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Altimeter Calibration

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Altimeter Calibration

Postby dilbert » Sun Jan 29, 2017 7:05 pm

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According to my understanding, any correctly calibrated pressure altimeter instrument should read 0 feet at 29.92 HG, and that's a matter of instrument calibration, not atmospheric pressure setting or location. Yet in FlightGear, the corresponding instrument calibration for 29.92 HG is 520 FT at KSAN and 680 FT at KSFO, when checked with either of the above aircraft. First of all, the instrument calibration (not the pressure setting) should not change with location. Second of all, at 29.92 HG it should be calibrated to always read 0 FT, or sea level. It seems to me something is amiss in the program. Please correct me if I'm wrong.
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Re: Altimeter Calibration

Postby Isaak » Sun Jan 29, 2017 7:53 pm

When setting the altimeter at 29.92 InHg, the altimeter will vary based on the altitude and the outside pressure. To read 0 ft at sea level, the altimeter has to be set to the local air pressure. 29.92 InHg is the standard pressure used to define Flight Levels. These are independend of the local air pressure, and thus the true (actual) altitude won't be the same as the indicated altitude. Because all (high altitude) pilots use them, everyone has the same offset to the true altitude and vertical separation is guarenteed. So setting the altimeter to 29.92 InHg has nothing to do with calibration.
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Re: Altimeter Calibration

Postby Gijs » Sun Jan 29, 2017 8:36 pm

Have a look at our wiki for some more info about altitudes as used in aviation: http://wiki.flightgear.org/Altitude There's a whole lot to it ;-)
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Re: Altimeter Calibration

Postby dilbert » Sun Jan 29, 2017 10:19 pm

Thanks for the reply
I understand what you are saying, but that is not the issue. The issue is how the instrument is calibrated. It is a bourdon tube gauge. When the outside pressure drops the tube straightens causing the needle to turn to a higher setting. To calibrate the instrument an instrument maintenance person has to adjust the physical linkage between the needle and the bourdon tube. It is adjusted according to standard atmosphere. When properly calibrated, the gauge will always read 0 altitude when the atmospheric pressure is 29.92 inches HG.
I don't know how this has been programmed in flight gear. I just went back and rechecked the A6 at San Diego. This time, quite inexplicably, it read 780 ft as at KSFO with the instrument setting menu showing 29.92 HG, which should not be. It's understandable that the gauge will read higher at a lower atmospheric pressure-that's what it does. But if it reads higher, the instrument setting menu should reflect a commensurate lower pressure, which it does not. It consistently comes up showing 29.92 at start, even though the altitude is 900 feet or so. Yes, you can adjust the altimeter to the field elevation ( around 20 feet) using the instrument setting menu, but when you do so, the pressure drops to around 29.0 or so inches, which is incorrect, since the instrument should be calibrated to read 0 feet at 29.92 pressure, not 900. Nevertheless that's not how it's programmed. At start the instrument adjustment menu always shows the pressure to be 29.92 HG; and Generally, the needle is at 900; though sometimes it will slowly drop to a slightly lower
figure (I don't know what in the program causes this).

Lets say the program imports the actual atmospheric pressure at the location, and the altitude reads 900 feet instead
of 20. If that's the case, the altimeter adjustment menu should show the pressure to be less than 29.92, something like 29.0.

Why is this important? Because arriving pilots depend on controllers, ATIS, Towers, etc. to be informed of the correct existing pressure, so they can set their altimeter accordingly. Should a flightgear pilot do that, his altimeter will always read about 900 feet high, and he would be in danger of hitting the ground or going in the drink.

I first discovered the problem doing night carrier approaches, and quickly discovered I had to be very careful to reset the altimeter to field elevation at departure; and when flying over sea make sure it agreed with the radar altimeter, not to visit Davey Jones locker. If I had depended on a pressure supplied by ATIS or the LSO, I'd would have needed my Mae West. Cheers
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Re: Altimeter Calibration

Postby Necolatis » Sun Jan 29, 2017 11:52 pm

I think you have misunderstood how it works.

Before takeoff, you calibrate the altimeter pressure so that 0 feet matches the pressure outside the aircraft, and no that won't be standard pressure. And yes, if there is a hill nearby you might crash into it if you rely on that.
When you get above a certain altitude you set the standard pressure, and yes, it will no longer match the ground where you took off from nor where you will land nor terrain below you, but it will match other planes nearby you that have done the same.
When you come in for landing the ATC will tell you the local pressure and you calibrate to that, so that when you have landed, if will show 0 feet.
So during your flight you will use 3 different settings.

Standard pressure I think should only show you 0 feet, when the outside pressure is at standard atmosphere, a situation which you probably never will encounter.

If you want to do low level flying and use altimeter to not crash into ground, you probably wanna rely on radar altimeter instead, if the plane have it.

Remember that 0 feet altitude at KSFO has a different pressure than 0 feet at KLSV, due to local weather and altitude differences in the 2 places. You don't automatically have standard pressure at any given 0 feet at some location.
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Re: Altimeter Calibration

Postby Johan G » Mon Jan 30, 2017 12:18 am

Some quick notes:

QNH, the pressure that is usually reported is based on the air pressure measured at the airport converted to sea level pressure. In essence, when you set the altimeter to the air pressure in the current METAR or ATIS, the altimeter should show an altitude in the near vicinity of the published airport elevation.

There is also a QFE air pressure, which is the actual air pressure at the airport (this is the one mentioned by Necolatis above). If you set the altimeter so that it will read 0 ft altitude, you have set the altimeter to QFE.


Gijs wrote in Sun Jan 29, 2017 8:36 pm:There's a whole lot to it ;-)

Not an understatement.

This is an illustration of just the most used terminology (though I have a hard time imagining a transition altitude below MSL): :wink:
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Vertical distances.svg by AronRubin and Dr. Wessmann at Wikimedia Commons
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Re: Altimeter Calibration

Postby sanhozay » Mon Jan 30, 2017 12:21 am

It's probably worth keeping a clear distinction here:

Calibration: by a maintenance engineer on the ground such that that the altimeter reads zero when the adjustment knob is set to QFE (which is derived from the observed pressure at the airport and known field elevation). Calibration is done with a tool after adjusting the altimeter to QFE with the knob.

Adjustment: by the pilot so that the altimeter reads zero at field elevation (QFE), field elevation at field elevation (QNH) or flight level as required.

I don't believe Flightgear models altimeter calibration, only adjustment.
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Re: Altimeter Calibration

Postby dilbert » Mon Jan 30, 2017 1:04 am

I agree with sanhozay

So, you just did long flight and are making a dark night approach. Most likely, the pressure at your destination is different from your departure point, so you don't have the luxury of resetting the altimeter at a known field elevation. You have to rely on what ATC, ATIS, the tower, or the ship tells you the local altimeter
setting should be. You set your altimeter accordingly, but not having a radar altimeter to cross check, you fly into the ground. When they pick up the pieces and find FlightGear's altimeter, they will discover that it reads 900 ft high; because, as programmed, it's is incorrectly calibrated (I.E. It registers 900ft at 29.92 HG, while it should read 0 ft. In the real world, if you entered the reported altimeter setting prior to take-off, and it then showed the field elevation to be 900 feet to high for that particular airport, you should immediately recognize a problem with the altimeter, and not depart until it's fixed.

Best Regards, Dilbert
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Re: Altimeter Calibration

Postby Necolatis » Mon Jan 30, 2017 1:43 am

Okay now I get what you are saying Dilbert, you seem to be correct, sorry.

I just did a test:

1 - set METAR to Q1013 at the airport where I am at
2 - adjust altimeter to 1013.0 hPa
3 - observe the indicated altitude is 139 feet at ground level.
4 - Go in property browser and observe that the pressure at the airport is 29.76, while at sealevel its 29.913

So it seems the altimeter is correct, its the pressure generated from METAR that is interpreted as being at sealevel.
If that is correct interpreted then, ATC should not give METARs pressure to you when they talk to you, but whatever pressure they measure..or is METAR interpreted wrong in Flightgear?
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Re: Altimeter Calibration

Postby Necolatis » Mon Jan 30, 2017 1:51 am

Just read up on QNH and QFE, it seems that METAR code is for QNH, so Flightgear is correct.

The problem is then that your ATC gives you QNH instead of QFE.
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Re: Altimeter Calibration

Postby Thorsten » Mon Jan 30, 2017 6:29 am

You set your altimeter accordingly, but not having a radar altimeter to cross check, you fly into the ground. When they pick up the pieces and find FlightGear's altimeter, they will discover that it reads 900 ft high; because, as programmed, it's is incorrectly calibrated (I.E. It registers 900ft at 29.92 HG, while it should read 0 ft


Unless you're flying a glider where you indeed normally set your altimeter to show zero on the field, barometric altimeters are otherwise almost exclusively used to display altitude above sea level.

So your mistake is to assume it should read zero where it should not - on the runway it should read the elevation of the runway over sea level in ft. And if you look into your approach charts, you will see the elevation of hills in ft over sea level, so if there's a hill marked with 2400 ft and your runway is 1800 ft, you know you'd better not go below 2700 ft as long as you're over the hill.

No chart shows hills and obstacle elevations above the runway elevation, so the information how high you are above the field wouldn't help you overly much.

The radar altimeter in contrast shows you your elevation above local terrain level, and I assure you, if you approach KGCN and have Grand Canyon right beneath you, it'll show you that you have a HUGE distance to the ground which will suddenly jump to a low number when you cross the rim - altitude above ground in rugged terrain isn't nearly as useful as you seem to think, and it most definitely is not altitude above runway either.
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Re: Altimeter Calibration

Postby sanhozay » Mon Jan 30, 2017 8:13 am

Necolatis wrote in Mon Jan 30, 2017 1:43 am:So it seems the altimeter is correct, its the pressure generated from METAR that is interpreted as being at sealevel.

I think you might find that if you use real weather vs a weather scenario, including entering a METAR string manually, Flightgear's behaviour is different.

This is some code I wrote that attempts to set the altimeter in a checklist binding so that the altitude reads height above sea level:

Code: Select all
addcommand("set-altimeter-qnh", func {
    var rwx = getprop("environment/realwx/enabled");
    var qnh = getprop("environment/metar/pressure-inhg");
    # This is probably a bug in weather scenarios. If the altimeter
    # setting is taken from the METAR string in the scenario, the altitude
    # is wrong, but if it is set from sea level pressure, it comes out
    # correct. At some point this will be fixed in FG and this fudge will
    # need removing.
    var slp = getprop("environment/pressure-sea-level-inhg");
    var setting = rwx ? qnh : slp;
    setting = math.round(setting * 100) / 100;
    interpolate("instrumentation/altimeter/setting-inhg", setting, 1.0);
});


The difference between environment/sea-level-pressure-inhg and environment/metar/pressure-inhg was discussed on the mailing list some time ago (within the last year, maybe two) but I can't find the message.
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Re: Altimeter Calibration

Postby dilbert » Mon Jan 30, 2017 10:07 am

Programming aside, this issue has absolutely nothing to do with Metar, the real weather or what ATC reported. If you look at the FlightGear instrument setting menu at start, you will see the listed altitude corresponding to 29.96 inches HG is 900 feet, which means that the mechanical calibration of the altimeter itself is wrong. If you set your altimeter to the correctly reported atmospheric pressure prior to take-off and it indicates an altitude 900 feet higher than the known terrain at the airport, you would not take-off until the altimeter was repaired (mechanically calibrated correctly by maintenance). As currently calibrated, the FlightGear pressure altimeter will always read 900 feet to high when the pilot sets it to the correct atmospheric pressure.

Best Regards, Dilbert
Last edited by dilbert on Fri Feb 03, 2017 11:25 pm, edited 2 times in total.
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Re: Altimeter Calibration

Postby Thorsten » Mon Jan 30, 2017 10:27 am

If you look at the FlightGear instrument setting menu at start, you will clearly see that the listed Altitude corresponding to 29.96 inches HG is 900 feet, which means that the mechanical calibration of the altimeter
itself is wrong.


I have no idea what the altitude listed to 29.96 in the Instrument setting menu is (I don't have any altitude listed there), but I do know that if you treat the altimeter like in a real aircraft, it shows the correct barometric altitude so there's nothing wrong with the simulation and your statement is just false.
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Re: Altimeter Calibration

Postby dilbert » Mon Jan 30, 2017 11:05 am

Please Thorsten. Sorry if I somehow offended you.

Start the program and look at the "instrument settings" under "equipment". On all four of my machines it shows an altitude of about 900 feet for an atmospheric pressure of 29.96 inches HG; when, according to my understanding, a correct altitude should read 0, if, in actual fact, 29.92 HG is the currently existing pressure (which it may not have been).

I just spotted the A6 at North Island. At start, " instrument settings" showed
29.96 inches HG whilst the A6 altimeter read 900 ft. So I used the arrows in the menu to adjust the altimeter down to 17 feet . The corresponding
atmospheric pressure in the menu dropped to 28.9 inches HG. Are you saying the then current pressure from Metar was 28.9 and that the program will show a different altitude in "instrument settings" for any particular pressure as the real pressure changes? If so I understand completely and apologize for the hassle.

My brain is getting a bit foggy at 81.

Best Regards, Dilbert
Last edited by dilbert on Mon Jan 30, 2017 12:02 pm, edited 2 times in total.
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