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kevinh

Pressurisation Defect in Climb

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Until recently I've not bothered to watch the pressurisation system in the NGX during climb and descent. On my last flight I did and noticed a flaw in the model. It seems to try and maintain takeoff airfield altitude in the cabin for as long as possible. Thus after takeoff there's no cabin rate of climb. Differential pressure increases until it reaches the yellow warning band (about 8.5 psi) at around 20,000ft. Then the cabin begins to climb at over 1000 ft/m, though the rate oscillates. As it nears the set altitude the rate decreases to 500 ft/min and Pdiff backs off slowly to settle at 7.9.

 

A real pressurisation system will start the cabin climbing slowly immediately after lift off, perhaps at 400-500 ft/m. The aim is to reach the cruise cabin altitude at the same time as the aircraft reaches it's cruise altitude and at a smooth, slow, rate for pax comfort. Differential pressure only reaches the value at cruise altitude. If the aircraft levels at an intermediate altiutude, the cabin rate also goes to zero. There is a programmed schedule for cabin altitude v aircraft altitude in the controller which sets the schedule for climbs and descents.

 

When descending, the NGX cabin altitude behaved more like I'd expect, beginning to descend as soon as the aircraft began to descend. I was too busy to noitice when cabin altitude reached the set landing altiutude but this should coincide with the aircraft nearing landing altitude.

 

I had both packs set to auto and the pressurisation set to auto. Field altitude at takeoff was about 250 ft, and cruise altitude was set to 35000 ft. I have all failures disabled.

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Could be a fluke. My pressurization works exactly as it should, about 400fpm from T/O.

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Could be a fluke. My pressurization works exactly as it should, about 400fpm from T/O.

 

Congratulations Willie, does not really help though does it. By the way my recirc fan is working just fantastic hows yours? Rolling%20Eyes.gif

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Until recently I've not bothered to watch the pressurisation system in the NGX during climb and descent. On my last flight I did and noticed a flaw in the model. It seems to try and maintain takeoff airfield altitude in the cabin for as long as possible. Thus after takeoff there's no cabin rate of climb. Differential pressure increases until it reaches the yellow warning band (about 8.5 psi) at around 20,000ft. Then the cabin begins to climb at over 1000 ft/m, though the rate oscillates. As it nears the set altitude the rate decreases to 500 ft/min and Pdiff backs off slowly to settle at 7.9.

 

A real pressurisation system will start the cabin climbing slowly immediately after lift off, perhaps at 400-500 ft/m. The aim is to reach the cruise cabin altitude at the same time as the aircraft reaches it's cruise altitude and at a smooth, slow, rate for pax comfort. Differential pressure only reaches the value at cruise altitude. If the aircraft levels at an intermediate altiutude, the cabin rate also goes to zero. There is a programmed schedule for cabin altitude v aircraft altitude in the controller which sets the schedule for climbs and descents.

 

When descending, the NGX cabin altitude behaved more like I'd expect, beginning to descend as soon as the aircraft began to descend. I was too busy to noitice when cabin altitude reached the set landing altiutude but this should coincide with the aircraft nearing landing altitude.

 

I had both packs set to auto and the pressurisation set to auto. Field altitude at takeoff was about 250 ft, and cruise altitude was set to 35000 ft. I have all failures disabled.

 

Many small to mid-sized business jet aircraft, (specifically Hawkers, which I maintain), do in fact employ a pressurization schedule exactly as what PMDG appears to have simulated in the NGX.

 

In these aircraft the cabin will climb at an extremely low rate, (much less than 500 fpm) achieving max delta-p at about 22-to-24,000 feet. In these aircraft, the cabin cabin altitude does indeed remain at approximately 1000 feet, from takeoff right up through the low 20,000 foot range. Once the cabin reaches maximum differential pressure, then, (and only then), will the cabin altitude begin to increase in order to maintain the maximum pressure differential... typically reaching a cabin altitude of about 7,000 feet, when the aircraft gets to about 39,000 feet.

 

That being said, the (real) NG is not a Hawker, and based on the diagram in the FCOM section 2.40 it appears that the NGX pressurization schedule should be more as you describe - i.e. with a steady increase in cabin altitude throughout the climb, with maximum delta-p only being reached at the scheduled cruise altitude.

 

Like you, I have never really paid much attention to the behavior of the NGX pressurization system in a climb to altitude. I'll check it on my next flight to see if I get the same result you did.

 

Perhaps a r/w NG driver or engineer could shed some light on how the system typically behaves in flight?

 

 

 

 

Sent from my iPad using Tapatalk HD

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Hope these help.

DCPCS= Digital Cabin Pressure Control System

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The described climb will let me think you set only the MCP cruise and not the pressurization panel cruise altitude.

If all is set correctly it will work.

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The described climb will let me think you set only the MCP cruise and not the pressurization panel cruise altitude.

If all is set correctly it will work.

if he didnt set it in the pressurization panel he would have gotten the warning horn etc

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The described climb will let me think you set only the MCP cruise and not the pressurization panel cruise altitude.

If all is set correctly it will work.

I see what you mean. I'm 99.9% certain I did set the cruise altitude correctly, but it's possible that I left it at the default value and that would explain things. I've been trying to reproduce the symptoms, but so far the NGX pressurisation has been behaving itself.

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if he didnt set it in the pressurization panel he would have gotten the warning horn etc

Wrong, cruise altitude on the panel is used only for cabin climb schedule, if set incorrectly it will work following its data, and, if maximum differential is approaching, then it will command a cabin climb equal to the aircraft vertical speed, this to prevent to reach the maximum values.

If you leave a value of 10000 for example, the pressurization panel will command a schedule that will mantain the pressurization at the destination altitude, if you set an higher value than your crz altitude it will climb as scheduled until your crz altitude, then, as you will descend without reaching the selected altitude, off sched descend will come on, however, the cabin will continue to be pressurized and the system will correct to prevent either damages and uncomfortable situations.

The 2 only ways (except for aircraft damages or failures) to get the horn sound is to set the controller to manual and open the outflow valve and set both packs off. A wrong value will not cause that.

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Wrong, cruise altitude on the panel is used only for cabin climb schedule, if set incorrectly it will work following its data, and, if maximum differential is approaching, then it will command a cabin climb equal to the aircraft vertical speed, this to prevent to reach the maximum values.

If you leave a value of 10000 for example, the pressurization panel will command a schedule that will mantain the pressurization at the destination altitude, if you set an higher value than your crz altitude it will climb as scheduled until your crz altitude, then, as you will descend without reaching the selected altitude, off sched descend will come on, however, the cabin will continue to be pressurized and the system will correct to prevent either damages and uncomfortable situations.

The 2 only ways (except for aircraft damages or failures) to get the horn sound is to set the controller to manual and open the outflow valve and set both packs off. A wrong value will not cause that.

thx for the clarification

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The pressurization in the NG is kinda weird and reacts quite dufferently to other aircraft like beechcraft for example. Robert got the details correct in this.

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The pressurization in the NG is kinda weird and reacts quite dufferently to other aircraft like beechcraft for example. Robert got the details correct in this.

There's nothing weird about the pressurisation in the NG, or any 737 for that matter. I'm well aware how it should work, it's just that on at least one occasion it hasn't worked like that in the NGX.

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There's nothing weird about the pressurisation in the NG, or any 737 for that matter. I'm well aware how it should work, it's just that on at least one occasion it hasn't worked like that in the NGX.

 

Actually, compared to aircraft like Beechcraft, yes it is weird.

 

JackColwill

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T

 

 

Actually, compared to aircraft like Beechcraft, yes it is weird.

 

JackColwill

Sorry Jack, but you'll have to explain what it is about Beechcraft pressurisation that makes the 737, along with all other Boeing and Airbus airliners, weird.

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T

Sorry Jack, but you'll have to explain what it is about Beechcraft pressurisation that makes the 737, along with all other Boeing and Airbus airliners, weird.

Think jack was having a laugh if my memory is alright jack is a rw pilot on the 737s, but i could be wrong jack can correct me on this if iam wrong

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Jack C is a RW pilot on the NG's and was a King Air jockey in his previous jobs.

 

The main different and wacky thing about the NG is the different delta P 'limitations' that the pressurisation systems schedules with, according to different cruise altitudes and such.

 

I'm only familiar with the pressurisation systems on the older 90's (superchargers baby!) and -100's but there all you can do is set what you want for cabin alt and play with the rate knob. The controller is most definitely not digital, with a mess of diaphragms, tubing, and springs.

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OK, without knowing Jack C's history I didn't spot the irony.

 

That doesn't really make the 737NG wacky though. Most modern airliners have similar pressurisation schedules. Even some much older ones. The 727-200 pressurisation control schedule isn't much different (though it has analogue electronics of course). The 727 system even had different delta P limits depending on which mode you were in.

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Congratulations Willie, does not really help though does it. By the way my recirc fan is working just fantastic hows yours? Rolling%20Eyes.gif

Oh god it's you again....

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