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Changing Cabin Pressure On Purpose?  
User currently offlineWNCrew From United States of America, joined Jun 2006, 1480 posts, RR: 10
Posted (7 years 2 months 1 week 5 days 23 hours ago) and read 4962 times:

I sincerely hope this isn't a stupid question but recently a passenger asked me if the pilots could alter the interior cabin pressure and I wasn't quite sure what to tell him.

He also wanted to know what our internal cabin pressure was and since we were at 30K I said around 6K, but that if we climbed up to 40K we could go as high as 8K inside the cabin. I also told him that yes the captain could alter the cabin pressure but only in the sense of "releasing it", not necessarily adding to it, that is automatic.

I wasn't able to verify my answer with the captain so did I completely lie to him? CAN the pilots in effect create a decompression? What would be the reason?


ALL views, opinions expressed are mine ONLY and are NOT representative of those shared by Southwest Airlines Co.
15 replies: All unread, jump to last
 
User currently offlineSlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 68
Reply 1, posted (7 years 2 months 1 week 5 days 23 hours ago) and read 4944 times:

Quoting WNCrew (Thread starter):
what our internal cabin pressure was

Depends very much on airplane type. Some jets have a limit of less than 7 PSI differential, others are well above 8 PSI.

Yes we "can" vary some values but normally we would not. We don't generally set the "pressure" per se but set the actual altitude at which we will cruise and perhaps the elevaton of our destination airport.

When we start up the air conditioning and pressurization system begins pumping air into our cabin. Some gets recirculated, some gets bled overboard by way of radio compartments (to cool the radios) some heats the bag pits, and some ultimately just gets released overboard through the outflow valve. It is this release through the outflow valve that is the means of controlling differential pressure. (difference between cabin and outside pressure)

The more automated systems (meaning last 30 years or so) will drive the outflow toward closed on takeoff roll to lock in departure ambient pressure. As the plane climbs conditioned air continues to be pumped in. Some is released as needed to prevent overpressure.

What the system aims for is a smooth cabin climb from takeoff to cruise "altitude" or internal pressure, then to maintain cruise until descent is begun. From there it will begin increasing pressure (decreasing cabin altitude) until it is more or less equal to expected arrival airport pressure. Result is that your ears get a smooth ride.

Normally at a high cruise altitude, the system will head toward maximum normal differential pressure giving the lowest possible 'cabin altitude' which is the healthiest.

So to vary it, all we could do would be to raise the cabin altitude. That would not be a good idea. We have passengers with pulmonary problems, smokers (but I repeat myself) even people with heart problems who might be adversely affected. As a matter of company policy we usually let it run itself.



Happiness is not seeing another trite Ste. Maarten photo all week long.
User currently offlineCosmicCruiser From United States of America, joined Feb 2005, 2255 posts, RR: 15
Reply 2, posted (7 years 2 months 1 week 5 days 22 hours ago) and read 4925 times:

On our emergency chk list for CABIN SMOKE it calls for MANUALLY raising the cabin alt to 25,000' and going on O2. This will inhibit the growth of the fire.

User currently offlineSlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 68
Reply 3, posted (7 years 2 months 1 week 5 days 21 hours ago) and read 4893 times:

Quoting CosmicCruiser (Reply 2):
MANUALLY raising the cabin alt to 25,000'

Obviously not for passenger operations!  Wow!

I actually knew of a 727 crew that flew around an airport for 25 minutes trying to work their way to the end of a cockpit smoke checklist. As you know they can be pretty long and drawn out. Management pointed out to them that fires are a lot more fun to watch from the ground and don't ever do that again. Apparently they thought they had to work all the way through it before they could start on the landing preliminary checklist.

The checklist was modified to include big, BOLD words: "Land as soon as practicable and..."

Over the ocean in a freighter. No prob!



Happiness is not seeing another trite Ste. Maarten photo all week long.
User currently offlineCosmicCruiser From United States of America, joined Feb 2005, 2255 posts, RR: 15
Reply 4, posted (7 years 2 months 1 week 5 days 20 hours ago) and read 4883 times:

Quoting SlamClick (Reply 3):
actually knew of a 727 crew that flew around an airport for 25 minutes trying to work their way to the end of a cockpit smoke checklist. As you know they can be pretty long and drawn out

Absolutely, In the sim in days of past they wanted to see you wander thru the chklist but thank goodness nowdays one can work on the chklist while the other does 300kts to the nearest arpt. We've seen with the few fires we've had that getting on the ground ASAP is paramount to the survivability of the crew.

Along the lines of what you're saying I had an old F/E get upset with me after the sim in debrief because on our 2-eng out app I lined up with the runway and said tell me what I've got when we land and he was pi**ed that I didn't HOLD so he could finish the 2-eng out chklist!


User currently onlineOB1504 From United States of America, joined Jul 2004, 3392 posts, RR: 6
Reply 5, posted (7 years 2 months 1 week 5 days 19 hours ago) and read 4860 times:

Quoting SlamClick (Reply 3):
Obviously not for passenger operations!

And for cargo operations with live animals on board (racehorses, etc).


User currently offlineCosmicCruiser From United States of America, joined Feb 2005, 2255 posts, RR: 15
Reply 6, posted (7 years 2 months 1 week 5 days 19 hours ago) and read 4845 times:

Quoting OB1504 (Reply 5):
And for cargo operations with live animals on board (racehorses, etc).

No, if the situation called for it the horses would not make it. You probably wouldn't have haz mat on the main deck w/ horses BUT if you did keeping the fire from growing and getting back on the ground is the most important thing.


User currently offlinePilotpip From United States of America, joined Sep 2003, 3150 posts, RR: 11
Reply 7, posted (7 years 2 months 1 week 5 days 13 hours ago) and read 4797 times:

The EMB-145 has two modes of operation when it comes to pressurization. Manual, and auto. Manual is a bear to operate. A knob on the controller is attached to a pneumatic line which opens and closes the pneumatic outflow valve. You have to watch the EICAS while doing this. Anytime you change power settings, climb or descend you have to adjust. It's basically a full time job. Auto is easy. We set the landing field elevation, and does everything on its own using static pressure readings from the ports in the back of the airplane, and Pitot/static 3 which has the computer that figures everything out which we call a CPAM. I'd tell you the name if I could remember, but it's 3am and my manual is in the other room. It's a very neat system. Much more simple than the kingairs and 400 series cessnas I've flown.

Also a dump button. This will raise cabin altitude to either 14,000 feet, or ambient if below 14,000. Beyond that you have to use the manual override and turn the nob to raise the pressure.

When training in the sim, we were taught there are basicalliy two events which you perform memory items, and get to the checklist when/if you can. The first is an emergency descent, which will usually allow you time to do it but you're not going to last long if you don't get the O2 on and communicating in those masks is next to impossible. The second is fire. That one is pretty self-explanitory.



DMI
User currently offlineTod From Denmark, joined Aug 2004, 1729 posts, RR: 3
Reply 8, posted (7 years 2 months 1 week 5 days 7 hours ago) and read 4734 times:

Quoting SlamClick (Reply 1):
some gets bled overboard by way of radio compartments (to cool the radios) some heats the bag pits, and some ultimately just gets released overboard through the outflow valve.

In most cases, you also lose cabin air via the sink/floor drain system out through the drain masts.
Vacuum waste systems also lose air overboard to create the vacuum.

On 742 and 743, in addition to lav/galley vent system dumping air near the outflow valves, some of the air is extracted overboard via a valve on the side of the plane.

Tod


User currently offlineSlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 68
Reply 9, posted (7 years 2 months 1 week 5 days 6 hours ago) and read 4708 times:

Quoting CosmicCruiser (Reply 4):
get upset with me after the sim in debrief because

One of my last LOFT periods we ended with the Airbus fuel leak scenario. (The Transat-Azores thing)

Well the problem manifested as we were about twenty miles abeam a major airport not on our flight plan. I was PF so I had him deal with it. He gets into the problem and the first time he looks up, maybe five minutes later, was when I put the gear down. We were 4-5 mile final.

He was a little out of sorts. He wanted to know more about the problem. I wanted to be on the ground. In the real world I would want to be on the ground. It can not end well otherwise.

Afterwards he sat silently and let me be debriefed. ("Kudos for getting us on the ground quickly. Nice ride.") then he asked the instructor to talk at length about the problem. He ended up missing our flight home. They might have been there 'til midnight!

Not that I didn't want to know more about the problem. It is just that I think I had extracted the most important single lesson from it. Easier to fix on the ground.



Happiness is not seeing another trite Ste. Maarten photo all week long.
User currently offlineKalvado From United States of America, joined Feb 2006, 491 posts, RR: 0
Reply 10, posted (7 years 2 months 1 week 4 days 18 hours ago) and read 4601 times:

Quoting Pilotpip (Reply 7):
The EMB-145 has two modes of operation when it comes to pressurization. Manual, and auto. Manual is a bear to operate. A knob on the controller is attached to a pneumatic line which opens and closes the pneumatic outflow valve. You have to watch the EICAS while doing this. Anytime you change power settings, climb or descend you have to adjust. It's basically a full time job.

For either manual mode, or in case of auto failure - is there a safety relief - I assume there should be something, but what would be the design - rapture membrane, spring-loaded valve or something else?
And yes, I remember photos of that KC-135 (?) ripped apart by pressurization..


User currently offlinePilotpip From United States of America, joined Sep 2003, 3150 posts, RR: 11
Reply 11, posted (7 years 2 months 1 week 4 days 16 hours ago) and read 4586 times:

Dual outflow valves. One is electopneumatic, the other is pneumatic. If the lights go out, the pneumatic valve is still operational via a totally independent system. The manual valve is a totally self-contained system.

Max differential pressure is 7.8PSI for the 145. Max differential overpressure is 8.1. We have a little margin there, also don't usually see 7.8 as we fly short legs and don't go up to FL370 very often.



DMI
User currently offlineFLY2HMO From , joined Dec 1969, posts, RR:
Reply 12, posted (7 years 2 months 1 week 1 day 23 hours ago) and read 4372 times:

Quoting Kalvado (Reply 10):
KC-135 (?)

Are you talking about this? Was a C-141


View Large View Medium
Click here for bigger photo!

Photo © Mark Baker



Thats what happens when you don't vent the tanks when fueling.


User currently offlineTod From Denmark, joined Aug 2004, 1729 posts, RR: 3
Reply 13, posted (7 years 2 months 1 week 1 day 7 hours ago) and read 4315 times:

Quoting FLY2HMO (Reply 12):
Quoting Kalvado (Reply 10):
KC-135 (?)

Are you talking about this? Was a C-141

Or this KC-135


Big version: Width: 756 Height: 540 File size: 25kb


Tod


User currently offlineCosmicCruiser From United States of America, joined Feb 2005, 2255 posts, RR: 15
Reply 14, posted (7 years 2 months 1 week 12 hours ago) and read 4230 times:

Quoting Kalvado (Reply 10):
I assume there should be something, but what would be the design - rapture membrane, spring-loaded valve or something else?

yes there are pressure relief valves. on the MD-11 (probably the same for DC-10) they open at 8.9psid and limit pressure to 9.1psid.


User currently offlineUAL Bagsmasher From United States of America, joined Sep 1999, 2147 posts, RR: 10
Reply 15, posted (7 years 2 months 6 days 20 hours ago) and read 4175 times:

CPAM= Cabin Pressure Acquisition Module.
The CPAM on the CRJ-200 provides the EICAS with the cabin alititude, cabin altitude rate of change, and cabin Delta P. It also causes the Fasten Seat Belts and No Smoking signs to illuminate if cabin altitude exceeds 10,000' with the switches in auto setting. The CPAM is also responsible for deploying the pax o2 masks automatically if cabin altitude exceeds 14,000'. The ERJ-145 probably has a similar setup.


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