Go Back  PPRuNe Forums > Flight Deck Forums > Tech Log
Reload this Page >

B738 - VNAV Forecast Descent Page

Tech Log The very best in practical technical discussion on the web

B738 - VNAV Forecast Descent Page

Old 2nd Apr 2021, 04:44
  #1 (permalink)  
Thread Starter
 
Join Date: Jan 2008
Location: England
Posts: 11
B738 - VNAV Forecast Descent Page

Does anyone know how the QNH entry on the VNAV forecast descent page is wired into the rest of the VNAV path construction? Specifically, I'm wondering how it's related to 1013 and box transition altitudes injected from approach selections.
GlassCollector is offline  
Old 2nd Apr 2021, 17:54
  #2 (permalink)  
 
Join Date: Jan 2018
Location: Amantido
Posts: 712
It's very important to fill that in when you have a high QNH and mandatory for a VNAV flown approach. The FMC receives data from the barometric altimeter, and at the transition level VNAV will recalculate its profile showing you are too high i.e. a QNH of 1023 will leave you 300 ft high.
A high QNH will move the T/D further away from the E/D point, a low QNH will move the T/D closer to E/D point.
Banana Joe is offline  
Old 3rd Apr 2021, 04:12
  #3 (permalink)  
 
Join Date: Jun 2008
Location: goofyland
Posts: 43
My experience is that putting the actual QNH usually in results in a “drag required” message closely followed by the auto throttle adding thrust or the opposite: thrust coming on followed by “drag required” so I do not bother anymore. If my learned colleague insists, I acquiesce but warn them that I have a strict quota of keystrokes for the day and they are rapidly approaching my limit. So far no one is walking thru the jungle after any of my flights and they have even been able to use the aircraft again!
Occy is offline  
Old 3rd Apr 2021, 05:57
  #4 (permalink)  
 
Join Date: Apr 2010
Location: IRS NAV ONLY
Posts: 1,085
Even if you put the actual QNH in before TOD?

I agree with Banana Joe, it's very important. FMC is all about garbage in, garbage out. If you don't supply the QNH, it will assume it's standard, so in case of very high or low QNH, the calculcated VNAV path could easily be wrong by 1000ft on either low or high side - until you switch from STD to QNH.
FlyingStone is offline  
Old 3rd Apr 2021, 08:24
  #5 (permalink)  
 
Join Date: Jun 2008
Location: goofyland
Posts: 43
Yep, only ever tried it prior to TOD (otherwise why bother). To clarify, the thrust/drag required stuff happens shortly after transition. Canít see how itís mandatory for VNAV approach, you have actual QNH set.

It may be cultural at our company, as some people enter the QNH but Iíve never seen anyone enter the temp so when guys/girls insist on setting/Changing QNH in FMC I ask why they donít enter the temperature as well?
Occy is offline  
Old 3rd Apr 2021, 11:41
  #6 (permalink)  
 
Join Date: Dec 2005
Location: Wherever I lay my hat
Age: 43
Posts: 561
Fancy pants with your QNH buttons! I'm assuming that only really matters during transition so you don't suddenly find yourself high or low on the path when you set QNH? I just try to anticipate and brief to expect it... Wouldn't you have to set QNH exactly when you told it you would in order for it to work anyway?
rudestuff is online now  
Old 3rd Apr 2021, 13:02
  #7 (permalink)  
 
Join Date: Jan 2018
Location: Amantido
Posts: 712
Occy

I can't recall that happening to me. As for the temperature, I enter the average ISA deviation between the CRZ level (as in PROG 2/4), halfway through the descent from the OFP and destination. Just a rough estimate, and usually only if the ISA deviation is warmer. Better be slightly low than high. The effect of temperature is not as big as barometric setting. FMC U14 should bring some improvements about temperature correction.

If during descent you notice that the wind is different from the forecasted descent winds, you can always recycle the WPT on top and VNAV will recalculate the descent profile with the actual wind.
Banana Joe is offline  
Old 6th Apr 2021, 08:20
  #8 (permalink)  
 
Join Date: Jun 2008
Location: goofyland
Posts: 43
Yeah ok, I don't get too fussed about entering any of that. If I have a high QNH and I'm worried about an at or below, I'll just be vigilant with monitoring, use VSD and the boards as necessary or like you do force a path re-calc with a direct / re enter descent speed..
Occy is offline  
Old 9th Apr 2021, 21:27
  #9 (permalink)  
Thread Starter
 
Join Date: Jan 2008
Location: England
Posts: 11
Nice info thanks.
GlassCollector is offline  

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Thread Tools
Search this Thread

Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service - Do Not Sell My Personal Information -

Copyright © 2021 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.