A320 DEST APPR Page
 

On the DEST APPR page, you have to enter the QNH, Temperature and Wind for weather. Regardless of QNH and temperature, VLS and Vapp stay the same. They are based on actual or predicated gross weight for a specific configuration. VLS and Vapp change after based on actual winds. Why do we than enter QNH and Temperature? I am sure it is trying to calculate density altitude but for what?

The pressurisation system?

Correct. The only required field is the pressure which is why it's in orange boxes. It's for the pressurisation.

QNH is for TOD/Wind is for GS Mini
 

Besides the pressurization system, there is another reason, why the QNH is needed on the approach page:
During Cruise, you are normal flying with Standard Baro-Setting. So your height above MSL depends on the QNH of the region you are flying through. For the FMGS to be able to calculate the correct Top Of Descent, it needs to know the QNH of the destination airport. If there is a huge low-pressure zone at your destination, the descent from FL350 will be significantly shorter than with a high-pressure zone.

Besides the increase in VAPP for large head winds, the wind information is also used to calculate the Ground Speed Mini function. (See FCOM 1.22.30, somewhere around page 66)

For the temperature, I am not sure, it is probably for the descent performance calculation, also.

Hope this info helps.

---- Beat to the punch on G/S mini.

If you take a look at the standard descent charts in the FCOM, you'll see that temp is factored in to get an accurate descent distance and fuel consumption. Has to do with the density of the air I suppose? I guess the aircraft already knows the temp at the current level, but knowing the temp on the ground probably allows for some fancy calculation that allows for a better picture of the entire descent profile. i.e. more accurate lapse rate.

Yes that's correct. For that reason re-entering the temperature will recalculate the descent path based on current conditions at the altitude at which it was re-entered.

Thank you everyone. You are all correct. I saw also within 180 nm the QNH entry goes from blue to amber. It is a required field.

Yes, as Dan quite rightly said, the QNH is the only technically required field.

On that note, Dan you clearly know your stuff with regard AB. Have you or any other operators here encountered situations where the destination data is correctly loaded in both the primary and secondary APPR pages, yet the FM generates an ENTER DESTINATION DATA message? I occasionally see that message come up, yet as mentioned everything is correctly loaded. I simply clear the message and continue on our merry way, but I'm at a loss to figure out why it's generating that message when all fields are correctly completed, and it would be nice to know if others encounter it. I don't like randomly generated messages! :p

I have seen it, many times. Why it happens - no idea!

D'Oh!!! That's my line. :)

Hi,
I just tried this in cruise, re-entering a hypothetical value (because changing 1or 2 degrees wont make any difference)in the QNH and in the temperature field....
This is what i observed,
All the speeds(F,S,O, Vls and Vapp) on that page went blank for a second and gave a different speed from the actual condition,furthermore the TOD arrow showed a huge jump.