LDG ALT (NNOs)
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LDG ALT (NNOs)
Hello,
Humble question once more from me at the depths of airline flying, this time regarding the checklist following a smoke detection/fire warning in the cargo hold (any one), onboard the 747.
In the QRH, it says to, if above 8,000ft:
LDG ALT switch... MAN
LDG ALT selector... Set ther landing altitude to between 8,000 and 8,500ft to command cabin altitude to 8,000ft.
May I simply ask what LDG ALT is exactly, what purpose it serves and how it has anything to do with the procedure following a smoke/fire detection?
Further down, it says in the Deferred Items checklist, under Before descent: LDG ALT switch... AUTO. I'd have thought that knowing you will land at the nearest suitable airfield, you're automatically put in a 'Before descent' situation, making the former checklist items irrelevent! (setting to MAN, putting 8,000).
Thanks in advance!
TP.
Humble question once more from me at the depths of airline flying, this time regarding the checklist following a smoke detection/fire warning in the cargo hold (any one), onboard the 747.
In the QRH, it says to, if above 8,000ft:
LDG ALT switch... MAN
LDG ALT selector... Set ther landing altitude to between 8,000 and 8,500ft to command cabin altitude to 8,000ft.
May I simply ask what LDG ALT is exactly, what purpose it serves and how it has anything to do with the procedure following a smoke/fire detection?
Further down, it says in the Deferred Items checklist, under Before descent: LDG ALT switch... AUTO. I'd have thought that knowing you will land at the nearest suitable airfield, you're automatically put in a 'Before descent' situation, making the former checklist items irrelevent! (setting to MAN, putting 8,000).
Thanks in advance!
TP.
Last edited by Tonic Please; 27th Jun 2012 at 19:05.
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TP
Without referring to my manuals (and not having flown the B744 for nearly 3 years!):
When the LDG ALT switch is pushed in (in AUTO), it takes the elevation of the destination airfield as programmed in the FMC and uses that known elevation to schedule the pressurisation profile for the cabin.
1. eg Depart LHR elevation 80ft, destination MEX elevation ~7500ft amsl.
the press controller will climb the aircraft to a cabin altitude of 7500ft even if that means the aircraft is below max pressure differential for the airframe. ie - if the aircraft is flying at 35000ft and it could maintain a cabin altitude of 5600ft, it would still schedule an altitude of 7500ft as that is the destination elevation.
2. eg Depart LHR elevation 80ft, destination JFK elevation ~20ft amsl.
the press controller will climb the aircraft to a cabin altitude appropriate to max press diff eg at 35000ft to a cabin altitude of 5600 ft.
So, what the QRH is doing is to say to exit AUTO mode and instruct the pressurisation schedule to set a cabin altitude of 8000-8500ft to keep it safe for pax whilst depriving any fire source of as much oxygen as possible.
Later, before descent, it is set back to AUTO to schedule proper pressurisation prior to landing to eliminate any unwanted pressrisation differential between the aircraft and the landing environment.
Without referring to my manuals (and not having flown the B744 for nearly 3 years!):
When the LDG ALT switch is pushed in (in AUTO), it takes the elevation of the destination airfield as programmed in the FMC and uses that known elevation to schedule the pressurisation profile for the cabin.
1. eg Depart LHR elevation 80ft, destination MEX elevation ~7500ft amsl.
the press controller will climb the aircraft to a cabin altitude of 7500ft even if that means the aircraft is below max pressure differential for the airframe. ie - if the aircraft is flying at 35000ft and it could maintain a cabin altitude of 5600ft, it would still schedule an altitude of 7500ft as that is the destination elevation.
2. eg Depart LHR elevation 80ft, destination JFK elevation ~20ft amsl.
the press controller will climb the aircraft to a cabin altitude appropriate to max press diff eg at 35000ft to a cabin altitude of 5600 ft.
So, what the QRH is doing is to say to exit AUTO mode and instruct the pressurisation schedule to set a cabin altitude of 8000-8500ft to keep it safe for pax whilst depriving any fire source of as much oxygen as possible.
Later, before descent, it is set back to AUTO to schedule proper pressurisation prior to landing to eliminate any unwanted pressrisation differential between the aircraft and the landing environment.
Last edited by TopBunk; 27th Jun 2012 at 19:56.
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How fascinating.
Unfortunately, you force me to pose a secondary question:
How does increasing cabin pressure from standard 5,600 (with a 'normal elevation' airfield such as your JFK example) to ~8,000, reduce the amount of oxygen to deprive a potential fire? Is this MAN LDG ALT only used during the cruise portion and NEVER during descent, or can AUTO be reselected even just a few thousand feet after descent has begun?
E.g: flying over the pacific from, say KLAX to YSSY, you get your smoke/fire warning relatively near to YSSY (say 250NM away), so you won't necessarily be diverting (or even if you are, my question still stands)... you would do the checklist to this point, then set LDG MAN, whack in 8,500 causing cabin pressure to increase from 5,600 and depriving any potential fire source of its needed oxygen (how?). This cabin pressure change occurs over what time frame? If you were to divert, T/D would be closer so the cabin pressure perhaps wouldn't have even made it to 8,500 by the time you start your descent - making it reduntant? Do you see my little confuzzlement?
Response(s) muchly appreciated,
TP
Unfortunately, you force me to pose a secondary question:
How does increasing cabin pressure from standard 5,600 (with a 'normal elevation' airfield such as your JFK example) to ~8,000, reduce the amount of oxygen to deprive a potential fire? Is this MAN LDG ALT only used during the cruise portion and NEVER during descent, or can AUTO be reselected even just a few thousand feet after descent has begun?
E.g: flying over the pacific from, say KLAX to YSSY, you get your smoke/fire warning relatively near to YSSY (say 250NM away), so you won't necessarily be diverting (or even if you are, my question still stands)... you would do the checklist to this point, then set LDG MAN, whack in 8,500 causing cabin pressure to increase from 5,600 and depriving any potential fire source of its needed oxygen (how?). This cabin pressure change occurs over what time frame? If you were to divert, T/D would be closer so the cabin pressure perhaps wouldn't have even made it to 8,500 by the time you start your descent - making it reduntant? Do you see my little confuzzlement?
Response(s) muchly appreciated,
TP
Last edited by Tonic Please; 28th Jun 2012 at 08:02.
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Originally Posted by TP
How does increasing cabin pressure from standard 5,600 (with a 'normal elevation' airfield such as your JFK example) to ~8,000, reduce the amount of oxygen to deprive a potential fire?.
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Hi BOAC,
Decreases from 5,400 to 8,500? I just realised when typing that why that is. The aircraft is at FL350, and the differential is usually 35,000 - 5,400 but now it's 'decreased' following the 'increase' from 35,000 - 8,500. Logical, really.
But I think I should go and read up on pressures affection oxygen supplies to fuel fires since I have no knowledge of that.
Thanks for the heads-up.
TP
Decreases from 5,400 to 8,500? I just realised when typing that why that is. The aircraft is at FL350, and the differential is usually 35,000 - 5,400 but now it's 'decreased' following the 'increase' from 35,000 - 8,500. Logical, really.
But I think I should go and read up on pressures affection oxygen supplies to fuel fires since I have no knowledge of that.
Thanks for the heads-up.
TP
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More Airflow
In our Fire, Smoke and Fumes checklist we raise the cabin to ~9,500 ft to help evacuate smoke. With the same amount of conditioned bleed air coming in, but less psid to maintain, the outflow valves let more air out. We crank up the rate controller as well to make the change happen quickly.
So long as it's not the pack thats the source of the smoke, this works pretty well...
So long as it's not the pack thats the source of the smoke, this works pretty well...
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Thank you for that, and your first PPRuNe post 
I have one further question: Pressing the ARM button for an AFT/FWD cargo fire warning, does this automatically change cabin pressure (as in, raise the cabin altitude from the ~5,600 to any particular altitude), even if you have not selected LDG ALT to MAN and manually selected ~8,500?
Thank you!
TP
I have one further question: Pressing the ARM button for an AFT/FWD cargo fire warning, does this automatically change cabin pressure (as in, raise the cabin altitude from the ~5,600 to any particular altitude), even if you have not selected LDG ALT to MAN and manually selected ~8,500?
Thank you!
TP
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TP
No it doesn't.
Arming the switch shuts off PACK 3 which is then backed up by manually moving the Pack 3 switch to OFF (amongst other items in the QRH drill).
Any change to the cabin alt is performed by the LDG ALT > MANUAL and selecting the desired altitude.
No it doesn't.
Arming the switch shuts off PACK 3 which is then backed up by manually moving the Pack 3 switch to OFF (amongst other items in the QRH drill).
Any change to the cabin alt is performed by the LDG ALT > MANUAL and selecting the desired altitude.
Last edited by TopBunk; 1st Jul 2012 at 09:05.
