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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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Re: Altimeter Calibration

Postby Robertfm » Wed Sep 18, 2019 3:28 pm

I've noticed several planes including the Piper PA28 and the Lioneau actually don't have either hPa (hectopascal) and inHg (inches of mercury on them. I have a chart that converts to +- feet and this can be set when plane is on the ground but how can you do it when flying and pressure changes.
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Re: Altimeter Calibration

Postby wkitty42 » Wed Sep 18, 2019 3:55 pm

once you get above a certain alt you use the standard 29.92 (i think that's the one)... you then adjust back to live pressure when you are inbound to your destination...
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Re: Altimeter Calibration

Postby tdammers » Wed Sep 18, 2019 4:21 pm

Robertfm wrote in Wed Sep 18, 2019 3:28 pm:I've noticed several planes including the Piper PA28 and the Lioneau actually don't have either hPa (hectopascal) and inHg (inches of mercury on them. I have a chart that converts to +- feet and this can be set when plane is on the ground but how can you do it when flying and pressure changes.


You should never have to convert to or from feet; if you do, then you're doing it wrong. Altimeter setting in any reasonably modern aircraft is going to be either in hPa or inHg, or there will be a switch selecting either. The PA-28 definitely has a QNH knob, though it may be hard to read in FG due to limited screen and texture resolutions. There may be a tooltip though that tells you the current setting when you hover over the knob.

So here's what you would normally do:

On the ground, you get your ATIS information on the radio. This will give you a QNH, either in hPa, or in inHg. If it's the wrong unit, you grab a conversion table from your glareshield (or wherever you have it) and convert it. Then you dial that number into the altimeter's QNH setting. If the ATIS says "QNH 1020", then that's what you dial into the altimeter. You may then want to double check the altitude displayed on the altimeter against the known airfield elevation; it should be within a few tens of feet. E.g., if you know the airfield is at 150 ft above sea level, and you have dialed in your QNH, then the altimeter should read something between 140 and 160 feet or so. You can also do it the other way around: turn the QNH knob until the altimeter matches the airfield elevation, then verify that the QNH setting is the same as what the ATIS said.

Once airborne, you don't need to do anything until one of the following happens:

a) You climb above transition altitude (18,000 ft in the continental US, often considerably lower elsewhere, e.g. 5000 ft in many parts of Europe); at this point, you switch to "standard" QNH (1013 / 29.92), and you leave it there until you descend below transition level again.

b) You cross into an airspace for which a different ATIS is available, or there is a significant weather change. In this case, you would get the new relevant ATIS information, and dial the new QNH in (again converting between hPa and inHg as needed).

c) You descend below transition level. This happens a good while after planning your descent, so you have plenty of time beforehand to get the ATIS for your destination, do conversions as necessary, and write down what you need to enter when you cross the transition level.

d) ATC informs you of a changed QNH for your current position. The most up-to-date QNH is often part of your approach clearance, but should rarely come as a surprise at this point - including it in the clearance is more of a sanity check than anything.

Again, you should never need to convert between QNH (whether hPa or inHg) and feet, there simply isn't a use case.
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Re: Altimeter Calibration

Postby Robertfm » Wed Sep 18, 2019 4:47 pm

Well I am currently flying the Lioneau and have checked the knob actually moves the big needle in either direction. I am sure the Piper is the same. As for converting this is done with a chart which actually shows pressure in various ways its called SensorsOne Altitude to Pressure Conversion table. As for the rest of your post, I know when to change pressures, according to Real Life Training material it's once every 100nm or so, Ie New ATIS. I use the Live weather meta data which shows changes regularly on FG. I concede the real planes may have the calibrations but we are talking this Sim.
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Re: Altimeter Calibration

Postby Thorsten » Wed Sep 18, 2019 6:17 pm

I've noticed several planes including the Piper PA28 and the Lioneau actually don't have either hPa (hectopascal) and inHg (inches of mercury on them.


Then you use the dialog to set air pressure as the instrument isn't properly modeled for that plane (looking at a real PA-28 cockpit, I see the hPa window in the altimeter just fine).

Unless you have a GPS, you can't set this correctly in the air without this pressure scale for the simple reason that you have no independent measurement of your actual altitude.

With a GPS altitude, you can set your altimeter to the GPS altitude and it will automatically be at the correct pressure setting.
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Re: Altimeter Calibration

Postby Robertfm » Wed Sep 18, 2019 6:39 pm

Sorry don't understand 'you use the dialog' . The GPS under Equipment tab doesn't have pressure setting. Doesn't the HUD show both AGL and varying terrain altitude
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