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DalSingh

Pressure Alarm? - How to fix?

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Hi AllI've had a very annoying problem with my recent flights on the NGX. Whilst the aircraft is climbing above 10,000ft, an alert (buzzer) will be raised. The most frustrating thing is that no error warning is displayed so you have no idea what is causing it. mad.gifExample Video:

I realised it was to due with the pressurization failure from the comments on that video. The thing is, I toggled some of the switches (L/R Air Packs) and the alert would not stop, then I toggled some of the bleed air switches and the alert still persisted. I even toggled with the 'Q' sound on/off key just in case FSX was stuck in a loop but that did not fix the situation.Eventually after more toggling of the air pack/bleed switches I managed to stop the sound but I have no idea how.I am able to start the 737NGX from a Cold/Dark setup (inc engines), so I have a fair understanding of the bleed switches but the air packs I cannot get my head around. The pressuraization panel always has 10,000ft for ALTITUDE and 1500 for LAND - but usually when this alarm is active the pressurization panel is inoperable and the digits are replaced by (-) hypens.airpacks.pngCould somone please provide me with the switch settings (Panel Above) for the following phases:COLD/DARK:ENGINE STARTUP:AFTER ENGINE STARTUP:CRUISE:I will take another look at the 737ngx checklist later today - (I'm not home right now).Thanks - I appreciate any help. wink.pngDal

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According to the normal procedures in FCOMv1: Cold & Dark: pack switches off, isolation valve close, all bleeds off, recirculation fans off.Engine startup: pack switches off, isolation valve open, all bleeds on, recirculation fans on.After startup: pack switches on, isolation valve auto, engine bleeds on, APU bleed off, recirculation fans on.Cruise: As above There are variations, but this is what is in the book.

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Hmm, if you turn your packs to auto and wait a few moments the alarm should stop and the digits should reappear, that's a pressurization alarm as you already know, but the alarm won't go off as soon as you switch the packs to auto, you need to wait a few moments. You should have a master warning light on the fo side though when this happens.

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You need to set the pressurization controller for your planned cruise altitude before takeoff. The 10,000 altitude is the default setting, but you should manually change it to whatever cruise altitude you plan to fly at. Likewise, you should set the "land" window to the elevation of your intended destination airport. All bleed air switches (engines and APU) should be ON prior to engine start. You can run the packs in AUTO during your preflight setup, but turn both packs OFF before starting engines, or you won't have enough bleed air pressure from the APU to spin up the engines during start. Once the engines are started and stabilized, turn the APU bleed air OFF, but leave both engine bleed air switches ON. Switch both packs to AUTO after engine start, and leave them there for the rest of your flight. If the packs are not turned on (by placing the switches in AUTO) there will be no flow of air into the fuselage to pressurize the aircraft during climb. Also, the recirculation fan switch should be placed ON once the aircraft is powered up from cold and dark, as well as the "Trim Air" switches located immediately above the bleed/pack panel. The "isolation" switch can remain in AUTO at all times.

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According to the normal procedures in FCOMv1: Cold & Dark: pack switches off, isolation valve close, all bleeds off, recirculation fans off.Engine startup: pack switches off, isolation valve open, all bleeds on, recirculation fans on.After startup: pack switches on, isolation valve auto, engine bleeds on, APU bleed off, recirculation fans on.Cruise: As above There are variations, but this is what is in the book.
Thank you very much trisager, thats exactly what I wanted to know. im%20Not%20Worthy.gif
Hmm, if you turn your packs to auto and wait a few moments the alarm should stop and the digits should reappear, that's a pressurization alarm as you already know, but the alarm won't go off as soon as you switch the packs to auto, you need to wait a few moments. You should have a master warning light on the fo side though when this happens.
Carlos, you are exactly right! - after giving up on the switches, walking out the room and coming back the noise went away. The master light switch does illuminate and I was hoping to see an error read out. - thank you for the help! biggrin.png
You need to set the pressurization controller for your planned cruise altitude before takeoff......
Thanks a lot Jim - that all makes much more sense now. :)

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Without setting cruise altitude pressurization system will continue to work. Also by setting 0 at crz altitude on the pressurization panel, it will work and you will never hear the alarm. The only problem is the crew/passenger comfort if climbing more than maximum differential pressure, in this case the cabin will climb at the same rate of the airplane, and 2-3000ft/min are a bit high for our bodies.

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With all the respect to the checklist, for the beginners is better to know how the system works.Why turning switches on or off if you don't know what they do?The pressurization system pressurize the plane to mantain a comfortable pressure for the passengers while the aircraft is flying with few air pressure around it.The system simply uses the air conditioning system to pump the air inside the fuselage. An outflow valve controlled by the pressurization system will modulate to pressurize the plane as programmed by the computer.The air pumped inside the cabin is taken from the bleed ducts. These ducts (left and right) are feeded by the engines or the APU. APU and LH engine feeds l bleed ducts, RH engine and ground cart pressurize the right one.To separate the 2 ducts there is an isolation valve.Ok, look at the panel, maybe you have already found all the switches in this explanation. But looking at it, you will see lines that starts from a switch to another, it is a schematic on how they are connected.To pressurize te aircraft you will need bleed/s pressure and pack/sLooking to the panel, you will see that the left bleed supplies air to the left pack.So if you want the left pack running basically turn on one of the onside bleeds. OR, following the lines, opening the isolation valve, you can pressurize using the right one.In normal flight condition you have both engine bleed ON, so, isolation valve can be closed (the auto logic will do for you) and each bleed feeds its onside pack. Pressurization will be automatically controlled if the controller is in AUTO.The pressurization must know the cruise altitude, this is needed to program the cabin pressurization rates because the aircraft is like a balloon, by climbing high where outside pressure is extremelly low, the difference between internal and external pressure can blow the fuselage.Pressure must be mantained to be under 8.3 PSI of difference.If the controller knows the cruise altitude before the departure will calculate a rate that will let the cabin pressure to be reduced at a low rate, comfortable, that, at crz altitude, is less than the maximum allowable difference from outside.If the controller is set incorrectly it will pressurize the cabin for the selected value, but, climbing more, will mantain the cabin pressure until limits are reached, as soon as the cabin reaches the limits, the controller will adjust the rate of cabin pressure to the rate of the aircraft (airplane climbs at 3000ft/min? cabin climbs at the same value or the maximum differential pressure will be passed).

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Thanks for the explantion Andrea. I am aware that there is a schematic just like the one features on the power bus for APU/GEN. My mistake was not realising that changes to some of the switches take time, I was expecting an immediate fix - that alarm sound was making my ears bleed.

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Oh, do I have the new sky interior? No, it's just that all my pax have been rendered cyanotic. LMAO.gif

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there is an alarm cutoff switch left of the pressurization panel.
If you cannot re-pressurize the plane after the alarm yes, you can use the cut out, in the simulation, yes, you can use ita s you have no passenger and your life is not in danger, but I think is better to solve the problem istead of suppressing a warning if you want to simulate correctly.I think that PMDG could simulate better the system by adding a complete loss of the plane control (pilot unconscious or death) after depressurization and if he didn't clicked on the oxy mask (or oxygen bottle empty)laugh.png

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