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Edited December 17, 20214 yr by Sethos

Finally, the modeling of pressure altitude changes due to temperature deviations, will make @jcomm happy (and I was looking forward to it too). 🙂

The influence of payload stations weight on moments of inertia is also a welcome improvement to increase realism.

27 minutes ago, Murmur said:

Finally, the modeling of pressure altitude changes due to temperature deviations, will make @jcomm happy (and I was looking forward to it too).

Make that three! 🙂

It is really well done - when you change the temperature at the nearest airport, the altimeter in your plane winds up (or down) as needed - and also the QNH reported at that airport changes (as it depends on an assumed pressure laps rate "beneath" the airport if it is not at MSL)...

WoW - Looking fwd into all this !!!

Well, Janov, actually, if the aircraft is on the ground, and with a given QNH setting given by ATC / MET, then varying "T" shouldn't change the reading in the altimeter.

The change applies only in terms of the lapse rate of pressure with "height" AGL or BGL... 

QNH, as we know, assumes altimeters are calibrated for a standard pressure lapse rate, the solenoid being calibrated for that variation. For an aircraft at the airport, with QNH properly set on the Kollsman window, no variations in T, no matter how intense, will change the reading, but with that same altimeter setting if you could descend towards MSL you would find out that you would reach it above 0 ft MSL under hot weather, and would get wet under cold temps 🙂

QFF is another altimeter setting, not used in aviation, that does correct for pressure lapse rate as a function of "T".

So, let's hope LR doesn't fall into the same mistake ASOBO did when trying to implement "From High to Low Watch Out Bellow" regarding "T"...

Edited December 17, 20214 yr by jcomm

10 hours ago, jcomm said:

WoW - Looking fwd into all this !!!

Well, Janov, actually, if the aircraft is on the ground, and with a given QNH setting given by ATC / MET, then varying "T" shouldn't change the reading in the altimeter.

The change applies only in terms of the lapse rate of pressure with "height" AGL or BGL... 

QNH, as we know, assumes altimeters are calibrated for a standard pressure lapse rate, the solenoid being calibrated for that variation. For an aircraft at the airport, with QNH properly set on the Kollsman window, no variations in T, no matter how intense, will change the reading, but with that same altimeter setting if you could descend towards MSL you would find out that you would reach it above 0 ft MSL under hot weather, and would get wet under cold temps 🙂

QFF is another altimeter setting, not used in aviation, that does correct for pressure lapse rate as a function of "T".

So, let's hope LR doesn't fall into the same mistake ASOBO did when trying to implement "From High to Low Watch Out Bellow" regarding "T"...

On CAE's simulator, when changing Temperature, you'll see on the IOS that pressure is also changing, thus keeps the QNH instructor inputted in the system remain it's value.

10 hours ago, jcomm said:

Well, Janov, actually, if the aircraft is on the ground, and with a given QNH setting given by ATC / MET, then varying "T" shouldn't change the reading in the altimeter.

I know what you are saying - the QNH as "given" by ATC will always make your altimeter indicate field elevation while parked on the field.

The question is (and I am not quite sure myself): If the airplane stays parked for a few days with the Kollsmann window untouched and the weather changes (it gets cold) but the pressure stays the same...would the altimeter needle move?

In other words - would the QNH as calculated for that airfield elevation - change if temperature changes?

I tested this at KDEN in XP12 alpha: The elevation of runway 16R is 5,450 feet MSL. If I change the temperature from ISA 0 (+3C) to ISA -40 (-37C) but leave the pressure unchanged at 1013...the altimeter now indicates 5,280 feet MSL. 170 feet less. Not sure if this is some instrument error, but it means that I need to change QNH to 1020 hpa to show accurate elevation again.

The real temperature difference for a 40C temp drop if flying at 5000 feet above the sea would be about 800 feet.

@Janov, then please report that to Austin, because it's wrong - just as ASOBO wrongly modelled it in MSFS 😕

We have to adhere to the meaning of QNH. What you are describing in your scenario of variation in T along a period of time does have an effect, not on QNH but rather on QFF.

Few rw pilots actually know about QFF, or mistakenly think whoever mentions it is making a typo and wanted to write QFE, unless they had to study for their PPL - ATPL licenses because it now makes part of the meteorology syllabus. QFF ( not QNH ) would show that variation indeed, but it is not used in aviation because it would introduce an additional level of complexity in managing altitude control and acquaintance.

Typical questions on the MET exams are:

- The QNH at an airfield located 200m above sea level is 1009 hPa. The air temperature is 10º C lower than a standard atmosphere. What is the QFF ?

A) Less than 1009 hPa; B) 1009 hPa; C) More than 1009 hPa; D) It is not possible to give a definitive answer.

- The QFF at an airfield in California located 69 metres bellow sea level is 1030 hPa. The air temperature is 10º C lower than a standard atmosphere. What is the QNH ?

...

When the QNH is reported by the aerodrome MET station, then altitude should stay put ! Actually in your XP12 test you are doing the change instantly, so that scenario of "a few days" does not even apply. It's wrong if they model it that way !

In flight simulation there are those professional multi-million full flight simulator platforms, there is also Aerowinx PSX, and IL-2 Great Battles as well as DCS World also model the effect of Temperature on pressure lapse rates, although the later two do it in a rather fixed / canned / simplistic way,  making the pressure levels shrink or dilate according to the flight situation Temperature.

Flight Gear's detailled / advanced weather model also models this effect, and geopotential height as a whole.

It's more or less simple to think of it like if the mechanical method used in a conventional altimeter was somehow ( and it actually in some sense is... ) like a hair-based hygrometer, where pressure levels are marked along the hair, under ISA conditions.

Setting the altimeter is alligning the ground level with one of those marks, but then as we vary T the hair shrinks or dilates around that mark so, while the mark itself stays aligned with the ground level, the remaining levels get further away under hot air, and closer if it's colder.

The effect is problematic only under considerable T variations, and the variations that are supposed to be taken into consideration are reported in approach charts.

Check bellow for a detailled explanation:

https://www.faa.gov/air_traffic/publications/atpubs/aip_html/part2_enr_section_1.8.html

And there's an excellent AOPA presentation on Cold Weather Ops also at the link bellow:

https://www.aopa.org/advocacy/advocacy-briefs/air-traffic-services-brief-cold-temperature-restricted-airports

Edited December 18, 20214 yr by jcomm

I think XP12 is right.

The discrepancy is probably due to the fact that (at least for v11) in XP weather settings, you don't set QNH, but you set the pressure at sea level. On the other hand, temperature is set at the closest airport elevation.

So, for a given set QFF (pressure at sea level), if you change the temperature at an airport and if the airport elevation is different from MSL, the QNH will change.

 

I don't care about all that stuff. It's completely irrelevant for a desktop flight simulator. Does it increase the fun and the visuals? 😂
Honestly, I also thought that "pressure is pressure", however after jcomm's description with the hair-based hygrometer I understood the additional dependency of temperature.
As a mechanical engineer, I also like that, amongst others,  the moment of inertia (something, a mechanical engineer deals with almost daily) is implemented, depending of the distribution of weight. It's even closer to reality.

So, that's what I call simulation and an "engineering tool". Didn't someone wrote something like "nobody wants to have an engineering tool ... but visuals ...." Engineers and real pilots do care.

 

Edited December 18, 20214 yr by uwespeed

1 minute ago, Murmur said:

I think XP12 is right.

The discrepancy is probably due to the fact that (at least for v11) in XP weather settings, you don't set QNH, but you set the pressure at sea level. On the other hand, temperature is set at the closest airport elevation.

So, for a given set QFF (pressure at sea level), if you change the temperature at an airport and if the airport elevation is different from MSL, the QNH will change.

 

But most aviation weather report are QNH, so unless you, or some 3rd part weather add-on, or even XP's own real-time weather do some fancy conversion, it will be very wrong....

.

 2 minutes ago, uwespeed said:

I don't care about all that stuff. It's completely irrelevant for a desktop flight simulator. Does it increase the fun and the visuals? 😂
Honestly, I also thought that "pressure is pressure", however after jcomm's description with the hair-based hygrometer I understood the additional dependency of temperature.
As a mechanical engineer, I also like that, amongst others,  the moment of inertia (something, a mechanical engineer deals with almost daily) is implemented, depending of the distribution of weight. It's even closer to reality.

So, that's what I call simulation and an "engineering tool". Didn't someone wrote something like "nobody wants to have an engineering tool ... but visuals ...." Engineers and real pilots do care.

 

It relevant between crashing or not when fly over a mountain in cold days, on a -20C day, 3500ft AFE mountain will caught you when your altimeter shows 4000ft

1 hour ago, jcomm said:

then altitude should stay put

yes.

But when the temperature at the airport changes the qnh changes. so without changing your altimeter the gauge will move.

Just now, C2615 said:

It relevant between crashing or not when fly over a mountain in cold days, on a -20C day, 3500ft AFE mountain will caught you when your altimeter shows 4000ft

Exactly - that's why pilots say that "in winter the mountains are higher" 😉.

3 minutes ago, C2615 said:

But most aviation weather report are QNH, so unless you, or some 3rd part weather add-on, or even XP's own real-time weather do some fancy conversion, it will be very wrong....

You are right, now that pressure deviations are simulated, XP ATC should account for them and give the correct QNH, different from QFF if there are temperature deviations.

Its ages since we had a good aviation physics argument.

14 minutes ago, mSparks said:

yes.

But when the temperature at the airport changes the qnh changes. so without changing your altimeter the gauge will move.

Wrong... QFF does change Sparks. You can have the exact same pressure level intercepting your "airfield ground level", and widely varying temperatures, and while QNH stays put, QFF does vary...

Give a look in your ATPL(H) Sylabus / Exams... they'll give you the basic theory on that....

20 minutes ago, Murmur said:

I think XP12 is right.

The discrepancy is probably due to the fact that (at least for v11) in XP weather settings, you don't set QNH, but you set the pressure at sea level. On the other hand, temperature is set at the closest airport elevation.

So, for a given set QFF (pressure at sea level), if you change the temperature at an airport and if the airport elevation is different from MSL, the QNH will change.

Then XP it's not right Murmur ... QNH is QNH, QFF is QFF... They have to model it in a coherent way in order for situations under which the very same QNH is being reported and gives airfield altitude in your altimeter when you set it on the Kollsman window, then even if T get's a good deal away from ISA that same airfield altitude should be shown on your altimeter...

Edited December 18, 20214 yr by jcomm