VNAV profile during tansition.

[loucat76]

Hello,

Not a real pilot but do enjoy the sim. I use the PMDG 737 NGX and have a question about VNAV. In descent using VNAV and on profile and transitioning from FL to altitude after adjusting to the current baro setting there may be a difference of several hundred feet.The aircraft immediately adjusts its rate of descent to correct this and get back on profile. I know the sim is not like the real thing and can be overly sensitive but I'm curious how the real aircraft behaves.Does the real aircraft adjust for this also? Is it normal procedure to immediately adjust the baro setting when transitioning? Thanks.

[galaxy flyer]

Yes, the VNAV profile will have to adjust for the change in altimeter. How smooth depends on the system and how the input is made. I slowly adjust the QNH to slow the change in gradient.

[Jwscud]

Set STD when cleared to a flight level.

Set QNH when cleared to an altitude.

If you put the QNH at destination in the DES FORECAST page (DES then <FORECAST at 6L) you shouldn't have any change at all in your VNAV profile.

[Denti]

Indeed, if the forecast page is correctly filled in the VNAV profile takes into account the difference between standard and QNH. There might still be some discrepancies if the forecasted average temperature deviation from ISA is too large and/or the downloaded wind data isn't quite right and of course if one misjudges the TAI use.

[framer]

Set STD when cleared to a flight level.

Set QNH when cleared to an altitude.

Just for interests sake, some companies do this, some do not. The ones that do not ( my current company included) have changed their SOP's after one or more incidents where pilots get a change to their clearance and forget to reset their altimeters and level off at the wrong altitude/ flight level.
Eg, you're descending through FL 220 and ATC clears you to decend to 6000ft and you go ahead and change to the local QNH as per the above SOP, then ATC amends your clearance to FL 150 but you forget to change back to standard. You'll level off at 15000ft instead of FL 150. It happens often enough that operators change their SOP's.

[Jwscud]

My lot take the view that the amber box around the QNH will help remind you, if your colleague doesn't.

Denti - "there might be some discrepancies" made me giggle. Yet to see VNAV get it much better with winds. Level Change, and V/S for finesse works just as well.

[Denti]

Actually, up to date forecast winds do help quite a bit, usually just download them just before TOD or shortly thereafter. However the biggest factor, especially in the coming time of the year, is getting the TAI right, and that is quite often, well, guesswork

Keeping ones own calculation up to date is not a bad idea, but i guess that is true in any plane.

[loucat76]

Did not know that, I haven't been filling out the FORECAST page. Thanks.