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B737NG: High Cabin Diff Pressure

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B737NG: High Cabin Diff Pressure

Old 30th Apr 2017, 14:41
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B737NG: High Cabin Diff Pressure

There is a lot of 737NG experience out there which I hope to tap into.

Here is the situation:
You are in cruise flight (ie FL370 or FL390), there are no known faults or MELs, all switches are in their normal position for cruise flight.

You notice that your Cabin Diff needle is in the orange band (8.4 PSI or higher).
There is no master warning
The AUTO FAIL light remains extinguished.

What is the proper reaction if any ?
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Old 30th Apr 2017, 14:46
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I'm not trying to be funny or deflectionist (new word) but I'll bounce it back to you. Firstly I assume you are a type rated B737NG pilot or something similar.

What do you think? What would you do?
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Old 30th Apr 2017, 14:46
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Sounds like a normal day at work...


Yes I know amber means caution. It's just that on every 737 I have ever flown you end up with the needle 0,05 mm into the amber band at high levels. If you're concerned, how about descending to a lower level where the max diff pressure is limited to a lower value?
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Old 30th Apr 2017, 15:23
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I don't know these new systems, but could you not just wind the cabin alt up a bit? I expect it would be below 8000ft.
I seem to recall that 8.0 diff gave us something like 8000ft cabin at 39,000ft.
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Old 30th Apr 2017, 16:08
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The cabin alt and diff pressure indicator isnt the most reliable gauge in the flight deck.

Changed quite a few of them. Have a look at the indication on the ground, lots of them sit at 0.2 diff or more with the doors open.
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Old 30th Apr 2017, 16:13
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I don't know these new systems, but could you not just wind the cabin alt up a bit? I expect it would be below 8000ft.

You can't set the cabin altitude specifically on that system. Before take off you tell the computer your cruise altitude and landing elevation. It works out the rest. I am thinking the OP could set a lower cruise altitude in the window. Below FL370 the max diff pressure is reduced slightly. That might relief some of the excess pressure inside. If so you'll end up with a higher cabin altitude. This is speculation on my part though, not sure if that is what will happen for real...
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Old 30th Apr 2017, 16:23
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Tap the gauge...
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Old 30th Apr 2017, 17:25
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Originally Posted by 172_driver View Post
You can't set the cabin altitude specifically on that system. Before take off you tell the computer your cruise altitude and landing elevation. It works out the rest. I am thinking the OP could set a lower cruise altitude in the window. Below FL370 the max diff pressure is reduced slightly. That might relief some of the excess pressure inside. If so you'll end up with a higher cabin altitude. This is speculation on my part though, not sure if that is what will happen for real...
If you set a LOWER cruise altitude what does the pressurisation system do?
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Old 30th Apr 2017, 17:27
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Originally Posted by Chesty Morgan View Post
Tap the gauge...
Word!

Failing that just don't look at it.

You will know if you go beyond the safety value by the blood running from the captains ears ;-)
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Old 30th Apr 2017, 22:03
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If you set a LOWER cruise altitude what does the pressurisation system do?
A lower cruise altitude on the pressure controller, while maintaining flight level.

I don't know the wiring, tell me if you do...

It's speculation, but as the max diff goes from 8,35 psi above FL370 to 7,80 psi below FL370 (I had to look up the numbers) I figured the controller starts aiming for a new, lower, diff pressure if you wind down the cruise altitude on the pressure controller. Normally you'd actually start a descent too.
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Old 30th Apr 2017, 22:06
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Originally Posted by 172_driver View Post
Sounds like a normal day at work...


Yes I know amber means caution. It's just that on every 737 I have ever flown you end up with the needle 0,05 mm into the amber band at high levels. If you're concerned, how about descending to a lower level where the max diff pressure is limited to a lower value?
So if you see this, you just ignore it ?

[IMG][/IMG]
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Old 30th Apr 2017, 22:32
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That looks like the 0,05 mm I was talking about. It also looks damn close to 8,35 PSI which is the limit differential pressure > FL370.
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Old 1st May 2017, 01:59
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Ok.

What if you see this?

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Old 1st May 2017, 02:45
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[/quote]

Perhaps you could climb until your differential is past 9, i.e. out of the red part of the scale?
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Old 1st May 2017, 03:32
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Your first image is normal (above FL370. Not at FL370 - should be 7.8).

Your second one isn't. In this case, my next step would be (after tapping the gauge!) to check the cabin vertical rate. If it is climbing, then consider whether you have just arrived at cruise altitude at a high rate of climb. If so, then the cabin is still catching up and the auto controller will resume normal diff shortly. Give it a few minutes. This is ok, no further action required.

If the cabin rate is descending, then there is a problem and you are not far away from pressure relief valves opening (9.1psi). In this case, the AUTO FAIL light should already have illuminated (>8.75 and auto not responding correctly), so this is an unlikely scenario. Have you seen this or are you posing a hypothetical?

In this case, I would conduct the "unscheduled pressurisation change" checklist, which will attempt the alternate controller first, and if that doesn't solve the problem, it will lead you to manual control. Take it slow. No rush here.

As an aside, I don't know whether the gauges get the cabin alt from the same sensor as the auto-controllers, or whether there are multiple sensors. So I don't know whether you could end up in this situation with a faulty gauge but yet the auto-controller is operating normally. Maybe someone can answer this.

But what I do know is that the Cabin Altitude Warning Horn does have it's own sensor, so you aren't going to unknowingly depressurise with a faulty gauge in manual mode. So in the case of a faulty gauge, I would try to keep cabin rate at zero until the outflow valve is fully open during descent, then land depressurised.
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Old 1st May 2017, 05:11
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Just tell the "Computer" your cruise altitude/flight level is going to be 1000' higher than actual. Just like we F/E's used to do. It will give you a slightly lower differential pressure and a slightly higher cabin altitude.

Last edited by Old Fella; 1st May 2017 at 05:22. Reason: clarity.
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Old 1st May 2017, 07:39
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Really happened...

I was flying a 737-200 back in the day.
The pressurisation mode, due to system faults, was manual.
Taking off from MEM towards the east, the storms were so bad, I told the FO I will take the radios. Didn't want to waste time explaining to him what I wanted and the palava of waiting for him to understand and also to have a rapid response if ATC had issues about my desired headings etc. Fought tooth and nail for a good 20 minutes or so avoiding storms and climbing. finally came out the other side into the clear and could take a breath, looked over and FO was dead to the world, asleep. I did a quick scan of the panels and came to the pressurization panel and was confused with seeing the differential needle sitting between the S and the 0. Thought a second, then hit the FO, closed the thrust levers, and told ATC we were descending now due to severe turbulence. I wanted to immediately reduce the input of any further air pressure, hence closing the thrust levers. The FO finally got to work on the outflow valve and we both watched the needle descend through the DIFF PRESS on down to the normal range. To this day I am amazed we didn't blow any windows or seals out. When I landed back at MEM, I wrote in the tech log, "check pressure relief valve"
The engineer asked why I wrote that up and I just looked at him with a big smile and said, "it doesn't work!" Yes, part my fault for not keeping the FO awake. Funny that on the same frequency, just after my telling ATC that I was descending now due turbulence, a lie for sure, someone else said the same thing, so there was turbulence somewhere in the area.

So, a bit into the yellow arc is fine.

Last edited by stator vane; 1st May 2017 at 07:40. Reason: Addition
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Old 1st May 2017, 08:14
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Just tell the "Computer" your cruise altitude/flight level is going to be 1000' higher than actual. Just like we F/E's used to do. It will give you a slightly lower differential pressure and a slightly higher cabin altitude.

I'm not so sure about that. These new fangled fully automatic systems are designed not to be fiddled with. I once had a pregnant lady board who was cleared to fly, but had some random pains. It was here flight home a few months before due date. I think this was back in my B757 days, or it could have been an NG. Either way, my fiddling with the control panel settings to try and create a lower cabin altitude achieved the opposite and it was back to fully automatic and she survived. The books don't really give you the insight to fiddle, because they don't want you to. Damn. "that's another fine mess you've got us into, Stanley."
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Old 1st May 2017, 11:44
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Originally Posted by Derfred View Post
Your first image is normal (above FL370. Not at FL370 - should be 7.8).

Your second one isn't. In this case, my next step would be (after tapping the gauge!) to check the cabin vertical rate. If it is climbing, then consider whether you have just arrived at cruise altitude at a high rate of climb. If so, then the cabin is still catching up and the auto controller will resume normal diff shortly. Give it a few minutes. This is ok, no further action required.

If the cabin rate is descending, then there is a problem and you are not far away from pressure relief valves opening (9.1psi). In this case, the AUTO FAIL light should already have illuminated (>8.75 and auto not responding correctly), so this is an unlikely scenario. Have you seen this or are you posing a hypothetical?

In this case, I would conduct the "unscheduled pressurisation change" checklist, which will attempt the alternate controller first, and if that doesn't solve the problem, it will lead you to manual control. Take it slow. No rush here.

As an aside, I don't know whether the gauges get the cabin alt from the same sensor as the auto-controllers, or whether there are multiple sensors. So I don't know whether you could end up in this situation with a faulty gauge but yet the auto-controller is operating normally. Maybe someone can answer this.

But what I do know is that the Cabin Altitude Warning Horn does have it's own sensor, so you aren't going to unknowingly depressurise with a faulty gauge in manual mode. So in the case of a faulty gauge, I would try to keep cabin rate at zero until the outflow valve is fully open during descent, then land depressurised.
Had this during cruise flight after we had been level at FL370 for about 45 minutes. The AUTO FAIL light was not on.
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Old 1st May 2017, 14:01
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Is that second picture at cruise level , something wrong there if it is . If you're still climbing it'll probably sort itself out .
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