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Question regarding QNH management in VNAV descent

Started by tango4, Wed, 5 Jul 2017 08:32

tango4

Hi guys,


I have a question regarding the moment you switch from STD to QNH during a VNAV descent, especially in continuous descent operations. The problem is most present in Europe where the transition altitude is usually pretty low, so quite close to final approach interception.


My problem is that AFAIK there is no way to make the FMC "QNH aware" in the Heavy Boeing FMCs. In the 737 FMC, even in the Classics, on the Descent forecast page, there is a field called "ISA DEV/QNH" where you can tell the FMC about temperature and QNH.


So here is an example of my problem:
Let's say I want to intercept ILS27R (which is intercepted at 5000ft) at LFPG from a night transition (so no vectors, those are closed procedures designed to be flown in LNAV/VNAV). At LFPG, transition altitude is at 5000ft. If I let the aircraft fly, it will aim 5000ft at my current baro setting. But when I cross the transition level, if the QNH on that day is high, all of a sudden, I can find myself easily 500ft high. If I am also a bit fast, this can be hard to stabilize the approach.


So the real question would be:
Is there a way using the systems that could avoid this ? Or is the only way making a mental note that the QNH is high on this day, expect the issue and move the switch from STD to QNH early, or getting out of VNAV?


Any real life tips on this appreciated !


Charles

Britjet

#1
Hi Charles.
You are correct - there is no way of programming the system to do this for you.
One tip is to insert the transition level into the FIX page - and it will appear geographically on the map. The PFD will also give you an amber warning if you descend through the TL without setting the QNH, but of course this may be too late. Another "situation awareness" tip would be to insert the last waypoint that is predicated as a FL in the fix page. That way you have a circle around it to remind you.
Many airlines (including my old one) set QNH as soon as descent is initiated to an ALTSEL altitude, regardless of whether you are above the TL at the time. I recommend this procedure.
The system isn't perfect, I'm afraid ..
Peter

tango4

Many thanks Peter, exactly the information I needed !

Charles


Dirk Schepmann

Hi Charles,

thanks for starting this interesting discussion.

I also find the "jumps" in the VNAV path also quite annoying, especially when there is a huge difference between standard pressure and local pressure.

When I notice a big difference in the weather forecast, I usually switch to "V/S" just before pressing the QNH button and make small adjustments to the V/S before re-engaging VNAV. This prevents abrupt pitch and thrust changes during the transition. Don't know if real pilots also use this trick but I like the idea that the coffee stays in the cups of my virtual passengers. ;-)

But overall, I'm impressed how smooth and reliable the VNAV descent path has become in PSX over time. There is a huge improvement if you compare the older PSX releases with the latest ones.

Makes me wonder, if there have been changes in the VNAV logic in modern planes like the 787 or if it still pretty much the same like in the 747-400, 747-8 or 777.

Best regards,
Dirk



Hardy Heinlin

Hi Dirk,

I see no "abrupt pitch and thrust changes during the transition", even when switchting by 20+ hPa. Which PSX version are you using?

The deviation indicator will scroll away, and after some seconds the path will be recaptured smoothly. No V/S mode required ...


Cheers,

|-|ardy

Dirk Schepmann

Hi Hardy,

perhaps I should have written it more precisely. :-)

You are right, at first only the VNAV deviation bar jumps. There are no abrupt pitch changes but you can certainly see how the aircraft (e.g. the AFDS) is trying to push the aircraft back on the path. Especially when the pressure change leads to a situation, where the aircraft is suddenly below the VNAV PATH, the thrust mode often changes to SPD and the autothrust kicks in (as it should). When the path is recaptured, it goes back to IDLE but quite often the aircraft is then again slightly below the path and thrust kicks back in. It looks like a small computer-induced thrust oscillation which eventually smoothens itself out after a while. I'll try to record a SITU next time.

But on the other hand, I think that's exactly how the system is designed and in reality it might not be noticeable at all. I "feel" that I can do it smoother and more anticipatorely than the system logic, though.

Hope this clarifies my comment. Other than that, VNAV works perfectly fine in PSX (especially after the modification where you increased the speed deviation tolerance). Sometimes I observe some jumps in the VNAV deviation bar directly after a waypoint change.

I guess it is very complicated to program a proper VNAV descent path, especially when weather add-ons modify the upper wind data in the background.

Best regards,
Dirk