B737NG: High Cabin Diff Pressure
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Also, did you ever get any feedback from maintenance as to what caused it?
Last edited by Derfred; 1st May 2017 at 14:43.
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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.
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.
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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?
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?
Last edited by Derfred; 1st May 2017 at 15:41.
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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......
Last edited by Minorite invisible; 1st May 2017 at 19:40.
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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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All I can find...
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.
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.
Last edited by stator vane; 2nd May 2017 at 08:49. Reason: correction
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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.
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.
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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?
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.
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.
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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.
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.
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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.
Last edited by Derfred; 2nd May 2017 at 16:49.
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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?
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?
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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......
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......
Last edited by Minorite invisible; 2nd May 2017 at 15:48.
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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......
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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.
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.
