A320 - pneumatics & pressurization doubts
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A320 - pneumatics & pressurization doubts
QUESTIONS ON - PNEUMATICS & PRESSURIZATION
Dear all . Kindly help me with the following A320 queries . Thank you for your time in answering
1. SOP - PARKING - If the APU is used, set the APU BLEED pb-sw to ON immediately before engine shutdown to prevent engine exhaust fumes from entering the air conditioning.
How can switching ON APU BLEED just before (immediately before) engine shut down prevent it from sucking engine exhaust into air conditioning ?? . My confusion is if i start APU bleed 10 minutes before engine shutdown , wont the engine exhaust still be sucked into the air conditioning ? I am not able to understand this logic
2. Regarding Air conditioning PACKS, what ever selected on the PACK FLOW SELECTOR, the system delivers higher flow during single pack operation. So if we have an engine failure the pack on the operating engine side will go on higher flow, but now if I open the cross bleed valve sine both packs are being supplied by operating engine , both packs go to normal flow (since this is not a single PACK case) . Am I correct ?
3.Safety valve relief setting is 8.6 psi (limitation) Does safety valve work for excessive negative Delta P also ? If yes where is this value given ?
4.FCOM SYSTEM DESCRIPTION, PRESSURIZATION CHAPTER - Note: The safety valve opens when the indicated ∆P is between 8.9 and 9.7 PSI. The range is due to the reduced accuracy of ∆P measurements (in MAN mode), combined with the
decrease in ∆P that occurs immediately after the safety valves open.
I understand this means that in MAN mode the safety valve will open when the Delta P indication is anywhere between 8.9 to 9.7 psi , This is applicable only to MAN mode of operation. When in AUTO mode the safety valve opens at 8.6 psi . Am i correct?
4. With both CPC failed the FAULT light on the CAB PRESS panel MODE SEL pb switch illuminates (indicating both SYS 1 & 2 failure)
When MODE SEL pb sw is put to MAN, will the FAULT light extinguish ? In the simulator I have seen the FAULT light remains illuminated , however some maintenance manual shown to me indicated that the Fault light will extinguish ( however this Maintenance manual shown to me was quite an old revision) .
Some A320 Question Bank also mentioned that the FAULT light extinguish when the MODE SEL is set to MAN
But as per my understanding when CPC MODE SEL put to MAN the FAULT light does not extinguish but stays illuminated . AM i correct?
Thank you for your kindness in taking the time to answer my questions.
Dear all . Kindly help me with the following A320 queries . Thank you for your time in answering
1. SOP - PARKING - If the APU is used, set the APU BLEED pb-sw to ON immediately before engine shutdown to prevent engine exhaust fumes from entering the air conditioning.
How can switching ON APU BLEED just before (immediately before) engine shut down prevent it from sucking engine exhaust into air conditioning ?? . My confusion is if i start APU bleed 10 minutes before engine shutdown , wont the engine exhaust still be sucked into the air conditioning ? I am not able to understand this logic
2. Regarding Air conditioning PACKS, what ever selected on the PACK FLOW SELECTOR, the system delivers higher flow during single pack operation. So if we have an engine failure the pack on the operating engine side will go on higher flow, but now if I open the cross bleed valve sine both packs are being supplied by operating engine , both packs go to normal flow (since this is not a single PACK case) . Am I correct ?
3.Safety valve relief setting is 8.6 psi (limitation) Does safety valve work for excessive negative Delta P also ? If yes where is this value given ?
4.FCOM SYSTEM DESCRIPTION, PRESSURIZATION CHAPTER - Note: The safety valve opens when the indicated ∆P is between 8.9 and 9.7 PSI. The range is due to the reduced accuracy of ∆P measurements (in MAN mode), combined with the
decrease in ∆P that occurs immediately after the safety valves open.
I understand this means that in MAN mode the safety valve will open when the Delta P indication is anywhere between 8.9 to 9.7 psi , This is applicable only to MAN mode of operation. When in AUTO mode the safety valve opens at 8.6 psi . Am i correct?
4. With both CPC failed the FAULT light on the CAB PRESS panel MODE SEL pb switch illuminates (indicating both SYS 1 & 2 failure)
When MODE SEL pb sw is put to MAN, will the FAULT light extinguish ? In the simulator I have seen the FAULT light remains illuminated , however some maintenance manual shown to me indicated that the Fault light will extinguish ( however this Maintenance manual shown to me was quite an old revision) .
Some A320 Question Bank also mentioned that the FAULT light extinguish when the MODE SEL is set to MAN
But as per my understanding when CPC MODE SEL put to MAN the FAULT light does not extinguish but stays illuminated . AM i correct?
Thank you for your kindness in taking the time to answer my questions.
Joined: May 2007
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From: Somewhere at the Milky Way
1- The APU flap lever is just below the fuselage and smoke from the running engines can go in, we processed by the APU and then thrown inside the cabin. The smoke itself is uncomfortable and many people don't like it. If the smoke is dense, it can trigger a false smoke perception on the passenger cabin with all it goes, and might end up with an evacuation drill.
the APU bleed is turned on to avoid another pack on/off cycle, to continue providing fresh air to the cabin when the engine bleed is stopped and to assist if a tailpipe fire event happens.
2- Correct, but high flow is not dependant on how many packs you have on, but it is the system which will provide high air flow to one or both packs with a single engine source or APU at ground.
3- I don't recall any value for negative pressure values, though there might be one.
4- Operation in manual mode is slow. As you said, the control in manual mode is less precise, it operates under DC motor opposed to an AC source, and the readings might be slow and difficult to read, plus some people might get impatient and keep ticking on the selector without waiting enough time, hence why there is that ample margin.
4b- The answer is in the FCOM.
the APU bleed is turned on to avoid another pack on/off cycle, to continue providing fresh air to the cabin when the engine bleed is stopped and to assist if a tailpipe fire event happens.
2- Correct, but high flow is not dependant on how many packs you have on, but it is the system which will provide high air flow to one or both packs with a single engine source or APU at ground.
3- I don't recall any value for negative pressure values, though there might be one.
4- Operation in manual mode is slow. As you said, the control in manual mode is less precise, it operates under DC motor opposed to an AC source, and the readings might be slow and difficult to read, plus some people might get impatient and keep ticking on the selector without waiting enough time, hence why there is that ample margin.
4b- The answer is in the FCOM.
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Thank you for your reply
Point 1 : I understand about the pack cycle and tail pipe fire .
However I wanted to know when FCOM SOP says putting the APU bleed ON just before engine shutdown will minimize odor (because like you said the APU inlet will suck the engine exhaust), how can this odor be prevented if APU Bleed is put ON just before engine shut down . Why will the exhaust fumes (from engine) NOT be ingested by APU if I switch on APU Bleed 10 minutes before engine shut down , I am not understanding this . In short why just before engine shutdown put the APU bleed ON (as it is specifically mentioned in the FCOM SOP - PARKING)
Point2: So you mean if I am on single engine even if i put the crosbleed ON , the packs will still be on HIGH FLOW. Though the FCOM very specifically says when on SINGLE PACKS - only then high flow . So in the case I mentioned , with single engine if I put the crossbleed ON both packs are now working (though from a single bleed source) Does that mean it will still be on High flow?
Point 3: Thank , i have understood now . grateful for your explanation.
Point4b : not able to find this in FCOM , thus asked here
Point 1 : I understand about the pack cycle and tail pipe fire .
However I wanted to know when FCOM SOP says putting the APU bleed ON just before engine shutdown will minimize odor (because like you said the APU inlet will suck the engine exhaust), how can this odor be prevented if APU Bleed is put ON just before engine shut down . Why will the exhaust fumes (from engine) NOT be ingested by APU if I switch on APU Bleed 10 minutes before engine shut down , I am not understanding this . In short why just before engine shutdown put the APU bleed ON (as it is specifically mentioned in the FCOM SOP - PARKING)
Point2: So you mean if I am on single engine even if i put the crosbleed ON , the packs will still be on HIGH FLOW. Though the FCOM very specifically says when on SINGLE PACKS - only then high flow . So in the case I mentioned , with single engine if I put the crossbleed ON both packs are now working (though from a single bleed source) Does that mean it will still be on High flow?
Point 3: Thank , i have understood now . grateful for your explanation.
Point4b : not able to find this in FCOM , thus asked here
Joined: Jan 2004
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From: Here and there
1. Airbus wants the APU bleed on prior to shutdown so it is available for a tailpipe fire and to avoid excessive pack cycles. The statement to do it immediately before shutdown to avoid fumes is in comparison to turning it on earlier. There is even less chance of getting fumes if you leave it until after shutdown down but this is not what Airbus recommend. They think it should be on, they recognise there may be fumes, to minimise the fumes they suggest putting it on as late as possible but still before engine shutdown.
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Joined: Mar 2018
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From: not in database
1. Airbus wants the APU bleed on prior to shutdown so it is available for a tailpipe fire and to avoid excessive pack cycles. The statement to do it immediately before shutdown to avoid fumes is in comparison to turning it on earlier. There is even less chance of getting fumes if you leave it until after shutdown down but this is not what Airbus recommend. They think it should be on, they recognise there may be fumes, to minimise the fumes they suggest putting it on as late as possible but still before engine shutdown.
If the APU Bleed is switched ON just before engine shut down , there will be some engine exhaust sucked in by the APU inlet but it will be minimum as compared to if you switch it on minutes earlier
Thank you
