B738 - VNAV Forecast Descent Page

[GlassCollector]

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.

[Banana Joe]

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.

[Occy]

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!

[FlyingStone]

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.

[Occy]

Yep, only ever tried it prior to TOD (otherwise why bother). To clarify, the thrust/drag required stuff happens shortly after transition. Can’t see how it’s mandatory for VNAV approach, you have actual QNH set.

It may be cultural at our company, as some people enter the QNH but I’ve never seen anyone enter the temp so when guys/girls insist on setting/Changing QNH in FMC I ask why they don’t enter the temperature as well?

[rudestuff]

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?

[Banana Joe]

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.

[Occy]

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..

[GlassCollector]

Nice info thanks.