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727 Pressurization Question  
User currently offlineHHHramp From United States of America, joined Sep 2001, 89 posts, RR: 0
Posted (11 years 2 months 3 weeks 6 days 21 hours ago) and read 1834 times:

At school today, one of my classmates was giving a demo of the ECS (AC and pressurization). While a group of us was in the aft cargo bin of a 727-100 going over the pressure relief valves. A question came up when he gave the pressure ranges for the cabin. He said, and these are rough numbers, the overflow valve allows for a maximum positive pressure of 8 or 9'ish PSID (with the cabin able to stand up to 12 PSID). He also said that the maximum negative pressure allowed was 1 PSID.

My question to him and the instructor that remains unanswered is:

How do you develop negative pressure differential?

This may seem dumb to some people here but, I could not see how it would be done.

Thanks in advance

Sean


5 replies: All unread, jump to last
 
User currently offlineL-188 From United States of America, joined Jul 1999, 29791 posts, RR: 58
Reply 1, posted (11 years 2 months 3 weeks 6 days 20 hours ago) and read 1803 times:

Easy, You have a cabin outflow valve get stuck closed when you have a say 9000 foot cabin. Then you try and decend below that altitude.

Intentionally I don't there is any time that would actually happen.


OH BTW. My research for a final systems paper I turned in today on the 727 give a max differential of 9.6 psi but the Automatic pressure controler is designed to allow for a max of 8.5 PSI.

I doubt that there have been any combi aircraft that have able to get an 8.5 differentail in 20 years  Laugh out loud



OBAMA-WORST PRESIDENT EVER....Even SKOORB would be better.
User currently offlineL-188 From United States of America, joined Jul 1999, 29791 posts, RR: 58
Reply 2, posted (11 years 2 months 3 weeks 6 days 20 hours ago) and read 1802 times:

I should add the the controled can also be manually set for any cabin altitude between -990 and 1990 feet.

If memory serves I got that out of an old AS Pilots Flight Manual for the 727.



OBAMA-WORST PRESIDENT EVER....Even SKOORB would be better.
User currently offlineScootertrash From United States of America, joined Aug 2001, 569 posts, RR: 9
Reply 3, posted (11 years 2 months 3 weeks 6 days 18 hours ago) and read 1786 times:

HHHramp...

Did you used to work on the ramp at Hilton Head?


User currently offlineHHHramp From United States of America, joined Sep 2001, 89 posts, RR: 0
Reply 4, posted (11 years 2 months 3 weeks 6 days 18 hours ago) and read 1776 times:

Thanks for the info.

I was close with the 8-9ish Psid. The 12 Psid was in the MM as a structural max not under the typical max allowed by the controler and the valve.

Scootertrash-- No.. I wish. I worked at MSP at the HHH terminal until the layoffs. Now I am a student located at TRF. As if it wasn't cold enough down there in the twin cities.

Sean


User currently offlineB747skipper From , joined Dec 1969, posts, RR:
Reply 5, posted (11 years 2 months 3 weeks 5 days 14 hours ago) and read 1681 times:

Negative relief ??? ...
Well - that happens in real life, with stupid F/Es when there were some...
As an example, cabin is 7500 feet high, aircraft descends from FL370, but our friend "forgot" to reset pressurization to landing field elevation...
Result is, painfull ears when the aircraft descends through 7,500 feet... then continues to descend at 1,000 fpm... insults of passengers... ooops !!!
xxx
Another type of problem... not negative relief...
There is the other type of idiots, such as landing in Denver, elevation 5300 feet, but our superior F/E resets cabin to sea level (as a habit, right?) -
When the airplane touches down... ear drums get cleared, in seconds...
With PanAm we used to say "Eaaaarrrrrwax and taaaaampax" for such cases.
xxx
Aviation has humour (some at least)  Smile
(s) Skipper


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