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Take-Off N1% - bug?

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Guest topcat

According to Boeing tables (and the tables in the PMDG manual) Take-Off %N1 is a function of pressure altitude (which is 1013,25 hPa or 29,92 inHg) and airport temperature.However, in the PMDG 747 the %N1 changes with the QNH/Altimeter pressure setting.Is this behaviour correct or am I missing soemthing here?Best Regards,Christian Grill

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Hi Christian,Its behaving correctly, the N1 will indeed change with the QNH and/or temperature.CheersJon

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Christian,You might wanna take a look at this link:http://en.wikipedia.org/wiki/Pressure_altitudePressure altitude is the indicated altitude when an altimeter is set to standard pressure. So it can and should fluctuate with QNH.Hope it helps,


Mats Johansson
PMDG Flight Test Dept
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Guest topcat

Correct: "Pressure altitude is the indicated altitude when an altimeter is set to standard pressure"That means, that pressure altitude is a constant value for a given weather condition at a given location (You set the altimeter to 1013,25 or 29,92 and can read the pressure altitude)Take-Off N1% values are given for a specific pressure altitude and should therefore be independent of your altimeter setting, as pressure altitude doesn't change, just because the pilot selects a different QNH/Altimeter setting.Best regards,Christian Grill

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Well what do'ya know.. Christian has a good point.The N1 values change when the kollsman window is changed, regardless of what the actual (sim-world) pressure is. And, it changes when you set different pressures in the FS advance custom weather settings. That begs the question why N1 varies in both cases. Either one or the other would make some sense, but more sense if it were only the latter. I expect the DFAEC to have it's own sensors and not rely on the pilot's altimeter setting.Verry interesting.(DFAEC: Digital Full Authority Engine Controller)


Dan Downs KCRP

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Christian et al,I think we are talking about the same thing but from two different points of view.The pressure altitude will be the same for any given airport at any given weather. Now any given weather means that with a change of weather and of pressure/QNH the pressure altitude will change as well.The formula is given below:http://upload.wikimedia.org/math/9/5/4/954...79ac0f11bf5.pngwhere: z = pressure altitude (feet)Po = static pressure (hPa)So to conclude; the altitude pressure will change with QNH at any given airport.I hope I got it right,


Mats Johansson
PMDG Flight Test Dept
Boeing777_Banner_BetaTeam.jpg

| Asus Z270-A | Intel i5-7600K @ 4.8 GHz OC/H2O | nVidia Geforce GTX 1070 8GB OC/O2|

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Guest michaelandre

Hi Matts,Yes you got it right!Michael

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The question is not concerning the physics of measuring air mass, but why the FADEC relies on the pilot's input to the kollsman window on the altimeter. Shouldn't the simulation use the ambient conditions regardless of altimeter setting?It does, in part. Changing the local settings affects N1, but so does changing the instrument on the panel. That doesn't seem right.


Dan Downs KCRP

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Guest topcat

Yes, of course Pressure Altitude changes with the change of ambient air pressure (QNH). But the Pressure Altitude does not change, just because the pilot changes the QNH/Altimeter setting in the Kolsman window...That's why I suggest that PMDG is behaving erratically here...Best Regards,Christian Grill

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Gotcha!Sorry for the confusion. The dev team is aware of this thread and discussions are ongoing...Thanks,


Mats Johansson
PMDG Flight Test Dept
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| Asus Z270-A | Intel i5-7600K @ 4.8 GHz OC/H2O | nVidia Geforce GTX 1070 8GB OC/O2|

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Christian,Looks like your fine observation made it into the -F release! Well done to you and the PMDG crew!Quote below is from the pinned post at the top of the forum:22) Adjusted EEC/ADC barometric pressure feed to prevent pilot barometric adjustment within MSFS from impacting FMS computation of takeoff performance.


Cheers, Søren Dissing

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