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# Minimum value, derate.

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Hi all,Two silly questions for today, but I'm not sure of what I'm doing, so let's get it as real as it is.Regarding the Minimums settings:- what difference and what / when to choose in the Minimum knob: Baro or the other ?- how to properly calculate the Minimums value ?Regarding the derate, I understood the role of derate, but in the FMC, I'm always setting 30° as temperature, as it's what read in a tutorial. But this mean that derate is only in use when temp is >30° or when we choose a % derate manually ?Edit: what Cabine altitude to set irl ? I normally set it to cruise Alt. is that right ?Thanks, I'll sleep less noob tonite.

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Regarding the Minimums settings:
From the appropriate approach plate, or set to 200 radio if you have no plate.
Regarding the derate, I understood the role of derate, but in the FM, I'm always setting 30° as temperature, as it's what read in a tutorial. But this mean that derate is only in use when temp is >30° or when we choose a % derate manually ?
There are two ways to derate.1. Select TO1 or TO2, these are fixed thrust derates. You are basically limiting the engines to a lower thrust rating, just as if the aircraft were fitted with less powerful engines. It's also possible for the engineers to configure the engine to a lower thrust rating before delivery.2. Assumed Tempreture Derate: This method tells the systems to set a thrust that would be available at higher ambient temperature.Obviously if the outside temperature is higher, then a jet engine isn't capable of generating so much thrust.So if the outside temp is 20 degrees, and you set an assumed temp [flex as Airbus call it] of 30, then you will only get the thrust the engine would generate if the outside temp was 30.It's also possible to double derate. In other words use TO1 or 2, and an assumed temp derate together.TO1 and 2 can be used on wet runways, but not an assumed temp derate.By reducing thrust in this way, you save fuel, save engine wear, and reduce the possibility of losing an engine on takeoff.Calculating the derate temp to use is complex, depending on runway length, ambient temp, aircraft weight etc. I belive TOPCAT can calculate the temp for you, although I don't have the product.Not sure if there are charts out there to do the same.Airbus use only flex.

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Radio altimeter is used to determine Decision Height (height above touchdown zone) on cat II or III approaches. You should check the published approach plates for the specific settings however. Some approaches do not allow use of the radio altimeter.

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In real world operations, as SOPs dictate, you would likely use derate and/or assumed temperature for most operations that allow it in terms of safety (runway length, TOW, temperature and humidity, elevation of field)If in doubt, don't derate... no point in reaching the end of the runway without having left the ground thinking, "I got my assumed temperature wrong"...Andrew

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Thank you about those answers.Some advice regarding the cabine altitude ? I can't find any safe tutorial about this, some say 10000, some say real altitude ... I'm always entering right altitude, but had 3 or 4 times failure of the system and had to switch to manual, so I got to be wrong somewhere.

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If you mean the dial on the right above the FO's head, you need to dial in your actual cruise altitude there, as the system operates automatically based on that cruise altitude in general, as well as the landing altitude rounded to the nearest 50 feet...You do not dial in the cabin altitude yourself in auto mode...Andrew

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Right, that's what I do. I still got to understand the origin of the regular failure, FMC gives an active "Pax 2 oxygen" (can't remember exactly) in the failure settings.

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That means you have set the cruise alt to 10K' or FL100, and then continued climbing to whatever... do you get an alt warning horn sounding off?As soon as you go above 14000' cabin altitude or similar, the pax oxygen system should deploy...You need to set that alt to your actual cruise altitude, whatever it may be. The system, when set to auto, will determine the correct cabin altitude and pressure differential and you should have no warnings, unless the system fails with failures set to active...Andrew

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Hi,The horn was on each time. Last nite, I triple checked: it was set to 33000 as my cruise FL was 330, and the pax oxygen was done.Maybe I'm not doing anything wrong, but it happens "too often" to be a service failure ...

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OK, did you have the engine bleeds switched on and the packs active? The left and right packs should be on to ensure that the cabin pressurization system works. That could be you issue, that the engine bleeds are not on, or you have not switched on the packs?Andrew

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Pressurization Mode Selector on AUTO or ALTERNATE?

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AUTO if no problems...Andrew

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AUTO, bleeds, pack on ON or AUTO if available.

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Sounds like you have an unpressurized aircraft to me. You sure you've flipped the bleeds/packs switches?

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