B737NG: High Cabin Diff Pressure

[Minorite invisible]

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 ?

[RAT 5]

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?

[172_driver]

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?

[dixi188]

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.

[Golden Rivet]

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.

[172_driver]

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...

[Chesty Morgan]

Tap the gauge...

[FE Hoppy]

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?

[FE Hoppy]

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 ;-)

[172_driver]

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.

[Minorite invisible]

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 ?

http://i1244.photobucket.com/albums/gg568/MinoriteInvisilble/651e8263-3ce7-4fb1-8128-1e0a7fa57994_zpsbdbriuu4.jpg (http://s1244.photobucket.com/user/MinoriteInvisilble/media/651e8263-3ce7-4fb1-8128-1e0a7fa57994_zpsbdbriuu4.jpg.html)

[172_driver]

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.

[Minorite invisible]

Ok.

What if you see this?

http://i1244.photobucket.com/albums/gg568/MinoriteInvisilble/fc63e367-8587-4ea9-a866-c2e1f596836e_zpshu6j4dir.jpg

[Pearly White]

http://i1244.photobucket.com/albums/gg568/MinoriteInvisilble/fc63e367-8587-4ea9-a866-c2e1f596836e_zpshu6j4dir.jpg[/quote]

Perhaps you could climb until your differential is past 9, i.e. out of the red part of the scale? :ooh:

[Derfred]

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.

[Old Fella]

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.

[stator vane]

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.

[RAT 5]

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."

[Minorite invisible]

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.

[Jonnyknoxville]

Is that second picture at cruise level , something wrong there if it is . If you're still climbing it'll probably sort itself out .

[Derfred]

Had this during cruise flight after we had been level at FL370 for about 45 minutes. The AUTO FAIL light was not on.

So what did you do next and how did the the pressurisation system respond? I'd be interested to know.

Also, did you ever get any feedback from maintenance as to what caused it?

[RAT 5]

Had this during cruise flight after we had been level at FL370 for about 45 minutes. The AUTO FAIL light was not on.

And yet there are many trainers I've heard of, and seen, who do not encourage pilots to scan the overhead panel using the philosophy that EICAS or CWS will monitor it for you. It was only when an SOP came in that pressurisation should be scanned every 10,000' in the climb that guys ever looked at the overhead panel, and then only at the pressurisation. I'm from the old school that made an overhead scan at TOC, and perhaps every so often thereafter. There are some things, like this scenario, which are not quite correct, but which do not activate any alerting system. It's a Mk.1 eyeball alert. My crew was being line checked and the F/O's L.C. is always a command assessment. At TOC I, as PF, made an overhead panel scan. On sector 2 the F/O as PF did not. In the de-brief the TRE commented that it was a good piece of airmanship and encouraged it. The F/O, of 3 years experience, in a hi-monthly-sector environment, replied that it was not an SOP. OMG! Another example of the thicker the SOP book gets the smaller pilots' brains become.
We complain about low quality instrument scanning; IMHO it's worse than just that. There is a creeping complacency. The same is true with keeping yourself aware of en-route Wx conditions. i.e. always having a bolt-hole in your mind when it's needed. Is it not professionalism and incumbent on us to always have high S.A. of the operation; i..e the status of the a/c, the status of the environment and the whereabouts of your a/c and a safe haven? I'm not advocating a constant nervousness that means checking every little minutiae every minute. There are those who do and it is very negative to a relaxed operation and doesn't necessarily mean a better awareness. I'm advocating an awareness that promotes a confidence that you have a firm yet relaxed grip on the reins.

[ajd1]

RAT 5 has summed up very nicely what all of us oldies know.

[Derfred]

Hi Rat,

Completely agree with your post.

It is interesting, however, that us "old school" folks can be also mislead by trying to "aviate" the more modern aircraft that are not so much designed to be "aviated".

This thread is a case in point with several posters suggesting techniques such as putting a different cruise altitude into the pressurisation system, or climbing above the set cruise altitude.

These techniques won't work on the 737NG, although they may have worked on the older 737's.

Unfortunately, as the SOP chapter widens, the systems description chapter narrows. FCOM Vol 2 is still fat, but these days it doesn't really tell you how the system really works. It just tells you the minimum you need to know.

As I pointed out in my earlier post, Vol 2 doesn't tell me how many cabin altitude sensors there are, and whether they are shared between gauges and systems.

It also doesn't tell those suggesting the alternative techniques why those techniques won't work.

I recall once (new to the NG, but not new to the 737), hearing the airconditioning go suddenly deathly quiet climbing through 8-9000 feet. We looked at the cabin climb rate and saw close to zero. We expected to see 300-500 fpm climb, as that was what we were accustomed to seeing. We leveled out at 10,000' thinking something must be wrong with the pressurisation, but couldn't work out what. Eventually we continued climb and all came back to normal.

It turned out that the NG didn't climb the cabin in the same way as the older 737's. The NG maintains close to zero cabin climb until above 10,000 feet. The older 737's didn't behave the same way. The Vol 2 didn't bother to point out that little nugget.

The deathly quiet was just a chunk of ice clearing from the duct, which was making the cockpit much noisier than normal, and the deathly quiet was actually just a noisier than normal cockpit resuming normal noise.

Boeing says do the checklist, and don't try to troubleshoot, unless the problem becomes more complex than the scope of the checklist. So that's my next question to the OP: what you saw shouldn't have happened, so what did it turn out to be?

[Minorite invisible]

So what did you do next and how did the the pressurisation system respond? I'd be interested to know.

Also, did you ever get any feedback from maintenance as to what caused it?

We did a quick check of the FCOM, which indicated that the AUTO FAIL light should have been on at this stage, but it wasn't.

We then decided to do the "Unscheduled Pressurization Change" NNC, but before we had time to do it we had a rapid depressurization when both Pressure Relief Valves suddenly opened with a loud bang in the tail that all passengers and crew clearly heard. There was a condensation cloud, the cabin became instantly cold and the masks dropped inside of about 30 seconds.
We initiated the Emergency Descent but then recovered pressurization around FL330 when the spring loaded Pressure Relief Valves closed.

This from maintenance:

"Boeing 737NG-FTD-21-10004; Digital Cabin Pressurization fault code 90, Cab Pressure switch activated. Operators have reported pressurisation issues on several airplanes due to improper activation of cabin pressure switch located in outflow valve with no flight deck effect"

Did anyone know that the two Automatic Cabin Pressure Controllers each have an independent Cabin Pressure Switch which is calibrated to automatically close the Outflow Valve when the Cabin Altitude is 14,500 or above and does so without triggering any Master Caution or Light in the Flight deck unless a certain modification is installed ? When FC90 is triggered, the Cabin Pressure Switch sends a "close" order to the Outflow valve which will stay fully closed until pilot action is taken. If action is taken quickly enough......

[Derfred]

Well, you are a sneaky fellow aren't you? You've been drip feeding us.

Thanks for the follow up. That is quite interesting.

[Skyjob]

So if you see this, you just ignore it ?

http://i1244.photobucket.com/albums/gg568/MinoriteInvisilble/651e8263-3ce7-4fb1-8128-1e0a7fa57994_zpsbdbriuu4.jpg (http://s1244.photobucket.com/user/MinoriteInvisilble/media/651e8263-3ce7-4fb1-8128-1e0a7fa57994_zpsbdbriuu4.jpg.html)

Yes, it's normal so ignore it unless other warnings tell you to do something.
As mentioned above, not the most reliable gauge.

[Old Fella]

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...

Derfred says that my suggestion of telling the "Computer" that the Cruise Level is 1000' above actual will not work (in giving a buffer) in the 737NG. If that is so, why bother telling the "Computer" anything about Cruise Level? Interesting to note the comment on scans. Seems to be very much a reliance on the aircraft monitoring itself. I just hope they are close to infallible.

[stator vane]

The cabin altitude warning switches, are on the ceiling in the forward EE compartment.

There are two digital cabin pressure controllers (CPCs). Each CPC has its own systems interface and valve motor system. This gives the AUTO mode of control a dual redundant architecture. Only one CPC controls the outflow valve at any time. The other CPC is a backup. The active controller changes for every flight or when there is an autofail event.

The manual control system has its own valve motor system. This gives the pressurisation control system a triple redundant architecture. A sensor on each CPC senses pressure in the cabin.

The outflow valve has three motors:
• Two AUTO motors with electronic actuators
• One MANUAL motor.

Altitude switches in each electronic actuator (at the valve itself) override CPC signals and close the outflow valve if the cabin altitude is 14,500 feet. (A bit late in the game?) This function does not affect the manual mode of operation of the outflow valve.

The cabin altitude and differential pressure indicator is connected to the alternate static system.

The rate of climb indicator detects pressure changes from a port on the back of the indicator.

These things cause the auto fail function:
• Power loss
• Cabin altitude rate of change is too high (>2,000 slfpm)• Cabin altitude is too high (>15,800 ft)
• Wiring failures
• Outflow valve component failures
• CPC failures
• Cabin differential pressure is too high (>8.75 psi). Obviously didn't in the above case.

Flush operations of the vacuum toilet system can cause the cabin rate of climb indicator to momentarily show a high rate of climb indication. This is normal.

[RAT 5]

Flush operations of the vacuum toilet system can cause the cabin rate of climb indicator to momentarily show a high rate of climb indication. This is normal.

Captain breaking wind after a heavy night on the Guinness can cause the cabin rate of climb indicator to momentarily show a high rate of descent. This is normal; and unpleasant. :O

[drooz]

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.

[GREATINDIAN]

Sir, 1st of all I will request to forgive me for making this post here as i am not at all a pilot, even never ever seen a plane closer then 2 kms and sorry to still dare to make this reply before so experienced pilots here.

Mr. Minorite invisible asked a problem and I curiously go through all the replies made here but for my disappointment, none answered here the exact solution to the exact problem that been asked. Just in single line of few words he asked that "What is the proper reaction if any?" and none replied to this question.

Anyways, though i am not a pilot still i would like to dare to reply to this question using my common-sense that what could possibly be done to get autocorrected. For this i would like to bring your attention that what pilots do when out of sudden cabin altitude is lost and experience low pressure!!! we immediately needed to descend down till safe cabin pressure altitude or upto Fl100 whichever is safer at it's highest.

Taking the reference of this condition, I think it wil be better to set the pressurization up to the flight level where the cabin altitude and difference get syncronized and then request ATC for descend up to that flight level informing him the situation. Once you recover the cabin pressure normally then again request ATC for your cruising altitude setting the cabin altitude back to the flight level. I think doing this procedure will reset the pressurization system as the system is set to auto instead of manual and you will get the correct difference when you will reach the cruising altitude.

This is not the reply for what been asked but I just tried to put my view using my common sense and once again I am sorry and beg your forgivings for daring to reply among experienced pilots.

[Derfred]

Derfred says that my suggestion of telling the "Computer" that the Cruise Level is 1000' above actual will not work (in giving a buffer) in the 737NG. If that is so, why bother telling the "Computer" anything about Cruise Level?

Old Fella,

I can't provide a reference because, as I said earlier, the systems description in the FCOM these days is very limited. But I'm reasonably sure that if you wound up the cruise level to 1000' above actual, exactly nothing would happen. The cabin altitude would not change, and the diff would not change.

Obviously, prior to a step climb, we would do exactly that (wind up the cruise alt). But the cabin would not start climbing until the aircraft started climbing.

The reason we set the cruise altitude in the pressurisation is mainly two-fold: (1) so it knows which diff to apply of the three different diffs, and (2) so it knows when to transition from "climb" mode, to "cruise" mode, and to "descent" mode. (A simplification perhaps, it also gives it a target to apply it's cabin rate algorithm to achieve the most comfortable cabin rate).

The system is largely foolproof (when it's working!). For example, you could set FL200 in the pressurisation and climb to FL410, and I'm pretty sure the system would happily keep a safe diff, safe cabin altitude and safe cabin rate. Not recommended of course, but the system would actually cope.

BTW: I've done the F/E gig in a past life, so I know what your're referring to in terms of ye olde semi-auto pressurisation systems. You can hack the 737NG pressurisation system by setting landing alt above current cabin alt - that will achieve the aim you were looking for (immediate cabin climb and diff reduction) and indeed that technique is used in non-normals such as cracked windows and smoke removal, but I wouldn't be recommending it in this scenario. Based on the OP's subsequent feedback, that wouldn't have helped either. Only manual mode was capable of saving the day on this occasion.

At the end of the day, the Boeing checklists kept everyone alive, even though a system failure occured that had not been properly thought out by the designers. So I'm going to keep the faith, although I will keep my "Ole Skool" scans going as described by Rat. :)

[Derfred]

Stator vane... thanks for all that info.

You mentioned the cabin alt/diff gauge is connected to the alternate static. That would be where the diff gets its aircraft altitude from. Do you know where it gets its cabin altitude from?

[FlyingStone]

I believe the cabin pressure (altitude) sensor is on each cabin pressure controller, so in the E&E bay.

[Derfred]

So you're saying the gauge gets it's data from the same source as the auto controllers?

[Minorite invisible]

When we had this problem, we put two snags in the logbook:
1) Failure of the CPC (and failure of the CPC to recognize its fault)
2) Failure of the AUTO FAIL light to illuminate above 8.75 PSI which was clearly the case here.

So maintenance had to find out why these two seemingly distinct problems occurred simultaneously.

It turns out that the real culprit was the failure of one of the Outflow Valve's two Cabin Pressure Switches.

"The cabin pressure switch located in the outflow valve is set to actuate when the cabin altitude exceeds 14,500 feet (8.46 PSIA) and will cause the outflow valve to close independent of the control system. "

".... the cabin altitude at the time the switch closed was much less than 14,500 feet ... the outflow valve closed unknown to the crew and the cabin altitude decreased until the differential pressure exceeded the cabin pressure relief valve threshold'"

When a true depressurization occurs, the sequences of faults in the CPC are:
FC17 (CAB_ALT_10000_FT)
FC18 (CAB_ALT_13,500_FT)
and last FC90 (CAB_PRES_SW_ACTIV) at 14,500 feet but only of no crew action is taken.

There is a cabin pressure switch located in each of the two Eboxes of the outflow valve, but only the active one controls the cabin and the outflow valve but both CPCs will independently record these faults in their memories.

If a FC90 fault occurs only in the active Ebox, the outflow valve will be commanded to close and the AUTO FAIL light will remain extinguished.

A software change (Nord Micro Service Bulletin CSB 21933-21-003) with case the AUTO FAIL light to illuminate in case of the failure of the outflow valve pressure switch).

As a final note, the cabin pressure switch that provides cabin altitude to the gauge in the overhead panel is obviously distinct from the one that controls the cabin and the outflow valve......

[Derfred]

As a final note, the cabin pressure switch that provides cabin altitude to the gauge in the overhead panel is obviously distinct from the one that controls the cabin and the outflow valve......

It may be, but I don't see any evidence of that from your incident. I would be interested in knowing exactly where the gauge gets its cabin alt from.

[Minorite invisible]

It may be, but I don't see any evidence of that from your incident. I would be interested in knowing exactly where the gauge gets its cabin alt from.

Lets hope someone produces that information. It's just a deduction I made.

[172_driver]

Derfred says that my suggestion of telling the "Computer" that the Cruise Level is 1000' above actual will not work (in giving a buffer) in the 737NG. If that is so, why bother telling the "Computer" anything about Cruise Level? Interesting to note the comment on scans. Seems to be very much a reliance on the aircraft monitoring itself. I just hope they are close to infallible.

I think the problem is that none here really knows the wiring inside the pressure controller. I speculated that the opposite might work, i.e. setting a lower cruise level in the window of the CPC. More specifically setting it to a level where the max diff pressure is 7,80 instead of 8,35. The aim being to somehow 'trick' the CPC so it increases cabin altitude until 7,80 is achieved.

Hand on heart it's happened more than once to me that we've flown the entire sector with the pressure controller set to a different cruise level than we've actually flown. That is not say that we're not monitoring the actual cabin altitude, vertical speed and differential pressure at regular intervalls.

Mr. Minorite invisible asked a problem and I curiously go through all the replies made here but for my disappointment, none answered here the exact solution to the exact problem that been asked. Just in single line of few words he asked that "What is the proper reaction if any?" and none replied to this question.

There is no direct checklist for the situation as described initially. The system should've activated the AUTO FAIL light and either the second CPC should've taken over or manual control of the cabin pressure should've ensued. It didn't happen. Discussion about the CPC with the information we've got in the FCOM/AMM has taken place. It's been a useful thread so far.

[Skyjob]

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.

[stator vane]

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.

[Derfred]

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/571035-737ng-flight-altitude-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...

[Derfred]

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.

[FE Hoppy]

Surely diff pressure gives a bit of a clue?

[Old Fella]

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 ?

[Derfred]

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.

[Jonnyknoxville]

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 .

[B2N2]

Ok.

What if you see this?

http://i1244.photobucket.com/albums/gg568/MinoriteInvisilble/fc63e367-8587-4ea9-a866-c2e1f596836e_zpshu6j4dir.jpg

Nothing going on boss.
Carry on.