14 minutes ago, jcomm said:

Sparks,

IF the reported QNH is the same, then you have to read the same alt... irrespective of T...

But here is the funny thing:

The REPORTED QNH is NOT the same 🙂

If I change the temperature as described in my experiment...and then check the ATIS again, they change the QNH (from 29.92 as set in GUI to 30.12) so that with the new QNH the altitude indication is correct again.

So I think for all practical purposes this is going to work out for the general use case.

The one caveat is someone setting up the temperature and QNH in the weather GUI...then changing temperature again....and then not listening to the new ATIS. But I have filed a report with Philipp on this - and while he will probably look down upon me again and shake his head why I can not understand even these absolute basic things - I hope we will get some enlightment and lecturing from this 😉

 

Edited December 18, 20214 yr by Janov

3 minutes ago, Janov said:

The REPORTED QNH is NOT the same 🙂

^^ this.

The altimeter moves because the QNH you need to set changes.

My guess is moving the temp should move the QNH slider.

Edited December 18, 20214 yr by mSparks

4 minutes ago, Murmur said:

The temperature offset changes the QFF (pressure reduced to sea level). So, if you keep the QFF constant (in XP you actually set the QFF), then the QNH (and altimeter reading) changes.

The problem is that using the QFF is not pratical. Austin should replace the entry in XP12 weather screen to modify QNH instead, not QFF.

 

 

If thats the case, that needs to change ASAP with this new implementation

10 minutes ago, mSparks said:

if there is xx feet per millibar instead of 30 because its a cold day for the same altimeter setting the altimeter will read half the altitude it would on an isa day.

Also could someone correct this so I have a chance of actually remembering it.

5 minutes ago, Janov said:

But here is the funny thing:

The REPORTED QNH is NOT the same 🙂

If I change the temperature as described in my experiment...and then check the ATIS again, they change the QNH (from 29.92 as set in GUI to 30.12) so that with the new QNH the altitude indication is correct again.

So I think for all practical purposes this is going to work out for the general use case.

The one caveat is someone setting up the temperature and QNH in the weather GUI...then changing temperature again....and then not listening to the new ATIS. But I have filed a report with Philipp on this - and while he will probably look down upon me again and shake his head why I can not understand even these absolute basic things - I hope we will get some enlightment and lecturing from this 😉

 

A good test would be fetching real world metar data into Xplane.Weather. in that case, QNH should.always give correct field elevation, no matter what the temperature.offset is etc etc. If in any way, Xplane does anything that doesnt allow.Metar QNH to indicate correct field elevation on the altimeter, then there.is.something.wrong in the implementation

 

1 hour ago, jcomm said:

Wrong... QFF does change Sparks. You can have the exact same pressure level intercepting your "airfield ground level",

but the temperature difference changes the number of miliibars between msl and the airfield.

e.g. on an isa day, an airfield 1000 feet above sea level will be  1013 - (1000/30) milibars height

on a cold/hot day (which?) it will be 1013 - (1000/15) millibars height.

All you can change on the altimeter is the 1013 part.

 

Edited December 18, 20214 yr by mSparks

10 minutes ago, mSparks said:

but the temperature difference changes the number of miliibars between msl and the airfield.

e.g. on an isa day, an airfield 1000 feet above sea level will be  1013 - (1000/30) milibars high.

on a cold/hot day (which?) it will be 1013 - (1000/15) millibars high.

If the QFF is the same (in your example, 1013 mb), then it is correct: QNH will change.

14 minutes ago, Murmur said:

If the QFF is the same (in your example, 1013 mb), then it is correct: QNH will change.

no qff or qne here. just pressure.

on an isa day an airfield 1000 feet above msl will have 1013 - (1000/30) = 979mb pressure, set to 1013 your altimeter will read 1000 feet.

when temperature changes the rate of change of mb with height from 30 to 15 mb per foot (still 1013mb at msl, just cold)

the airfield will have 1013 - (1000/15) = 946mb and set to 1013 your altimeter will read 2000 feet.

looks like a cold day to me.

BTW check this math, because I always get it mixed up, its like my aviation nemesis. it looks wrong to me.

Edited December 18, 20214 yr by mSparks

Even QFF isn't the "best" measure as it assumes an isothermal layer between MSL and the airfield surface.

What Janov added above is important though, saying the ATIS also changed QNH with T variation.

But nonetheless we have to make sure that under situations for which the QNH actually stays the same, then the altitude should, OFC, read the same in your altimeters.

As a glider pilot I use mostly QFE. If I want an approximate value for the local QNH all I do is wind up or down ( usually up 🙂 )  the altimeter adjustment knob until I get the airfield reported elevation. The pressure reading in the Kollsman window will be an acceptable QNH.

Actually QFF = barometric pressure at the airport ( QFE ) reduced to mean sea level ( MSL ) with regard to the actual outside air temperature and assuming an isothermal layer from MSL to the airport surface, while QNH is the QFE reduced to MSL without any further adjustments, hence assuming ISA pressure lapse rate from MSL to the airport surface.

Also, for an airfield situated at sea level, QNH = QFF irrespective of T.

The following are valid Above Sea Level:

T > ISA ==> QFF < QNH

T < ISA ==> QFF > QNH

The following are valid Bellow Sea Level:

T > ISA ==> QFF > QNH

T < ISA ==> QFF < QNH

 

Edited December 18, 20214 yr by jcomm

2 minutes ago, jcomm said:

As a glider pilot I use mostly QFE

which, btw, is really easy because you just set your altimeter to zero feet before you take off 😛

30 minutes ago, mSparks said:

no qff or qne here. just pressure.

on an isa day an airfield 1000 feet above msl will have 1013 - (1000/30) = 979mb pressure, set to 1013 your altimeter will read 1000 feet.

when temperature changes the rate of change of mb with height from 30 to 15 mb per foot (still 1013mb at msl, just cold)

the airfield will have 1013 - (1000/15) = 946mb and set to 1013 your altimeter will read 2000 feet.

looks like a cold day to me.

BTW check this math, because I always get it mixed up, its like my aviation nemesis. it looks wrong to me.

Right, lets try this again.

https://skybrary.aero/articles/international-standard-atmosphere-isa

Quote

• Pressure of 1013.2 millibar - Pressure is taken to fall at about 1 millibar per 30 feet in the lower atmosphere (up to about 5,000 feet).
• Temperature of +15 °C - Temperature falls at a rate of 2 °C per 1,000 feet until the tropopause is reached at 36,000 feet above which the temperature is assumed to be constant at -57 °C. (The precise numbers are 1.98 °C, -56.5 °C and 36,090 feet)

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

Quote

Temperature has an effect on the accuracy of barometric altimeters, indicated altitude, and true altitude. The standard temperature at sea level is 15 degrees Celsius (59 degrees Fahrenheit). The temperature gradient from sea level is minus 2 degrees Celsius (3.6 degrees Fahrenheit) per 1,000 feet. For example, at 5000 feet above sea level, the ambient temperature on a standard day would be 5 degrees Celsius. When the ambient (at altitude) temperature is colder than standard, the aircraft's true altitude is lower than the indicated barometric altitude. When the ambient temperature is warmer than the standard day, the aircraft's true altitude is higher than the indicated barometric altitude.

So.

Airport is 1000 feet true altitude

1. If your true altitude remains the same and the temperature becomes colder than standard then barometric altitude (what the altimeter says) rises, you will need to change it to get back to 1000 feet.

My math looks OK, its 30 to 15 feet per millibar, so the pressure falls faster.

QNH

https://skybrary.aero/articles/altimeter-pressure-settings

QNH - The pressure set on the subscale of the altimeter so that the instrument indicates its height above sea level. The altimeter will read runway elevation when the aircraft is on the runway.

Is corrected for temperature, and the number you have to SET on the altimeter to read runway elevation when you land.

Edited December 18, 20214 yr by mSparks

18 minutes ago, mSparks said:

Is corrected for temperature, and the number you have to SET on the altimeter to read runway elevation when you land.

which is on a scale of 30 feet per mb, so the QNH becomes 33 less than 1013. QNH = 979

now the airfield has 946mb, your alitmeter sees 979-946mb =33mb difference

33mb*30 feet = your altimeter now reads 990feet (10 feet of rounding error)

In xplane now

setting 979mb on the altimeter makes it read 1000 feet below true altitude, regardless of temperature.at the airport

Edited December 18, 20214 yr by mSparks

In practice for airliners operations it works like this.

Low Tempreture altitude  corrections are applied at -10C and below or if an approach chart has a specified minimum tempreture and the tempreture is below this. 

Corrections are applied to the MDA, platform heights, SSA, go around altitude , and distance to threshold heights.

Charts are available to obtain the required correction and I also have an app on my phone for getting the correction. For -20C for example the correction of 14% would need to be added to the required heights.

For 3D RNAV approaches where the tempreture is below the minimum specified on the chart the approach cannot be flown using VNAV . Instead LNAV and VS or FPA must be used, the FPA must also be increased by the percentage increment, so in the -20C example of a 14% increment then a 3.0 degree FPA must be increased to 3.4 degrees.And the LNAV only minima should be used.

An ILS can still be used as this is a fixed radio beam and obviously wont change due to tempreture, however platform, FAF and DH all need corrections ( not cat 2 and 3 DH as they use radio altitude.)

The heights are manually adjusted using MCP intervention, there is no authorisation yet where I work at least to amend the waypoint altitudes in the FMC though approval is being sought.

This applies at least to all Boeing types, other FMC VNAV set ups are available which can automatically compensate for tempreture and pressures.

Finally altitude  given by ATC while under radar vectors do not need to be compensated.

We flew for years with no one even knowing about all this stuff, at least on a practical  level outside the classroom it was ignored , now the regulators have highlighted its required use.

All very interesting I’m sure, but what about the choice of dramatic stirring music that’s to be played while XP12 is loading? All good sims need a dramatic piece of music while loading … apparently/annoyingly 😉

Edited December 18, 20214 yr by jon b

jon b couldn't have put it better ! And the music !!!!!

45 minutes ago, jon b said:

An ILS can still be used as this is a fixed radio beam and obviously wont change due to tempreture, however platform, FAF and DH all need corrections ( not cat 2 and 3 DH as they use radio altitude.)

Not much to add to Jon´s summary...the one thing that is making an entry these days are the GBAS 3D approaches, also called GLS. A small GPS receiver/transmitter at the airport receives GPS position, computes the positional error to it´s own (known) position and transmits this error correction to approaching aircraft.

These aircraft can then home in on the known 3D position of the runway (while displaying a virtual GS and LOC deviation) much like a smart-bomb homes in on a GPS position. As we are finding out in X-Plane these days, these approaches will not work well if the elevation of the threshold is not correct (sometimes airports in X-Plane are "flattened" to deal with undulating terrain).

These approaches are also independent of temperature (except for correcting the minima like Jon pointed out) - one thing you DO notice while flying ILS and GLS approaches in warm or cold weather is the different distance at which the approach "seems" to start. If you fly in at 3000 feet AAL, the nominal glide-path intersection is at 10NM from the threshold. During cold weather you will actually fly lower...so it seems to start "closer in" to the runway.

Edited December 18, 20214 yr by Janov