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Uncontrollable Cabin Pressurisation B737NG

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Uncontrollable Cabin Pressurisation B737NG

Old 19th Oct 2018, 09:45
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Uncontrollable Cabin Pressurisation B737NG

Hi

is there a Cabin ROC that you will constitute as uncontrollable in Cabin Alt Warning/Rapid decompression situation?

or is the best indicator simply Cabin altitude?

500fpm ROC

regards

pin
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Old 19th Oct 2018, 12:11
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No. If it doesnít do what you tell it to do then itís uncontrollabel.
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Old 19th Oct 2018, 12:28
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Agree with Chesty - does the ROC react to commanded movements. This is usually quickly evident in the sim anyway. Caution when looking at the ROC gauge - full cabin climb which reverts to full cabin descent can be easily missed and misinterpreted as uncontrollable when itís in fact controllable.
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Old 19th Oct 2018, 20:31
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In addition to the above...

Watch the DIFF needle, itís the biggest and easiest to see.
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Old 20th Oct 2018, 01:05
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Should rate of change react reasonably quickly? If so how long would you give it?
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Old 20th Oct 2018, 21:54
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Originally Posted by Pin Head View Post
Should rate of change react reasonably quickly? If so how long would you give it?
It depends on what is causing the issue largely, and also the thrust setting to a smaller degree.
It is important to take your time.

Basically, your only avenues to control it are, switching to Alternate mode and seeing if the computer can have a second go at it, or Standby mode and controlling the outflow valve yourself.

If you have an Unscheduled Pressurization Change, take the time to look at the Cab Alt, Diff and cabin VSI, but also take note of the Outflow Valve position.
If the valve is wide open due to a controller fault, then Alternate or Manual mode may gain control.
If the valve is already been driven fully closed by the controller, then you have a leak in the airplane, and you probably will not gain control of it.


If you hear a bang and the diff needle is racing towards zero, there is not much chance you are going to control it by closing the outflow valve.... but you still need to take the time to analyse it fully. Once you have gone to manual mode and confirmed that the outflow valve is fully closed, but the diff needle is still falling toward zero rapidly, itís safe to say you donít have control of it.

If you go to Manual mode, close the outflow valve and the diff needle slows or stops, then you may have control of it. If the needle starts going back the other way then you are pressurising the airplane.

With the outflow valve fully closed at cruise or climb thrust, you will pump the airplane up fairly quickly, remember to MAINTAIN control of the cabin.
It becomes a full time job for the RHS pilot....

Long story short, take your time....

Last edited by Switchbait; 20th Oct 2018 at 21:57. Reason: typos
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Old 21st Oct 2018, 01:20
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Long story short, take your time....
Sound advice indeed. The majority of simulator exercises involving a depressurisation problem seem to be a rapid depressurisation scenario where everything is rushed as a matter of dire urgency. This can lead to a bit of a trap in that if the actual problem in the real aircraft is nothing more than an uncommanded pressurisation change, rather than a obvious rapid decompression requiring an emergency descent, there may be a tendency for the crew to revert to their simulator experience and get going down fast without first evaluating the real problem. Been there done that in a 737-200 in the middle of the night over the South Pacific ocean.

In our case, the first indication was a cabin altitude warning horn as the cabin altitude climbed through 10,000 feet as we were climbing through 31,000 feet. The warning caught us completely by surprise as there was no ear distress.

In those days, the Boeing SOP required the captain to take over control, call the drills aloud and among other things call for the F/O to close the outflow valve. In the meantime I commenced the emergency descent even though the initial cabin rate of climb was less than 1500 fpm. It turned out later the problem was a faulty pressure controller. The closing of the outflow valve was the killer for our ears and the ear pain was very bad during the rapid descent. It was only when I realised I had cocked up the diagnosis (which I conveniently blamed afterwards on poor simulator training in that everything was rush-rush-rush), that I saw the cabin altitude had gone from circa 12,000 feet to 3000 feet in under 90 seconds. That's what happens when you close the outflow valve fully when there was no need to.
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Old 21st Oct 2018, 02:34
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One needs to follow the procedures.

No tinkering or messing around if you get Cabin Alt warning!
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Old 21st Oct 2018, 06:37
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Pinhead,
The procedure for Cabin Alt warning horn requires Oxygen on,, establish crew comms, manual mode, valve closed;
THEN an assessment of whether or not the cabin altitude is controllable. Some tinkering may be required at this point.

The most important actions when the CABIN ALTITUDE WARNING goes off are, oxygen on 100% (for obvious reasons), establish crew communications (so you can make a joint assessment of the problem).

The majority of in service pressurization problems are controller faults, and in the NG these are rare, but there is a good chance that if you take your time you will regain control of the cabin.

The majority of SIMULATOR exercises are faults designed to force you in to an Emergency Descent, to tick that box..... But depending on the syllabus or the fault induced by the TRE, you might just avoid all the drama if you take the time to assess things. This can be done with you Oxygen mask on after the horn has gone off.

The humble and honest account given above by Tee Emm is a great example to learn from.

Cheers
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Old 21st Oct 2018, 15:41
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A 400ft per min ROC was good enough for me to go down after outflow value closed and a period of time elapsed.
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Old 22nd Oct 2018, 04:48
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I would rather know about a developing problem than wait until the cabin alt warning goes off. Therefore I check the system periodically during climb, cruise & decent - more often during climb & descent.

During one descent in a B737NG, I was surprised to find the cabin climbing at around 500fpm when I checked the system at around 30,000í. After watching for a short period of time to see if it was a temporary condition, it was obvious we had a problem as the ROC was constant. I directed the F/O to disconnect the A/T & push the power up to around 60% N1. The cabin then went from a 500fpm climb to a 600-700fpm descent. The system stayed in the normal mode, so hadnít detected a fault. We used speed brake against the power & with a lap of the pattern on the way in (ATC requirement, not ours) we had plenty of track miles to get the cabin down before landing.

The engineers checked it out & found a sticky outflow valve was the issue. Too much foreign material had built up in the mechanism. They couldnít tell me why the system hadnít detected the fault & swithed to ALTN. They did concur that it wouldnít have made any difference anyway, given the nature of the problem.
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Old 22nd Oct 2018, 12:47
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Originally Posted by Oakape View Post
During one descent in a B737NG, I was surprised to find the cabin climbing at around 500fpm when I checked the system at around 30,000í. After watching for a short period of time to see if it was a temporary condition, it was obvious we had a problem as the ROC was constant. I directed the F/O to disconnect the A/T & push the power up to around 60% N1. The cabin then went from a 500fpm climb to a 600-700fpm descent. The system stayed in the normal mode, so hadnít detected a fault. We used speed brake against the power & with a lap of the pattern on the way in (ATC requirement, not ours) we had plenty of track miles to get the cabin down before landing.

The engineers checked it out & found a sticky outflow valve was the issue. Too much foreign material had built up in the mechanism. They couldnít tell me why the system hadnít detected the fault & swithed to ALTN. They did concur that it wouldnít have made any difference anyway, given the nature of the problem.
In addition, the 737NG is very prone to the vacuum toilets being flushed, which can give rates of up to 800 fpm momentarily.
In addition, the 737NG cannot cope well with selecting a higher FL (a new selection above FL370 while previous selection was at or below), this can cause up to -1000 fpm until stabilised.
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Old 7th Nov 2018, 20:38
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POSTED 4 YEARS AGO BY B M.

A little thinking outside the TRIs box and the cabin alt/diff guage is soooh easy to interpret!

It reads like a clock, 12 o'clock on the ground at sea level airports.

By the time the after T/O checklist is actioned, the "clock" reads 1210 to 1215 with the "minute hand" ALWAYS leading the "hour hand", just like any normal analogue watch/clock.

By 10,000 ft, it reads 1225, by 20,000 ft it reads 1235 and at 30,000 ft it reads 1340ish.

If it doesn't, it is blindingly obvious, especially when the little hand moves faster/further than the big hand.

The same obvious trend, with slightly different "times", will be obvious after takeoff from non-sealevel fields.

In various companies I tried to get the TRIs to try this as a method for teaching new to type trainees, but alas the not-invented-here syndrome ensured this easy unambiguous way of interpreting the 2 pointer instrument was never adopted.

Unless since then Mr Bong has changed the press'n instrument indicator............?

And I have forgotten the new generation of airframe managers is unable to read an analogue watch/clock so I'm "wasting my sweetness on the desert air".

S'not rocket science.
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