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Altimeter Calibration

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Re: Altimeter Calibration

Postby Richard » Tue Jan 31, 2017 8:57 am

The problem is reading the A-6 altimeter. The flag section is redImage - which indicates a -ve amount; so this should be read as -10 feet (or thereabouts). I think this is the standard FG altimeter - so it maybe that the instrument needs fixing - I'm not quite sure which particular altimeter this is modelling.

I just checked the F-14 and as far as I'm concerned it's pretty much spot on; 17feet on the runway at KSFO is within the bounds of plausibility. Plus I performed a "landing on water" and it was about -1 feet. The F-14 altimeter should be as per F-14AAD-1 because I made it based on that document.
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Re: Altimeter Calibration

Postby dilbert » Tue Jan 31, 2017 10:59 am

Image
Thanks Richard. Now obvious to me, I was the problem. Speculate I was confused by a negative altimeter reading, sitting on the carrier deck at night.

Wasn't an issue landing, as I use the radar altimeter on approach.

To answer Thorsten's question, when I last tested at KSAN, I used the 2:45 Pacific time pressure as reported on the internet for KSAN, which was 30.1.

The above shot shows a negative reading for the F-14, which I probably confused as being 990 feet on my first test, with the altimeter setting in default (29.92).

Thorsten, apologize for the hassle and thank you for your forbearance and help.
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Re: Altimeter Calibration

Postby Johan G » Tue Jan 31, 2017 12:06 pm

Richard wrote in Tue Jan 31, 2017 8:57 am:The problem is reading the A-6 altimeter. [...]

dilbert wrote in Tue Jan 31, 2017 10:59 am:Thanks Richard. [...] Speculate I was confused by a negative altimeter reading [...]

Sometimes seemingly daunting problems can have a very simple cause. ;)

Good to see this puzzle solved. :)

Maybe or maybe not a screenshot of the instrument and the involved properties (shift-click on a property in the property browser to get them shown on screen) could have solved this sooner. Someone will hopefully help a user with that another time.
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Re: Altimeter Calibration

Postby Thorsten » Tue Jan 31, 2017 12:44 pm

Thorsten, apologize for the hassle and thank you for your forbearance and help.


You're welcome.
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Re: Altimeter Calibration

Postby david.megginson » Fri Feb 03, 2017 5:58 pm

Glad you all sorted this out. It's also worth noting that in real life (vs FlightGear), the altimeter will almost never actually show your altitude ASL, for a few reasons:

1. Unless the barometer is falling or rising fast, the altimeter setting used by ATC will be changed only every hour (or left for even longer periods), so you might be 20-50ft off even at ground level.
2. ATC uses altimeter settings over fairly wide areas. If you're enroute, you might be using the altimeter setting for an airport 50+ miles away, where the pressure is quite different (the goal is to keep all aircraft on the same altimeter setting in a zone, not actually to have the true altitude).
3. At altitude, the altimeter shows something close to the actual altitude only in an international standard atmosphere. In cold air, you're much lower than you think (in Canada, we're required to add 1,000 or 2,000 ft to published altitudes over mountainous regions in cold weather, and also to add to all the altitudes in instrument approaches).
4. At FL180 and above (with some regional variations), you set the altimeter to 29.92 inHg regardless of conditions on the ground. Not that my Warrior ever goes that high. :)

Add all these together, and it's perfectly normal for the altimeter in my PA-28 to read 8,000 ft enroute using the altimeter setting ATC gave me, while my WAAS GPS might tell me that I'm actually at 7,600 ft (in the winter) or 8,400 ft (in the summer).
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Re: Altimeter Calibration

Postby Robertfm » Tue Sep 10, 2019 11:50 am

Came across this thread and would just ask if their is a conversion chart that actually shows both HG & I assume Ft. The Meta Weather shows something like 1015, which appears on altitude meter. So why isn't it in pressure and what is 1015, is it feet and why doesn't Meta show it as HG. And yes I have looked this up and can't find anything that explains this. Lots of Conversion Charts.
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Re: Altimeter Calibration

Postby tdammers » Tue Sep 10, 2019 12:29 pm

The most commonly used scales for QNH are hPa (hectopascal) and inHg (inches of mercury). A quick web search pops up this: https://www.convertunits.com/from/hpa/to/inhg, which seems to work well enough. Some aircraft models (especially airliners) will also allow you to switch between hPa and inHg, so you can enter it in either format. METAR will show whichever unit is preferred at the location in question; in Europe, this will usually be hPa, while in the US, you will generally get inHg. 1015 is hPa, not ft; the fact that 1015 is also a valid altitude in feet is a mere coincidence.

So it actually *is* pressure - the pressure at sea level, at a given location.
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Re: Altimeter Calibration

Postby Alant » Tue Sep 10, 2019 1:09 pm

Pressure is also given in Atmospheres. This unit has the same value as the hPa.
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Re: Altimeter Calibration

Postby Robertfm » Tue Sep 10, 2019 1:30 pm

Thank you a lot clearer than other explanations I've read.
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