# ISA DEV / QNH

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Hi captains,

Just have a little question about the ISA DEV / QNH entry in the FMC's forecast page. I am not entirely sure on what are correct values to enter here.

So I was flying in to LOWI today. Weather was: wind 160/8, temp 18, QNH 1019.

As far as I understand from the tutorial flight, ISA is 15C at 0 ft ASL, correct? Ryan explained in the tutorial flight how to calculate the ISA tempature for other altitudes. The formula for this is: 15 - ( (altitude / 1000) x 2 ) . So in this case, airport elevation was 1900 ft, so: 15 - ( (1900 / 1000) x 2 ) = 15 - ( 1.9 x 2 ) = 15 - 3.8 = 11.2C. So ISA for 1900 is 11.2C

ISA DEV = actual air tempature - ISA tempature = 18 - 11.2 = 6.8C.

So, ISA DEV = 6.8, rounding that up makes it 7C

Not sure if this is correct...

For QNH I just assume I've got to enter the local QNH level of 1019 HPA?

So then for the ISA DEV / QNH field I enter: 7 / 1019.

Correct?

Arjen Vandervelde

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Correct?

Arjen

That is the way I have been doing it for almost 2 years now. I believe it is the correct way.

Michael Cubine

Michael Cubine

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Correct?

Yep.  That or look at a weather report where it has that generated for you (though the quick method you used won't be too far off).

Kyle Rodgers

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All good. Just a quick response to mention that, strictly speaking, you should be entering the average ISA deviation for the descent. In the real world this is probably more relevant because ISA deviation can be higher at altitude than on the deck.

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All good. Just a quick response to mention that, strictly speaking, you should be entering the average ISA deviation for the descent. In the real world this is probably more relevant because ISA deviation can be higher at altitude than on the deck.

Recently over Egypt with AS2012 I encountered ISA DEV changing gradually from +25 on ground to +15 at FL330, then more rapid change to +5 at FL400. Seems true, even though if I can't find aloft charts for that area.

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.

Recently over Egypt with AS2012 I encountered ISA DEV changing gradually from +25 on ground to +15 at FL330, then more rapid change to +5 at FL400. Seems true, even though if I can't find aloft charts for that area.

Seems about right rsvit. I assume AS has the benefit of real data for those profiles. I have seen ISA +20 on ground and with ISA+22 from around 9000 to 22000! If climbing the FMC will issue UNABLE FL3X0 messages until the ISA deviation drops back down in the mid to high 200s

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mid to high 200s

Did you mistype 20s as 200s?

Michael Cubine

Michael Cubine

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Learning something still. Thanks. But equal important is why this forecast? Airpressure calculations in the cabin?

Michael Moe

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Learning something still. Thanks. But equal important is why this forecast? Airpressure calculations in the cabin?

VNAV calculations. Pressurization system uses data from altimeter.

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Learning something still. Thanks. But equal important is why this forecast? Airpressure calculations in the cabin?

Let's say the baro at the field is 29.87". If you don't put the forecast in, when you hit the transition from STD to local, you'll be a whopping 100' low. Catastrophe, I know.

Matt Cee

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Let's say the baro at the field is 29.87". If you don't put the forecast in, when you hit the transition from STD to local, you'll be a whopping 100' low. Catastrophe, I know.

I think you are mixing things with the EFIS BARO settings here. FMC forecast has nothing to do with the altimeter/backup altimeter settings/readings display

Michael Moe

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FMC forecast has nothing to do with the altimeter/backup altimeter settings/readings display

It doesn't on the NGX, because that aspect isn't simulated. It does, if I'm not mistaken, on the real thing, whereby the forecast entry is used to avoid a sudden vertical path deviation when switching between the STD and QNH baro settings (the effect of which becomes more significant the more QNH differs from the standard 1013 mB value, which is what Matt is referring to). Disclaimer: I Am Not A Pilot™, so I could be taking through my hat here.

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I think you are mixing things with the EFIS BARO settings here. FMC forecast has nothing to do with the altimeter/backup altimeter settings/readings display

It doesn't on the NGX, because that aspect isn't simulated. It does, if I'm not mistaken, on the real thing, whereby the forecast entry is used to avoid a sudden vertical path deviation when switching between the STD and QNH baro settings (the effect of which becomes more significant the more QNH differs from the standard 1013 mB value, which is what Matt is referring to). Disclaimer: I Am Not A Pilot™, so I could be taking through my hat here.

Exactly as Petja says. Since VNAV is BARO based, it only knows what you give it, and if you don't tell it there's going to be a shift at the Transition Level, it can't account for non-std QNH.

Matt Cee

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Which of the following statements is correct?

1. Enter the ISA DEV/QNH at the Destination Airport
2. Enter the ISA DEV at TOC and the QNH of the Destination Airport?
3. Enter the ISA DEV at TOD and the QNH of the Destination Airport?

SimBrief shows me the AVE ISA for the trip as well as deviations (TDV) at TOC and TOD but NOT at the destination airport.

4. Can I enter either in Hg or hPa for the QNH?

Thanks

AVG ISA

TOC / TOD

Edited by toby23

Ryzen 5800X3D, 64GB RAM, RTX 4090, Windows 10, Varjo Aero, DOF H3

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On 9/16/2023 at 3:20 AM, toby23 said:

Which of the following statements is correct?

1. Enter the ISA DEV/QNH at the Destination Airport
2. Enter the ISA DEV at TOC and the QNH of the Destination Airport?
3. Enter the ISA DEV at TOD and the QNH of the Destination Airport?

SimBrief shows me the AVE ISA for the trip as well as deviations (TDV) at TOC and TOD but NOT at the destination airport.

4. Can I enter either in Hg or hPa for the QNH?

You need to enter average ISA over descent. You can get airport one by calculating from METAR.

Either inHg or hPa is accepted.

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