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PerWel

QNH and AP

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HiI have seen one little thing that might be wrong or maybe it is me.Right now I am doing a Canadian tour and the trans. alt is 18000.If I am in at a lets say 14000 approach and ATC (RS4) calls me and say thatthe QNH have change up or down and I change it lets say from 3002 to 2986and of cours the altimeter strip changes to. But the NGX does not adjust thealtitude so I have done app. 2-300 feets above or below assigned alt.But if I am at FL330 and the pressure is changing the NGX adjusts the altitude.Is this the way it works or do I have to press some button somewhere,oris it a bug.It might be somewere in the manuals but I ask here before I spend some hourstrying to find it.Rgds,/Per W Sweden


Per W Sweden

 

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You need to adjust your QNH on your own. There's a knob for that to the left of the MCP (autopilot panel). You might want to visit the tutorial, as it is mentioned in there.If you're above 18,000 in the United States (not sure about Canada), your QNH should be 29.92inHG/1013mb. This is to keep all aircraft operating at those altitudes on the same page. Otherwise, if I was flying up there with my altimeter set at 30.42 and someone else was set at 29.92, we'd only be 500' apart (assuming I was the higher aircraft), which is a problem. Setting everyone at 29.92 ensures everyone is using the same standard for determining their altitudes.If the pressure changes in FSX, your aircraft will climb or descend to be at the altitude indicated by the standard 29.92. Normally, this isn't so sudden as the pressure gradients are normally pretty smooth in real life, but the weather engine in FSX isn't that great, so it can be sudden at times....and there's no need to spend "hours" in the manuals. CTRL + F allows you to search the PDFs. It shouldn't take more than 5 minutes, which beats the nearly 3 hours you waited for my reply.


Kyle Rodgers

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Below transition altitiude, you should set the QNH manually. You will see the altitude changing but the plane will not descend or climb without instruction. By just testing this, I see that you first have to change your altitude in the MCP, then select a pitch mode and set the altitude back to what it should be.Above transition level, you altimeter should be set to STD. when you change the QNH then, you'll see it changing in small white letters but the altitude indication shouldn't change.Bert Van Bulck

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This isn't a bug - in real life you don't change the altimeter setting a bunch of times, you get it from ATC before you descend through transition altitude on approach. We checked this and the AP does not adjust itself when in ALT HLD if you change the altimeter setting.


Ryan Maziarz
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a small hint, if you press "b" under 18000ft, your altimeters will correct themselves to current pressure.


--Peter Fabian 
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The NG uses uncorrected baro altitude for altitude hold modes. It's all in the manuals. Whenever the AFDS maintains a certain altitude that you had set before and then change the QNH setting, that doesn't quite change the outside pressure so the AP doesn't really care for that because outside pressure is the only input it has. Hence you'll have to figure something out in order to get the AFDS climb or descent to your adjusted altitude.

FCOM Vol II 4.10.8Altitude Hold (ALT HLD) SwitchPush -
  • engages ALT HOLD command mode
  • commands pitch to hold uncorrected barometric altitude at which switch was pressed
  • [...]

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Thanks Ryan !Exactly what I was asking about. RC4 sometimes say one QNH before I go below 18000 and after a while tells me a different one.And it is at the second change when I am in a level flight I have seen what I asked about.So if it works like this in a real 737 I am ok with it.Case closed :-)And to all other answers, thank you for them. But I have been flying all kind of simulators since the very first MS Flightsim, only a Green screen. I have also takensome real lessons in a PA28. Almost made it to a license but it was to expensive so I had to stop, it was back in the beginning of the 80sI know all about regulations and how it works up in the air. Have flying Heavy IFR airplanes for the last 20+ years, and of course a lots of reading about it.So I have to say that PMDGs 737NGX is the very best I have flown, state of the art in the MS world. Have no problems like the ones I have read about in this forum.Everything in it works exactly as stated in manuals, no weird behaviour or anything else crazy. Just perfect. Must have done 100+ hours now in it.Have sold the iFly, feelt like a toy and ran on rails in the air. + lots of crazy behaviour.Keep up the good work PMDG. I will get your next plane what ever it is. No question about that.I only have a 32-bit WinXP with a Core2 Quad, a couple of SSDs. And a Nvidia 465 GTX.REX, UTX, GEX, UT2 and ASE. And I run it with 25-30 FPS without to low settings. Just a bunch of tweaks in fsx.cfg.rgds/Per W Sweden


Per W Sweden

 

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The altitude hold functionality on this and almost every other jet autopilot follows a specific target atmospheric pressure --- the one prevailing at a specific altitude when ALT HOLD (or VNAV PTH) engages. In other words altitude hold follows an isobar.When you change QNH in the plane through the EFIS knob you change your display but of course NOT the outside air...Note also that unless you have enabled the left/right side BARO synchronisation option, altitude information is received from the FD master side. For example if CMB B is engaged but you turn and change the captains' side EFIS knob this will have no effect on the autopilot.


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E M V

Precision Manuals Development Group

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Thanks badderjet for the manual part, I did suspect it would be somewhere about what I saw.And emvaus for the explanation. I makes sense, if I do not touch anything it will asc and desc afterthe pressure/isobar. This behaviour (if I may say so) is not simulated by some cheaper toys, if you changeQNH it will adjust to have the altimeter at the same as the MCP. Never seen this before. Really good work of youto deal with it. The last sentence I have not thought about or tried, will do that some time. So this is whythe FD that is switched on first is the Master. Great job!! And that CMD A and B makes a difference !!!I always use A and sometimes in bad wheather I do a CATIII with both A and B. Must try the only B andplaying with FD switches and the other way around.This tells me on what level you are. You really know what you are doing and make sure that you come asclose to reality that is possible in FSX.Any plans for an X-Plane airplane sometime in the future? It is the best when it comes to aerodynamics but lacksa lot of the good looking candys FSX have. Also a lot more difficult to fly, did recall the feeling from my realtrainings in the PA28. Understand why FAA have said OK for it for pilot training.In the end I think that ASE and RC4 fools me a little. ASE sends in a new pressure and RC4 is programmed to reportchanges when it would be enough to report the destinations QNH in the last part of the approach or one time beforeI go below transition level. So next time I will only write it down and maybe change it on the next descend.Regards,/Per W Sweden


Per W Sweden

 

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