was because the fuselage was soooo long (I believe that it is still the longest narrow body aircraft made... yes, longer than the 757-300) that it needed to flex between the nose and tail ends so that it wouldn't crack.
I know that the A300-600 takeoff and landing is done pressurized. I can't say why. There was an incident not too long ago (and the NTSB wants changes made to the airplane because of this) when a flight attendant opened a door of a still pressurized A300-600 on the ground. The attendant was sucked out very quickly, and fell to their death. The A300-600 is supposed to depressurize during the taxi to the gate.
Most all pressurization is computer controlled today. There is an entire map schedule that the computer will follow to maintain certain cabin levels at certain flight levels. Ultimately, most pressurization on large jets is not controlled by the air being pumped in, but by how the air that is pumped in is allowed to escape. This is called an outflow valve. The more it is opened, the more air escapes, the cabin pressure goes down, the higher the cabin alt will be. Close the valve down some, less air escapes, the cabin pressure goes up, effectively reducing cabin altitude.
Also, (last thing, I swear!) the air that is pumped in does come off of the compressor section of the engine. Then the air conditioning sytem dresses it up (dries it, cleans it and adjusts the temp) and puts it into the cabin. If you used the air just as it comes out of the compressor section, I think it would be... ahem... uncomfortable.
JT-8D From United States of America, joined Dec 2000, 423 posts, RR: 4 Reply 5, posted (10 years 11 months 1 week 3 days 8 hours ago) and read 2702 times:
Space, it seems that differant airlines do it differant ways. Our planes begin to pressurize when the takeoff roll is started. On the dc-9s, switches built into the pedestal close when the thrust levers are pushed forward to take off. The outflow valve will close, and the plane will pressurize to about .125 psi. As far as how long it takes, thats a bit more complex. The pressurization controller(s) will control the rate of cabin climb (pressure drop) based on the info entered by the crew before the flight. If the crew dials in that the cruise alt will be 35,000, then the controller will plan to have a specific differential pressure (lets just say 7 psi) at that altitude. Think of a graph with 2 bars on it. The left is altitude of the plane, the right is cabin alt. Now think of both bars moving up, but the right one moves slower and will stop at a preset cabin alt (lets say 10,000 ft) at about the same time as the left one reaches 35,000. In the event that this plan is not followed, there will be some method of warning the crew. On a lot of the Boeing planes, there is a light marked "off scedule decent", that tells the crew that something just aint right. On the dc-9, there is a "flow" light. This indicates that the outflow valve is fully closed, indicating that either there is low airflow into the cabin, or the plane is leaking too fast for the air conditioning to keep up. There are a few backup systems to it. On the 727, and others, there is auto mode, standby mode and a/c, and d/c manual. Auto is like I described, standby will require the crew to control cabin climb rate manually (think of changing the speed of the right bar on the graph), but will still limit maximum differential pressure. In manual mode, the outflow valve position is controlled by hand, and is "steered" by watching the climb rate indicator. Crews tell me this is a major pain in the butt. A D/C manual mode is added in case of loss of all A/C power. On planes with 2 controllers, one will be in control, while the other watches. If there is a fault in the controlling one, the system will usually switch over to the other controller. On the dc-9, there is a switch to do this manually if the crew isnt satisfied with the performance of the system. Hope I answered your question, if not, ask..JT
727PFE From , joined Dec 1969, posts, RR: Reply 6, posted (10 years 11 months 1 week 3 days 5 hours ago) and read 2680 times:
All good information. Several of the A/C I've flown would pressurize on the ground to .125 psi. The pressure kept the outflow valves closed and prevented pressure "bumps" that might be uncomfortable to the passengers. The .125 psi differential could be easily overcome when opening the cabin door. It seemed to depend on the manufacturer, whether or not the jet pressurized on the ground or on takeoff roll, or even after takeoff through the gear safety switches.
JT-8D From United States of America, joined Dec 2000, 423 posts, RR: 4 Reply 7, posted (10 years 11 months 1 week 3 days 5 hours ago) and read 2658 times:
727PFE, welcome to the family--lol. I saw that this is your first post..JT
Chdmcmanus From United States of America, joined Mar 2001, 374 posts, RR: 3 Reply 8, posted (10 years 11 months 1 week 3 days 4 hours ago) and read 2656 times:
727, Ditto on the welcome, not many of us FE's left out there.
JT hit the nail on the head. As far as additional stress from being at sea level or 41K, differential pressure is just that, a difference in pressure. Basically if you have 8.6 psid, whether it is 8.6 higher than 14.7 or 2.1, the stress is still only 8.6 psi. The technical name for the method most airliners use is "isobaric-differential". This method provides the best combination of passenger comfort and fuselage stress.
JT-8D From United States of America, joined Dec 2000, 423 posts, RR: 4 Reply 9, posted (10 years 11 months 1 week 3 days 4 hours ago) and read 2647 times:
Isobaric curve, thats the term I was trying to remember, thanks chd..JT
Mr Spaceman From Canada, joined Mar 2001, 2777 posts, RR: 18 Reply 10, posted (10 years 11 months 1 week 2 days 11 hours ago) and read 2621 times:
Hi guys. Like always you're feeding me excellent info!
Another question has entered my mind...Can you pressurize the cabin with the bleed air from only 1 engine? I suspect the system would be designed that way for safety reasons. In other words...What would happen if your 757 lost an engine at altitude, and the pressurization system required both engines to be running.
Welcome aboard 727PFE. You mentioned that you pressurize your aircraft on the ground (I think it was to .125 psi differential), to help stop the passengers from feeling pressure "Bumps". What are pressure bumps?
JT-8D From United States of America, joined Dec 2000, 423 posts, RR: 4 Reply 11, posted (10 years 11 months 1 week 2 days 7 hours ago) and read 2609 times:
When we pressurize for maint purposes, we use the apu for air. Some planes takeoff with "bleeds off" and use the apu as well. The engines would be able to supply enough air one at a time, and most planes use only one source in flight anyway. This is to save fuel. Bleed air=money. Pressure bump is the change in cabin press when the plane first leaves the ground. By pre-pressurizing, there will be no change until ordered by the press controller..JT
JETPILOT From United States of America, joined May 1999, 3128 posts, RR: 37 Reply 12, posted (10 years 11 months 1 week 2 days 5 hours ago) and read 2615 times:
The 727 pressurizes to .125 difff. on the ground. After all th engines are started the enginer will perform his afterstart flow which includes pressurizing the aircraft. The PAC's go on and the outflow valve is modualted either electricly with the electronic pressurization or in auto mode on the pneumatic pressure controller by dialling the cabin altitude 200 ft below field elevation.
After takeoff the electronic pressure controller will schedual the cabin climb. The pneumatic pressure controller should be set to the flight altitude after climb power has been set.
The 727 should be able to hold SLP up to 24,000 feet if the airframe is well sealed.
The outside air doesn't push in against the fusealage as you described, but instead the inside air presses out.
Pressurizing the cabin is a constant process until the plane reaches its final cruising altitude. The pressurization schedual if working correctly will reach 8.6 PSI diff when you level off. At 35,000 feet your cabin altitude will be about 6,000 feet.
VC-10 From United Kingdom, joined Oct 1999, 3656 posts, RR: 42 Reply 13, posted (10 years 11 months 1 week 2 days 4 hours ago) and read 2595 times:
JT,
All the a/c I have been involved with will bleed air off all the available engines. In fact on the A320 if you only have one bleed air source available you are restricted to a max Flt Alt of 31500 ft.
JETPILOT From United States of America, joined May 1999, 3128 posts, RR: 37 Reply 14, posted (10 years 11 months 1 week 2 days 3 hours ago) and read 2598 times:
The 727 only bleeds off engines 1 and 3 normally. Two can be used in case of an engine failure.
JT-8D From United States of America, joined Dec 2000, 423 posts, RR: 4 Reply 15, posted (10 years 11 months 1 week 1 day 15 hours ago) and read 2579 times:
VC-10. The 737 has a "dual bleed" light to make sure the crew knows that they have 2 bleed sources being used. The light is blue in color, and Im sure you know this means that its just for information, not a safety thing. The same restrictions apply to the 737 (and others) for mel purposes. If they lost the remaining bleed source in flight (or airconditioning system), it would be difficult to maintain pressurization, thus the alt restriction. The 727 is a strange case because it had 8th and 13th stage available from #1 and #3 but 2 seperate 8th stage bleeds from #2. From what I understand, two bleed sources are nessasary on the 727 because of the low (almost zero) rate of recirculation (excuse spelling)of cabin air. If it had been filtered and returned to the cabin, the bleed demand would have been reduced..JT
JT-8D From United States of America, joined Dec 2000, 423 posts, RR: 4 Reply 17, posted (10 years 11 months 1 week 1 day 13 hours ago) and read 2561 times:
You know, that stinkin light might be amber? The "ram door full open" lights are right next to it, and they are blue. Maybe Im confused again? Seems Im confused a lot more often these days--lol..JT
UAL Bagsmasher From United States of America, joined Sep 1999, 2116 posts, RR: 12 Reply 18, posted (10 years 11 months 6 days 4 hours ago) and read 2532 times:
Speaking of pressurization, today I was having the damndest time opening the rear cargo door on one of our 737's. The handle was hard to turn and I couldn't get that door to budge at all. Turns out they forgot to depressurize the aircraft. About a minute later all was well and I was able to get the door open once the crew was notified.
FBU 4EVER! From Norway, joined Jan 2001, 998 posts, RR: 8 Reply 19, posted (10 years 11 months 5 days 22 hours ago) and read 2532 times:
As JT-8D wrote in an earlier post,the DC-9,MD-80 and MD-90 all start to pressurise the cabin at a controlled rate as soon as take-off thrust is added.This is to avoid the pressure "bump" which would normally happen if the plane was unpressurized during T/O and pressurization started afterwards.
Now,if the plane has not become airborne within 60 seconds,de-pressurization will take place automatically.It will also de-pressurize if throttles are retarded in case of an aborted take-off.The pressurization system works via the ground shift mechanism,amongst other systems,and this means that as soon as the A/C is on ground,residual cabin pressure will be bled off at a constant rate as soon as the nosewheel strut is compressed after touch down.
To pressurize these planes on the ground,you have to pull the ground shift relay c/b's first to "trick" the plane into believing it is airborne.
JT-8D From United States of America, joined Dec 2000, 423 posts, RR: 4 Reply 20, posted (10 years 11 months 5 days 14 hours ago) and read 2509 times:
FBU, we just put it in manual, and roll the little wheel closed. If you are op checking the system, then it has to be in auto. Ever test the wrong system because it switched after "landing" from a fake flight? Then you have to trick it into switching back, then test the other system. Lots of fun..JT
174thFWff From , joined Dec 1969, posts, RR: Reply 21, posted (10 years 11 months 4 days 17 hours ago) and read 2498 times:
Well I guess some airlines land pruessurized because of the male f/a opened the door too erily and got sucked out and hit the tarmack at ungodly speeds. Anyone remember that?
FDXmech From United States of America, joined Mar 2000, 3251 posts, RR: 44 Reply 22, posted (10 years 11 months 4 days 14 hours ago) and read 2489 times:
That was an American Airlines A300-600 in Miami. The aircraft air-returned to KMIA and the aircraft failed to depressurize as it normally does.
The F/A was able to open open the door while the a/c was still pressurized and the rest is tragic history.
The A300 pax and cargo doors are able to be opened while still pressurized (albeit a low differential). This is in part due to the large amount of force that the door operator can impart to the lock/unlock handle.
For this reason the A300 has a warning light by each door to warn the door operator of residual pressure in the aircraft and to not attempt opening the door.
Mr Spaceman From Canada, joined Mar 2001, 2777 posts, RR: 18 Reply 23, posted (10 years 11 months 4 days 13 hours ago) and read 2489 times:
Hello gentlemen. I just want to let you guys know that the information I have learned from you about this topis is excellent. I apreciate it!
Also, I want you to know that I feel bad for not being more involved with the conversation, reguarding this topic. The reason for this is because my Grandfather (my Mom's Dad), just had a major stroke, and is in the hospital...and then 3 days later my dear Grandmother (my Dad's Mom), died of cancer. So, as you can imagine, I've been very busy lately...trying to be strong for my family, and help them, any way I can.
I think the world of you guys. I feel very lucky to be learning from you.
I still have many questions that should get you're Grey Matter working. I'm looking forward to it...and I hope you are too.
JT-8D From United States of America, joined Dec 2000, 423 posts, RR: 4 Reply 24, posted (10 years 11 months 4 days 8 hours ago) and read 2479 times:
Space, sorry to hear the news. Get back when you can?..JT
25 Mr Spaceman: Thanks for the kind words JT-8D. My Grandfather is 87 years old. If my family losses him...well, that has to do with the BIG GUY up above! I'm trying
26 HAWK21M: The dual bleed lt on the P5 overhead panel of a B737 is def "Amber" as it should be.The consequences of having the Engine operating & thrust levers mo
27 TimT: Rocket Scientist #1 ( or a candidate for the DARWIN Awards) Aircraft landed and didn't depressurize, Junior went and got the crash axe. With a mighty
28 Dc10hound: >>For this reason the A300 has a warning light by each door to warn the door operator of residual pressure in the aircraft and to not attempt opening
29 FDXmech: Hi Dc10hound, At FDX, they're functional. When I worked at AA (91-93), I don't recall if they were or weren't functional or installed.
30 Dc10hound: Hi FDX, >>At FDX, they're functional.When I worked at AA (91-93), I don't recall if they were or weren't functional or installed
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