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B737NG: High Cabin Diff Pressure

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B737NG: High Cabin Diff Pressure

Old 2nd May 2017, 19:40
  #41 (permalink)  
 
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Originally Posted by drooz View Post
Slightly off-topic but what are your airline's policy in regards to the 'flight altitude' when requiring a step climb? Would you set the highest flight level anticipated or would you start with the initial cruise level and then increase it as you climb further?

My company suggests entering the initial cruise altitude and then changing it as you climb, to avoid unnecessary 'off-scheduled descent' warnings in case you forget to climb. However this means that you suddenly get a quite annoying pressurisation change in the middle of the cruise when the pressurisation logic changes from 7.8psi diff to 8.35psi (i.e changing cruise level from FL360 to FL380).

Would be interesting to know other operators thoughts around this.
Boeing practise advises to set the highest expected cruise level. Other methods of operations are also acceptable but a bulleted list of problems and risks arise and thus are not recommended.
If operating as per Boeing FCOM and FCTM, set the highest expected level. The descent checklist item AIR COND & PRESS should be used to catch it if not climbed to planned maximum. Worst case scenario you get an UNSCHEDULED DESCENT, a non-issue in most airlines these days.
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Old 2nd May 2017, 20:02
  #42 (permalink)  
 
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Originally Posted by Derfred View Post
So you're saying the gauge gets it's data from the same source as the auto controllers?

From the book. Physical Description
Each CPC (there are two) has these items:
• Cabin pressure sensor
• BITE instruction plate
• Two-line LED display
• BITE control buttons.

And I'm not "saying" anything. Just quoting the book. The chapter on the cabin altitude panel does not go any further than shown.

Last edited by stator vane; 2nd May 2017 at 20:08. Reason: spacing
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Old 3rd May 2017, 15:52
  #43 (permalink)  
 
Join Date: Jun 2006
Location: Brisbane
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Slightly off-topic but what are your airline's policy in regards to the 'flight altitude' when requiring a step climb? Would you set the highest flight level anticipated or would you start with the initial cruise level and then increase it as you climb further?
Initial cruise level. But this has been discussed at length in a previous thread: http://www.pprune.org/tech-log/57103...indicator.html

Skyjob and I agreed to disagree in that thread. Furthermore, airline operators differ in their interpretations of Boeing's recommendations, and implement their own SOP's accordingly. Please re-kindle that thread if you wish to discuss it further rather than take this one off topic...
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Old 3rd May 2017, 16:20
  #44 (permalink)  
 
Join Date: Jun 2006
Location: Brisbane
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Originally Posted by stator vane View Post
The chapter on the cabin altitude panel does not go any further than shown.
Thanks, Stator.

That cabin altitude/diff gauge is an interesting one.

I recall a 737 incident where they did a rapid depressurisation and emergency descent after a window crack, and subsequently found themselves at a low altitude with the cabin altitude above 40,000'.

Very confusing. Except that the cabin altitude wasn't above 40,000' of course, it was way below sea level. You see, they didn't actually have a rapid depressurisation. So they closed the outflow valve manually as per the checklist, descended as per the checklist, and consequently pressurised the cabin way below sea level. That gauge is capable of reading negative, but it doesn't have negative graduations. The needle just moves anti-clockwise until it shows a high cabin altitude. No mention of that in the manuals either. Hmmm.

Last edited by Derfred; 3rd May 2017 at 16:39.
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Old 3rd May 2017, 22:24
  #45 (permalink)  
 
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Surely diff pressure gives a bit of a clue?
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Old 4th May 2017, 04:15
  #46 (permalink)  
 
Join Date: Feb 2008
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Clue to Cabin Altitude

As well as the Differential Pressure being a clue Hoppy, I am sure the lack of any Cabin Altitude Warnings (Visual & Aural) as well as the lack of a "Rubber Jungle" in the cabin would/should have told them something ?
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Old 4th May 2017, 11:33
  #47 (permalink)  
 
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Yes, you would think. It's a while since I read the report. It was bit of an embarrasing read from memory, but the interesting point I got out of it was the behaviour of the gauge.
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Old 4th May 2017, 15:39
  #48 (permalink)  
 
Join Date: Apr 2016
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There is also a modification available from boeing which gives a higher differential and a lower cabin altitude , not sure on the differential numbers , but the cabin stays at around 6500 . Could be one of those .
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Old 4th May 2017, 17:01
  #49 (permalink)  
 
Join Date: Dec 2001
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Originally Posted by Minorite invisible View Post
Ok.

What if you see this?

Nothing going on boss.
Carry on.
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