Altimeter operation: Display of atmospheric pressure

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Altimeter operation: Display of atmospheric pressure

Hello,
Altimeter operation: Display of atmospheric pressure varying with altitude that the needle transcribes into barometric altitude in feet.
With the help of an adjustment knob, a reference atmospheric pressure is defined as a height of mercury in inches (inHg).
By convention, mean sea level (MSL) is reference elevation 0. The atmospheric pressure is 101,325 Pa = 1013.25 hPA = 1.01325 bar = 1 atmosphere at 15°.
By definition of the millimeter of mercury (mmHg) and the torr (Torr) that is equal to it, the normal atmospheric pressure is exactly 760 mmHg = 760 Torr.
Since one inch is 25.4 millimeters, it is 29.92 inHG. At altitude 0, an altimeter must therefore display a reference atmospheric pressure of 29.92 inHG.

How do you explain that on the Cessna 172 parked at LFMH, its altimeter reads 1325 feet and 28.97 inHG?

How do you explain that on the B 777-300 ER parked at LFMH, its barometer set at 1325 feet shows 1028 hPa and 30.38 inHG?

With the help of https://www.sensorsone.com/elevation-st ... alculator/

Data :
Hp = 1325 ft
QNH = 1 ATM

QFE = 28.516 in Hg
QFE = 965.665 hPa

Thanks

Back on the Cessna 172: set to 28.6 inHg negative altitude read with the small needle between 0 and 9 i.e. 9.5 and with the large needle between 7 and 6 i.e. 6.5.. Directly I read 650 feet or I read with mental correction that this would be equivalent to negative 350 feet, which would be completely consistent! So there's a problem?

Back to the B 777: set at 28.57 inHg the armospheric pressure is 965 hPa, it's consistent!
But the barometer shows a negative altitude, i.e. -370 feet! So there's a problem with that too?
Last edited by S-KIMO on Fri Feb 09, 2024 4:37 pm, edited 1 time in total.
S-KIMO

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Re: Altimeter operation: Display of atmospheric pressure

Probably it is the long standing weather bug fixed recently for the next release. See: https://wiki.flightgear.org/w/index.php ... _bug_fixed

benih

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Re: Altimeter operation: Display of atmospheric pressure

Hello,

<property>instrumentation/altimeter/indicated-altitude-ft</property> is the property that gives the altitude.
<property>/instrumentation/altimeter/setting-inhg</property> is the property that gives the atmospheric pressure that corresponds to...
https://www.sensorsone.com/altitude-pre ... onversion/ is the table that gives the correspondences!
A computer scientist should be able to easily fix this defect?
S-KIMO

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Re: Altimeter operation: Display of atmospheric pressure

What Version of FlightGear are you running?

benih

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Re: Altimeter operation: Display of atmospheric pressure

Good morning,

I'm using FG 2020.3.19

I am developing "my" Crusader F-8E(FN) which I took over on my own based on Gérard Robin's model designed before 2007. FG which I have been using since 2013 was itself under development. In fact everything was problematic at that time...

So I'm thinking of making the altimeter work as it works on the B777, that is to say with an adjustment of the barometer by the altitude of the runway that I know without reading a map by querying the "Position" tab then "Choose a airport” for example LFMH: “Elevation” = 1325 feet which will give the atmospheric pressure QFE...

I don't know all the evolutions of FG...

With the B777-300, I see that the generic HUD gives an altimetry of 1300 feet at LFMH threshold 18.
When launched, the barometer displays:

920ft
“HPA” = 1013 hPa
“IN” = 29.92 in Hg

So I correct with the wheel button 1300 feet to obtain:

hPA = 1027
inHg = 30.35

The generic HUD gives an altimetry of 1340 feet at LFMH Threshold 36.
When launched, the barometer displays:

950ft
“HPA” = 1013 hPa
“IN” = 29.92 in Hg

So I correct with the wheel button 1340 feet to obtain:

hPA = 1028
inHg = 30.36

The map of french SIA AD 2 LFMH ATT 01 mentions threshold 17 at 1286 feet and threshold 35 at 1325?

There is no connection with METAR information as stated in the link: https://sourceforge.net/p/flightgear/codetickets/2022/

I only compare a FG display with a calculation by https://www.sensorsone.com/elevation-st ...alculator/ which reveals that the <property>/instrumentation/altimeter/setting-inhg</property> is wrong!

We need to know how the inHg height is developed as a function of altitude?

The use of METAR data gives realism but complicates a flight because we then consider a variable wheather instead of a constant weather!
Last edited by S-KIMO on Sun Feb 11, 2024 10:34 am, edited 2 times in total.
S-KIMO

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Re: Altimeter operation: Display of atmospheric pressure

Do you try this on the „fair weather“ setting?

benih

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Re: Altimeter operation: Display of atmospheric pressure

Hello

This link copied from your message doesn't work! I've put the one for the explanation...

So I had my son do the exercise who owns MFS... He introduced the METAR which gave 1003 hPa at LFMH threshold 35 = 1325 feet and his altimeter then shows a PAtmo = 29.6 inches of HG!

When I experiment with the FG B777-300, I read HPA = 1003,
IN = 29.63 and the altimeter reads 670 feet! So the atmospheric pressure correspondences are OK and the altitude has a defect of 1325 - 670 = 655 feet! Is that clear... There's a bug! Give it a try and you'll see how I see it!
Otherwise I'm on the generic environment with no modification on my part!
S-KIMO

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Re: Altimeter operation: Display of atmospheric pressure

On my FG it works properly positioned at LFMH
Aircraft: Boeing 777-300ER, Airbus A320, Cessna 172P, DHC-6
Airport: LIPQ, LGTS

I-PIC68

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Re: Altimeter operation: Display of atmospheric pressure

Hello,
Thank you, it works!
Last edited by S-KIMO on Mon Feb 12, 2024 12:12 am, edited 1 time in total.
S-KIMO

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Re: Altimeter operation: Display of atmospheric pressure

benih wrote in Sat Feb 10, 2024 10:21 pm:Do you try this on the „fair weather“ setting?

benih

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Re: Altimeter operation: Display of atmospheric pressure

Hello,
There is a specificity to air transport which is that it happens in an element, a fluid, air which is a gas and which does not follow the theory of ideal gases! So in its movement within the air following the temperature and altitude, EVERYTHING varies and yet from this air we must find ways to maintain the relative movements of the plane and the fluid...Geniuses have invented the theories and instruments to obtain the speed by a sensor _ the Pitot tube system _ which informs the analog anemometer and the altitude using another sensor _ the barometer _ which informs the analog altimeter. .. Why analog? Because they were needed and invented from the very beginning of aviation! Digital instruments are somehow less complex and will not work the same way due to the fact that you can combine equations and calculators to indicate values! I'm building an F-8E (FN) which has analog instruments while a B777-300 ER has digital instruments, human machine interfaces, managed by computers! There is therefore a problem in my approach which is to know if in FG the properties make this distinction? So to be explicit on a plane from the 45/75 years the altimeter must be associated with a barometer...the barometer will measure the atmospheric pressure in terms of variation because there is no standard value there only exists a reference value! We must therefore, as in mechanics with a comparator to measure a height, calibrate, and this calibration is done at a given altitude which will serve as a basis for developing a difference in altitude by a difference in atmospheric pressure... An altimeter is therefore an instrument which has a fixed scale of altitudes _ the dial_ so we make a value _ by adjusting the needle _ coincide with an intrinsically variable pressure: at altitude A, that of the threshold of a runway of the aerodrome prevails the atmospheric pressure Patmo...When the plane flies in the environment close to this aerodrome my micro variation of the Patmo immediately translates into variation of altitude which in this restricted space will be linear...
The ICAO standard atmosphere defines that the Patmo variation of 1 hPa is equivalent to 27.31 feet of altitude difference. So in France, it is said that the variation of Patmo of 1 hPa is worth an altitude variation of 28 feet or 8.5 meters...and vice versa! To facilitate mental calculation, we say that there is a variation in atmospheric pressure of 1hPA for every 30 feet of altitude. 1 Pa worth 0.0002952974 inches of Hg therefore 1 hPa worth 0.02952974 inches of Hg, We thus obtain that the variation of Patmo of 0.02952974 inHg is worth an altitude variation of 27.31 feet... So Patmo's change of 1 inHGg is worth an altitude change of 27.31/ 0.02952974 = 924.83 or 925 feet! So with an analog altimeter calibrated at QFE, its needle is at 0! A small passenger plane that circles an airfield for a short period of time or a Crusader that takes off and lands on its aircraft carrier only has to make this adjustment if the weather is stable. If it changes only the variation in atmospheric pressure matters between departure and arrival and by mental calculation the pilot can adjust his altimeter: There is no point in considering an anticyclone (good weather) which causes the Patmo to rise...visual flight is all the easier! But a sudden depression (bad weather) which may occur causes the Patmo to descend and which may obscure the view when approaching the aerodrome, it will be necessary to land on the instruments after having obtained the current QFE by radio from the ATIS which will make it possible to make the altimeter correction necessary for landing. Thus if the atmospheric pressure has fallen by an inch of Hg between two moments of the flight, the altitude displayed will be equivalent to an incorrect altitude of the plane, that is to say that the pilot without having acted on the nose-up pitch of the aircraft he plane will think it is flying 925 feet higher...than it actually is! During instrument landing it will impact the planet (on land or in the sea) at the altitude indicated by the altimeter of 925 feet! It was therefore necessary to reduce the altitude initially read by 925 feet!
In the context of the time, on November 1, 1944, this is what happened, near my home, to a C47 serial 42-92700, 17th troop carrier squadron belonging to the 64th transport carrier group of the 12th Air Force USAAF, based in Istres, during a medical evacuation flight between Luxeuil and Istres in bad weather conditions (intense fog, frost and rain) which crashed around 2:30 p.m. at a place called "Le Chirat de l'Escoutay" on the commune of Doizieux. : Drill Info - Crash! (drill-info.com)! In this vast inventory of aircraft accidents, this one is missing: Bad weather appears to be the cause of the accident of the three Mirage F-1s (lemonde.fr) Let's go on... Regarding a modern aircraft, that is to say with digital instruments, I would take the example of the B 777-300 ER at threshold 35 of LFMH whose altitude is 1325 feet. When I launch it I display threshold 36...and once launched the PFD altimeter is not set to 1325 feet! However, it would be possible to display a value of 1325 feet at threshold 35 which is part of the official aeronautical documentation and for the altimeter to be at 1325 feet at threshold 35! We would then adjust the Patmo at this point by consulting its current value on the METAR...This is completely realistic, right? What I mean is that a pilot knows that at the threshold of a runway, consulting the map will provide him with the altitude value to adjust the altimeter: it's hard work, it doesn't vary! Once the Patmo value is known, he matches the altitude/pressure scales for his flight: for example with a QFE of 1017 hPa = 30 in Hg, according to the ICAO rule, when his barometer will indicate in flight 1 inHg less than its setting is 29 inHg it should read on the altimeter 1325+ 925 = 2250 feet! When we practice the method which gives the solution to my question and which consists, as explained by I-PIC68, in obtaining a value of the QFE by querying the METAR (to be configured with “Environment > Weather > Detailed Weather > Weather conditions > Live data > Apply") or 1023 hPa = 30.21 inHG for example, then turn the barometer dial until you see it displayed and correspond to 1340 feet, this is not realism! Ground altitude and inherent atmospheric pressure are independent! Altitude is geography and Patmo is meteorology! In flight, we use a variation of the Patmo to correct and recalibrate an instrument!
So there is some computer magic: my Boeing is at threshold 36 (35) of LFMH...
I start “Autostart” and the PFD shows: 950 feet, the barometer shows 1013 HPA and 29.92 IN 950 feet!

The ground is marked as an orange zebra at 1325 feet! So IT managed this data without my knowledge!
This confirms that this data is tabulated in "scenery" to effectively position the aircraft...Why then not directly adjust the position of the aircraft to the threshold altitude? Then to match this intangible altitude with the specific Patmo to the real weather by turning the barometer dial to the QFE value to carry out the calibration?
S-KIMO

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Re: Altimeter operation: Display of atmospheric pressure

Wow, that was hard to read.

I would take the example of the B 777-300 ER at threshold 35 of LFMH whose altitude is 1325 feet. When I launch it I display threshold 36...and once launched the PFD altimeter is not set to 1325 feet

I just spawned with FGFS 2020.4 (compiled today) with the C182 (recent git master) at EDDM RW 08L.
That runway threshold is, according to the charts at 1467 ft.
My ATIS METAR tells me QNH 1023 (which is 30.22inHG).
When I calibrate my Altimeter to 30.22, I get ~1465.
That part looks completely OK to me.

When I dial in 0ft altitude (QFE) , I get 28.65 inHg in the Kolsmans window.

Honestly, I don't get what your question/issue really is.

Can't test LFMH unfortunately, because the ongoin terrasync issues.
But, either its a scenery bug, or the mentioned bug in the weather engine that was recently fixed.
To check the latter: Be sure to set the "Tile selection mode" to METAR in the Detailed Weather config.

Maybe also of help: https://skybrary.aero/articles/altimete ... e-settings

benih

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Re: Altimeter operation: Display of atmospheric pressure

Hello

I have FG versions since version 2.10.0.3. from 25/03/2013...
I reinstalled version 3.0.0. of 20/02/2014 and I compare the first screen of the B 777-300 ER at KSFO threshold 28 R with that of the version 2020.3.19 of the same aircraft!

See for yourself the result!

[img][img]https://i.postimg.cc/44Sp4bRw/Capture-d-cran-2024-02-15-085859.png[/img][/img]

Before, the altimeter was set and the METAR of this version was invalid...
Today, the altimeter is not settled!

There is a difference that I can't explain to myself in terms of the coherence of an instrument setting that has nothing in common with reality!

Well, let's leave it at that!
S-KIMO

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Joined: Mon Dec 07, 2020 3:01 pm

Re: Altimeter operation: Display of atmospheric pressure

No! I will be back...
Here at LFMH and the B777 at threshold 36(35) I set the barometer to QNH or 0, I should read 29.92 in HG and I read 28.92!

Look for the error!
S-KIMO

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Joined: Mon Dec 07, 2020 3:01 pm

Re: Altimeter operation: Display of atmospheric pressure

S-KIMO wrote in Thu Feb 15, 2024 9:50 am:Here at LFMH and the B777 at threshold 36(35) I set the barometer to QNH or 0, I should read 29.92 in HG

Why should it be 29.92? This is only the case if the actual pressure at your location is equal to 29.92... Can you check /environment/pressure-inhg (not the sea level pressure!) in the property browser (I suspect it was 28.92 in the screenshot that you just posted)? That will allow us to confirm what the actual in-sim weather is at your location, which may be different from the METAR that you see in the Weather dialog. If you set your instrument to the value from /environment/pressure-inhg, it should give you an altitude read out of zero (and only then!).

If you set your instrument to 29.92 inHg, you get pressure altitude (which is used for flight levels) based on a standard atmosphere. This is completely independent of the airport elevation and does not guarantee your altimeter to read 0 ft when you are on the airfield...
Airports: EHAM, EHLE, KSFO
Aircraft: 747-400

Gijs
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