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Altimeter Calibration

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Re: Altimeter Calibration

Postby Thorsten » Mon Jan 30, 2017 11:45 am

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;


There's no altitude shown anywhere on my dialog. I get pressure in mbar, in InHg and an offset.

What's bugging me is that when the atmospheric pressure is actually 29.96 inches HG, the altimeter should read sea level-not 900 feet.


Which it does here.

Maybe it'd be time for a screenshot of your Instrument settings dialog, with property browser showing /position/altitude-ft and an altimeter instrument in view showing indicated altitude and pressure setting.

Did you fiddle with the instrument failure dialogs by any chance?
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Re: Altimeter Calibration

Postby dilbert » Mon Jan 30, 2017 1:18 pm

Oops Thorsten.

I just checked the actual atmospheric pressure at KSAN and it's 30.15 inches HG. Started FlightGear with A6 at KSAN. Set the pressure to 30.15 using the "instrument settings menu" under "equipment", and the altimeter in the A6 read 920 FT while on deck at about 20 feet elevation. Something's not right, or I've gone bonkers. Cheers
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Re: Altimeter Calibration

Postby Johan G » Mon Jan 30, 2017 3:02 pm

dilbert wrote in Mon Jan 30, 2017 1:04 am: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.

dilbert wrote in Mon Jan 30, 2017 10:07 am:As currently calibrated, the FlightGear pressure altimeter will always read 900 feet to high when the pilot sets it to the correct atmospheric pressure.

Oh. Have you been trying this whole time to say that the altimeter always will show about 900 feet to high altitude? :oops:

sanhozay wrote in Mon Jan 30, 2017 8:13 am: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.

I vaguely remember being it discussed on the forum.

Maybe not relevqnt here, but is perhaps sea-level-pressure-inhg interpolated over some time so it do not jump when a new METAR is read in?
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Re: Altimeter Calibration

Postby Michat » Mon Jan 30, 2017 3:38 pm

About QFE.

On FG calibrate instrument dialog. Inches lacks of one decimal position. Flying at Siberia with reported QFE 747 is 29.74 Inches, so we are loosing accuracy 007 :)

Having only adf app, low temperature and visibility. Is a good idea to get equipped with 3 altimeters, 2 based on pressure plus 1 radio alt. Cross back-up, and GPWS alert activated.

Could be great been able to set QFE on the instrument setting altimeter dialog.

Cheers
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Re: Altimeter Calibration

Postby Thorsten » Mon Jan 30, 2017 3:45 pm

Maybe it'd be time for a screenshot of your Instrument settings dialog, with property browser showing /position/altitude-ft and an altimeter instrument in view showing indicated altitude and pressure setting.

Did you fiddle with the instrument failure dialogs by any chance?
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Re: Altimeter Calibration

Postby dilbert » Mon Jan 30, 2017 4:09 pm

Yes Johan with regard to the A6 and the f14

I have recently checked some other aircraft at KSAN at 30.15 inches HG (the currently prevailing pressure):

The F14 registered 780 feet.
The 172 registered 50 feet, which is close enough.
The 777-200 registered 30 feet, which is quite accurate.
The DR-400 registered -200 feet.
The f16 registered 990 feet
The PC-9M registered 980 feet

It's now clear to me that altimeter reading disparities exist between various aircraft, and that I was incorrect in thinking it might be
an across the board FlightGear programming problem. Now speculate the disparities relate to older, newer, and recently updated models. Will check other aircraft when I have the opportunity.
In the meantime, I stand by my comments regarding the A6 and F14 altimeter functions as being accurate and appropriate.
Last edited by dilbert on Mon Jan 30, 2017 6:45 pm, edited 2 times in total.
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Re: Altimeter Calibration

Postby Johan G » Mon Jan 30, 2017 4:15 pm

Michat wrote in Mon Jan 30, 2017 3:38 pm:Could be great been able to set QFE on the instrument setting altimeter dialog.

You use the same knob to set either QNH or QFE. :wink:

I guess "Altimeter Setting" was a too long label to fit in the dialog, and since the most commonly used pressure is QNH anyway they used that as a label instead.
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Re: Altimeter Calibration

Postby Thorsten » Mon Jan 30, 2017 5:14 pm

The F14 registered 780 feet.


Watch the F-14b altimeter show just fine:

Image
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Re: Altimeter Calibration

Postby dilbert » Mon Jan 30, 2017 6:02 pm

I don't have a problem with anything you are showing on your f14; and, by the way, I still have an E6B, for that matter. What I do have a problem with is that my f14 and A6 altimeters indicate around 800 feet on deck at KSAN, even though they both are set at the right atmospheric pressure, which today was 30.15 inches HG, and even though the 172 and 777 altimeters read correctly. Have now checked 7 aircraft, the six tabulated above plus the A-6, with results as shown. Tired of beating a dead horse. Enough said.
Respectfully, Dilbert
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Re: Altimeter Calibration

Postby Thorsten » Mon Jan 30, 2017 6:26 pm

I don't have a problem with anything you are showing on your f14


Except that you claimed it is 750 ft off -

The F14 registered 780 feet.
The 172 registered 50 feet, which is close enough.
The 777-200 registered 30 feet, which is quite accurate.


(or how else should I take the comment that 30 ft is 'quite accurate' and the F-14 has 780 ft - did you expect that, is that also accurate?)

which the screenshot shows to be false. So why don't you _reproduce_ the settings of the screenshot and post yours with all involved properties displayed so that we can get a glimpse at what's happening rather than repeating 'it's off' and shifting your description of the problem all the time? We used to be at 'all aircraft are 900 ft off', now we're at the list of aircraft all showing differently (never mind the fact that instruments all run internally over the same code).

So, let's cover this again:

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.


Obviously whenever you change the atmosphere pressure setting on the altimeter, the indicated altitude changes, so obviously the altimeter can not do that independently of pressure setting.

If outside pressure happens to be really 29.92 and if you set the altimeter to 29.92, then it should conclude that you are at zero altitude. Whereas if you change the setting while actual pressure remains what it is, you should get to read different altitudes.

Now, note that you don't really have a means to control *actual* pressure at your location in FG short of dialing sea level pressure via the weather interface. So it's not really clear what you did to test *actual* outside pressure of 29.92 at different airports or whether you did this in any way correct.


Maybe you don't see it, but this is growing into a waste of time.
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Re: Altimeter Calibration

Postby dilbert » Mon Jan 30, 2017 6:53 pm

What I did was set the various aircraft on deck at KSAN, with the altimeter correctly set at the then existing atmospheric pressure. All the aircraft should
have indicated a pressure altitude close to the field elevation; nevertheless, they did not; and while the Boeing 777 did so accurately and the 172 did so adequately,
some of the other aircraft were way off. What else can I say? I won't waste any more of your time.
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Re: Altimeter Calibration

Postby Alant » Tue Jan 31, 2017 12:11 am

Am I correct in thinking that this problem only exists with certain aircraft, but not with others?

KSAN is more or less at sea level, so any QFE/QNH confusion error will hardly be visible on the altimeter.

If it is aircraft specific, try to contact the author or maintainer of that aircraft.

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Re: Altimeter Calibration

Postby dilbert » Tue Jan 31, 2017 12:41 am

Image
On these screen shots the F14 looks ok and when I flew it over the ocean the pressure altimeter was fairly close to the radar. The shot of the A6 looks strange-apparently the 900 represents 100 below zero rather than 900 feet above, which caused me some confusion sitting on the deck. Sorry guys.
Last edited by dilbert on Tue Jan 31, 2017 4:14 am, edited 3 times in total.
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Re: Altimeter Calibration

Postby dilbert » Tue Jan 31, 2017 2:19 am

Image
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Re: Altimeter Calibration

Postby Thorsten » Tue Jan 31, 2017 6:23 am

What I did was set the various aircraft on deck at KSAN, with the altimeter correctly set at the then existing atmospheric pressure.


And your knowledge of the existing atmosphere pressure came from where? Did you use the property browser to actually check what it is, did you assume it would be 29.92, did you tune into ATIS?

In the A6 shot, I see GUI set to 30.1 yet the in-cockpit instrument shows 92.93 - did you check which of these is considered valid by FG ?

The most likely explanation if you're seeing a large discrepancy in altimeter readings to what basically everyone else is seeing is that there's a flaw in your procedure. If you think about it, how likely is it that hundreds of flightsim enthusiasts miss the fact that altimeters are habitually 900 ft off and unusable over more than a decade?

So I guess we still have no clue what's going on (note that your shots don't show the internal properties, so we can't know what the altimeter should be showing). Can the fact that negative altitude isn't always displayed the same way on every instrument explain all? We don't know. Do you feed wrong ambient pressure to the altimeter? We don't know. What is the mysterious altitude indication on the Instrument settings dialog you referred to twice? We don't know.
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