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amahran    279

This isn't a support question, but more of a curiosity thing. What exactly does entering the QNH, temperature, and winds in the MCDU PERF APPR page do? I never notice it doing anything, and I've tried upper and lower bounds just to see what would happen to the Vapp speed (if at all), and nothing happens.

 

I tried going through the manuals to see what it does, but there's nothing there that talks about WHY we do it. Is it just to force the pilots to brief themselves on the arrival, and contribute nothing to the functionality of the approach?

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DaveCT2003    796

I've asked Hans to weigh in.  I'm hopeful that a few of our resident CRJ drivers will weigh in as well.

 

Many thanks for starting this topic... it's a very good one, and the type of topic I enjoy the most!

 

Best wishes.

 

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skelsey    96

The short answer is -- yes, the aeroplane uses this data for various reasons, not just the VApp calculation. How much is functional in the Aerosoft bus, I'm not sure!

 

The QNH value is used to compute the cabin pressurisation schedule, and the temperature is used to refine the VNAV descent profile (as it gives an indication of ISA deviation).

 

The entered wind is very important (and I am sure that this is modelled on the AS A320); this is used for the VApp and Ground Speed Mini calculations. Vapp is defined as VLS + 1/3 of the headwind component based on the entered wind with a minimum of VLS + 5 and maximum of VLS +15 (of interest -- note that the aircraft expects an entry in degrees magnetic, as would be passed by the tower; METAR winds are in degrees true and depending on where you are in the world there may be a significant difference). 

 

Because the minimum value of VApp is VLS +5, you won't see any effect until the entered headwind component exceeds 15 kt (in practice obviously a headwind component of 18 kt is required before you get a VApp value of VLS +6).

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Simon is spot on (that's never been any different, has it ...)

 

In addition to Vapp only "moving" with HWC higher than 15 kts, Joshua also modeled GS mini (target speed) only "moving" with unexpected/temporary HWC higher than 10 kts. (At least last time I checked that. FSL did that differently, but I'm more with the AS Airbus here.)

 

 

 

7 hours ago, DaveCT2003 said:

I've asked Hans to weigh in.  I'm hopeful that a few of our resident CRJ drivers will weigh in as well.

 

Many thanks for starting this topic... it's a very good one, and the type of topic I enjoy the most!

 

Best wishes.

 

 

Airbus topic.    :P

 

 

 

EDIT:

 

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amahran    279

Neat! How much of a change does the temperature impart on the descent profile though? I tried 50 and -50 and barely noticed any difference.

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