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martinlest2

QNH/ATC question

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It sometimes happens that after I pass transition altitude and switch to STD pressure, 29.92, ATC (and the onscreen information too, in red at the top of the screen) do not seem to take account of this. So when I am at cruise altitude, I am, according to ATC, several hundred feet (depending on the actual value of the QNH of course) off my assigned altitude. It is only corrected when I switch back to the below-TA QNH from the STD pressure I had set after FL180 (or whatever).

 

Is this due to something I do wrong when I set up my flight? It doesn't always happen by any means, so I am wondering what the variable is. Can't recall which a/c I am flying at the time, or if that makes any difference, but at the moment it's a PMDG 737-700...

 

Thanks,

 

Martin

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FS ATC uses a transition altitude of 18,000 feet in its worldwide universe. (The "B" key is hard coded to this BTW.)

 

One clue of when to use standard pressure is if ATC issues a command to change to an altitude in feet, use QNH surface pressure. If they issue a command to change to a flight level altitude use standard altimeter pressure.

 

http://en.wikipedia.org/wiki/Flight_level#Transition_altitude has good information. Read the paragraph that starts with:

 

"The transition level is the lowest flight level above the transition altitude." (Emphasis mine).

 

When adjusted for separation minima where the transition altitude is 18,000 feet the QNH adjusted transition level would not provide minimum separation required so the first practical flight level would be FL190 or above. So at 18,000 feet you would use QNH in global FS transition altitude of 18,000 feet terms.

 

For any RC users (see my banner) RC has a table of transition altitudes built in for airport areas in its data base. All FAA airports are fixed at 18,000 feet. Others are defined by local authority.

 

I also use the PMDG B737NG FS9 fleet which has the Standard button on the altimeter pressure knob. This lets me set expected QNH while still in the FL zones by keeping in STD mode. You'll see both settings on the PFD. Look at the QNH in the standby altimeter and see if it follows the STD button. I don't recall.

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Hi. Thanks for the reply.

 

The problem is that ATC is using a below transition altitude QNH when I am at cruising altitude, well above the transition altitude - say 38000'. When my instruments show 38000', the onscreen data may show 38800 (for instance) and ATC tells me to descend the 800' difference. So my altimeter in the PMDG would then show 37200'. Not sure, when that happens, if ATC ask me to descend to "FL" something or other, or to a value in feet. I'll listen out for it next time there is a problem.

 

When that happens (not all the time - the last few flights have been fine), I just switch back from the STD setting (which I press when passing FL180) to the barometric pressure below that altitude. Not what should happen of course.

 

This is only an issue on my Dell XPS laptop, which I use for FS9 when I am abroad (for long periods, for work - I am away two years at the moment). Back home my FS PC has PFE installed and I don't use FS9 ATC. I couldn't get PFE working properly on the laptop for some reason, so use the out-of-the-box ATC (with some Russian tweak so that at least I am not nagged to expedite this that or the other or my flight plan is cancelled if I don't do what they say!).

 

M.

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What happens if instead of pressing STD, you dial in 29.92 as the QNH?  Results should be the same, but there was some discussion on the PMDG 747 forum about the STD button not working as it should. Perhaps it is a PMDG/your laptop issue ?

 

I use FS9.1 and dont have this issue.

 

What weather program are you running?

 

What is the QNH before you reach transition altitude ? Is it an extreme away from 29.92 ?


Peter Schluter

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What happens if instead of pressing STD, you dial in 29.92 as the QNH?

I'll try that and post back.. Thanks.

 

What weather program are you running?

On this laptop, just the native FS9 weather.

 

What is the QNH before you reach transition altitude ? Is it an extreme away from 29.92 ?

Variable, sometimes not so far away from 29.92...

 

M

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it's more often than not when climb / descending thru the transition layer that std 1013mb / 29.92" is not the area QNH ... and can vary by as much as a 1000' near a frontal system.

 

you should read ron's wiki link above & digest what it says !!


for now, cheers

john martin

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John, I think the OP is aware of  transition altitude/Level.  His problem is that when he is in the cruise above 18,000 feet and is using STD or 29.92 as the altimeter setting, ATC is telling him he is too high/low by a number of 100's feet. If he then resets the altimeter to the QNH he was using below the transition layer, ATC are happy !

 

So his problem is that the sim is using local QNH to determine his altitude even when he has climbed above transition layer.


Peter Schluter

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thanks peter & apologies martin ... I misunderstood what you were discussing, I guess mainly because I had not seen the issue myself !!


for now, cheers

john martin

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The PMDG FS9 B737NG series uses FSUIPC3 for some functions. Be sure it is current from:

 

http://forum.simflight.com/topic/66139-updated-modules/

 

installing the full and current interim update as necessary. Then download free Traffic Look and install the .exe in any convenient folder. It is standalone application that shows your and ai aircraft status in your area:

 

http://forum.simflight.com/topic/66136-useful-additional-programs/

 

Start up FS and be sure it is set to not pause on task switch. Load Traffic Look. Start a flight using the clear weather theme which gives a surface pressure of 29.92. Alt-tab to Traffic Look and it should show your parked aircraft at the airport elevation and agree with your altimeter set to 29.92 for this test. . Alt-tab back to FS. Climb to FL340 and hold. Alt-tab to Traffic Look and see what altitude it reports for your user aircraft. Switch between STD and QNH of 29.92. there should be no change.

 

Repeat with a flight climbing to FL340 but create the flight with a non-standard surface pressure.

 

What we have been checking is your altitude as seen by FSUIPC reported by FS to apps using FSUIPC.

 

I can't find the thread on the RC4 support forum here on AVSIM, but a PMDG B737NGX or B777 FSX user had this problem. You can try this debugging method to check your altimeter.

 

Start FS with a flight parked and menu bar showing. Click modules, FSUIPC. Click the logging tab. Set it up per this image:

 

https://www.dropbox.com/s/2dm0zo9ftsvgw7n/fsuipc3%20altimeter%20debug.jpg?dl=0

 

Close FSUIPC. In the title bar of windowed FS now appears a debugging test code Monitor IPC0330 (U16) = 16211 (approx.) for altimeter pressure of 29.92. Change the altimeter pressure to lower. The value should go with your altimeter pressure change. If on the PMDG click the STD button. The value should return to 16211. You can try this in flight. This is just to confirm your altimeter pressure setting operation. Try this in your cruise at FL340 to see if the number still matches.

 

Now I have the user paid registration. I am not sure if the logging tab shows in the free version licensed by PMDG.

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Thanks for all the information, I will certainly follow it up. Not sure though that this only happens with PMDG a/c - perhaps it does - I just haven't noticed. I only get the issue occasionally anyway, so I'll note carefully which plane I am flying in the future. It may well be (I have been flying PSS a320s the last few days and all has been well, QNH-wise!).

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What ATC programme are you using? It seems to me that your settings there need some adjustment.

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Just the FS9 generic on this laptop. I use PFE on my main PC but I haven't installed it here.

 

M.

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Default ATC will always assume the transition level is 18,000ft wherever you are in the World.

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Hi. Yes, I know it will, that's not a problem. The 'problem' (hardly a huge one) is that above 18,000', and as high as I care to cruise thereafter, my altitude is only shown as correct in the onscreen data and according to ATC if I maintain the below transition altitude barometric setting. So if I start off at 29.85, let's say, ATC is happy as long as I don't change to STD or 29.92 after passing FL180. If I do, I am told I am at the wrong altitude.

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