Local altimeter setting & station temperature

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MrManH

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MrManH
Hey everyone,

Something I always wondered is if the altimeter setting we get in the US from the ATIS (or any AWOS/ASOS for that matter) is corrected for temperature at the field? The FAA Pilot handbook says "Altimeter setting is defined as station pressure reduced to sea level" so I'm inclined to think it's not corrected for temperature.

The first time I flew in Europe I was surprised to hear they had different kinds of altimeter settings, one called QNH that explicitly corrects for temperature. So basically if you set a 100% accurate altimeter to the QNH, when on the ground your altimeter would indicate true altitude which would equal field elevation.

Thanks!
 
I looked the European wikipedia and came across a French page that says the QNH corrects for temperature.

So I don't know which page is right but I'm asking about the US anyway. Am I correct in thinking that our altimeter setting does not correct for non standard temperature?
 
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Thanks for confirming my thoughts. Would it be right to say that the altimeter setting corrects for non-standard pressure at the station elevation?
 
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MrManH said:
Hey everyone,

Something I always wondered is if the altimeter setting we get in the US from the ATIS (or any AWOS/ASOS for that matter) is corrected for temperature at the field? The FAA Pilot handbook says "Altimeter setting is defined as station pressure reduced to sea level" so I'm inclined to think it's not corrected for temperature.

The first time I flew in Europe I was surprised to hear they had different kinds of altimeter settings, one called QNH that explicitly corrects for temperature. So basically if you set a 100% accurate altimeter to the QNH, when on the ground your altimeter would indicate true altitude which would equal field elevation.

Thanks!
Click to expand...

You don't want it corrected for temperature, or you could crash!
The idea is that both altimeters, the one in the plane and the one in the tower, should read the same once you land. So you don't want any extra "compensations", just a calibrated unit both in the plane and in the tower.
 
If your altimeter reads true altitude at field elevation, how would that be dangerous?
 
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They take the station pressure at the airport with their barometer. They correct the station pressure to sea level pressure (the pressure at sea level as if there is a hole dug to sea level at the airport and the barometer dropped down the hole). Then the airplanes flying around above the airport that don't know their altitude can put that sea level pressure into THEIR altimeter. The plane knows the station pressure through its static port and PRESTO! The planes altimeter reads the correct altitude!! Note that sea level pressure doesn't change with altitude. It does change with weather differences.

There three things
1. Station pressure
2. Sea level pressure
3. Altitude (or elevation, same thing)
Know any two, you can calculate the third. Only ATC knows how to get from station pressure to sea level pressure though. The airplane knows its station pressure and sea level pressure comes from ATC and the airplane's altimeter gives the correct altitude.

Temperature affects air density. Density and pressure are not the same thing. A given column of air has so much pressure. Temperature goes up, the column of air can have the same pressure (weighs the same) but its density decreases. The top of the air has gone up higher into the troposphere. The air has expanded due to increase in temperature. Higher temperature, less density, same pressure. It is density altitude that changes with temperature. That's a different creature.

Hope that helps.
 
Yeah this page details the formula
http://www.srh.noaa.gov/images/epz/wxcalc/altimeterSetting.pdf

When going through private pilot training, I wrote down what I think is an oversimplification that the altimeter setting is the setting at which the altimeter will read field elevation. In real life, whenever I've gotten an altimeter setting it's always been within 40ft of the field elevation. What I'm trying to find out is under what conditions would the altimeter read the exact field elevation assuming the altimeter doesn't have any errors.

Would this be correct:
Altimeter Setting + Standard Temperature = True altitude while on the ground at the station where the altimeter setting was calculated.
 
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The simple manual altimeter setting equipment is two certified altimeters on a vibration table. One knows the altitude of the two altimeters by the local survey. Turn on the vibration table and adjust both altimeters to read field elevation. Read the two Kollsman window values and average them. Usually they are identical. Temperature is not directly taken into account, but effectively it is since the altimeter setting is determined at the ambient temperature and pressure. One could say that the altimeter setting is corrected for field elevation and an altimeter in an aircraft should read the correct value of field elevation when the altimeter setting matches. As one climbs in altitude over the field elevation, the altimeter will progressively increase the amount of error introduced by the temperature difference from standard temperature. On a cold day the altimeter will read higher than the MSL altitude and a warm day the opposite. Fortunately, when descending on an approach to the DA or MDA, the height above the field elevation, the error will generally be small. On an extremely cold day, cold weather altimetry correction should be applied and the FAA has recently published a NOTAM on the topic.

Automated equipment sense the pressure and corrects it based on a standard atmosphere to determine the altimeter setting. The value of the indicated altitude is known.
 
John I fully agree with the 2nd half of your paragraph, all bets are off once off the ground unless we were in a truly standard atmosphere. So are you saying that if I had a perfect altimeter, the altimeter setting would in fact read true altitude/field elevation while on the ground?
 
MrManH said:
John I fully agree with the 2nd half of your paragraph, all bets are off once off the ground unless we were in a truly standard atmosphere. So are you saying that if I had a perfect altimeter, the altimeter setting would in fact read true altitude/field elevation while on the ground?
Click to expand...

I am not John, but that's absolutely correct.
 
MrManH said:
John I fully agree with the 2nd half of your paragraph, all bets are off once off the ground unless we were in a truly standard atmosphere. So are you saying that if I had a perfect altimeter, the altimeter setting would in fact read true altitude/field elevation while on the ground?
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It should match the field elevation. Your GPS altitude should also match when on the ground. When airborne, the ships altimeter should match other altimeters in other aircraft that are using the same altimeter setting. The GPS altitude will read very close to what the altimeter would read with the correct altimeter setting when the atmosphere had a standard temperature and pressure lapse. It will often differ by quite a large value on warmer or colder days. During the next three months it will generally read above the altimeter reading and at higher altitudes it could easily differ by a good part of a 1000 feet. I did a recent test flight and at 10000 feet, the GPS altitude was almost +800. When on the ground, I corrected for temperature and pressure and had agreement within 60 feet.
 
Since I don't have to worry about rising terrain in the Midwest I've rarely even calculated by true altitude in flight, however are you saying that GPS altitude is a pretty good indication of true altitude in flight?
 
MrManH said:
Yeah this page details the formula
http://www.srh.noaa.gov/images/epz/wxcalc/altimeterSetting.pdf

When going through private pilot training, I wrote down what I think is an oversimplification that the altimeter setting is the setting at which the altimeter will read field elevation. In real life, whenever I've gotten an altimeter setting it's always been within 40ft of the field elevation. What I'm trying to find out is under what conditions would the altimeter read the exact field elevation assuming the altimeter doesn't have any errors.

Would this be correct:
Altimeter Setting + Standard Temperature = True altitude while on the ground at the station where the altimeter setting was calculated.
Click to expand...

How do you know your altimeter is correct? It can be up to 75 feet off and still pass inspection.
 
Steven8385 said:
How do you know your altimeter is correct? It can be up to 75 feet off and still pass inspection.
Click to expand...

Most airports have "known" locations that are surveyed to within a foot or so of correct elevation.. Taxi on top of them, add 3 feet to match your panel height and see what your altimeter says based on the ATIS altimeter setting....

Easy Pleasy...
 
MrManH said:
Since I don't have to worry about rising terrain in the Midwest I've rarely even calculated by true altitude in flight, however are you saying that GPS altitude is a pretty good indication of true altitude in flight?
Click to expand...

It is, this is one reason why terrain warning systems prefer to use GPS altitude as they are not affected by altimeter setting or temperature.
 
John, I don't know when you'd use that method to determine altimeter setting. Here's a screenshot of the Allweather AWOS 3000 maintenance manual showing how to field check the altimeter setting. It's just math.
 

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Old Geek said:
John, I don't know when you'd use that method to determine altimeter setting. Here's a screenshot of the Allweather AWOS 3000 maintenance manual showing how to field check the altimeter setting. It's just math.
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In the olden days when you were a young geek. Before the ASOS was installed at our airport, that is the equipment we used.
 
And bear in mind the published field elevation is the highest point on a runway, and no airport is perfectly flat, so unless you are at that highest point, your altimeter should read below field elevation (if you could read it with that much resolution).

And if you want to get even nit-pickier, your static port is not on the ground either, so if you are sitting on the point that coincides with the published field elevation, your altimeter should read a few feet high.

But again, there is insufficient resolution in the Kollsman window or the altimter dial to see these differences. Don't overthink it.

(My observations are based on steam gauges; maybe some glass panel guys have had different experience)
 
Haha Van Johnston yeah I fly on steam gauges as well but I've started using Foreflight with the Stratus 2 ADSB receiver about a year ago. I just wanted to nit pick enough to better understand the relationship between temperature, altimeter setting and altimeter reading on the ground. Thanks everyone for the help, that helped clarify something that was a little blurry dating back to private pilot training.
 
MrManH said:
Since I don't have to worry about rising terrain in the Midwest I've rarely even calculated by true altitude in flight, however are you saying that GPS altitude is a pretty good indication of true altitude in flight?
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I would abide by AIM 1-1-18(a)(4) and ignore GPS altitude.

Bob Gardner
 
bobmrg said:
I would abide by AIM 1-1-18(a)(4) and ignore GPS altitude.

Bob Gardner
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GPS is not magic. It has measurement errors like anything else. They are generally LARGER than altimeter error.

Just today, I measured GPS altitude near the 28R threshold at Oakland. It's surveyed at 6 MSL, but the GPS said I was swimming in the Bay at -30.

GPS presumes the earth is exactly ellipsoidal. That's not good enough for altimetry.

WAAS can improve that somewhat, but it's still not good enough, and your altimeter tells you where your precision instrument approach terminates.
 
bobmrg said:
I would abide by AIM 1-1-18(a)(4) and ignore GPS altitude.

Bob Gardner
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I don't agree with the AIM on that point. WAAS corrected GPS altitude has a 95% accuracy around 3 meters and unaided is close to 5 meters. That compares with Baro altitude that at 8000 feet can vary by up to 1000 feet from the true altitude.

Although not usable for vertical separation with other aircraft, it is much more accurate for terrain or obstacle separation.
 
MrManH said:
The first time I flew in Europe I was surprised to hear they had different kinds of altimeter settings, one called QNH that explicitly corrects for temperature. So basically if you set a 100% accurate altimeter to the QNH, when on the ground your altimeter would indicate true altitude which would equal field elevation.
Click to expand...

QNH does NOT take temperature into account. In the ICAO system it is the same as in the FAA system. Its the altimeter setting given in Hectopascals/millibars. So when descending through the transition level, you set the local QNH(altimeter setting). Climbing through the transition altitude(which is 18,000 in the US) you set 29.92. In ICAO that is the QNE(1013 or 2992). If you are given a QFE, your altimeter will read zero when you are on the ground.
 
MAKG1 said:
GPS is not magic. It has measurement errors like anything else. They are generally LARGER than altimeter error.

Just today, I measured GPS altitude near the 28R threshold at Oakland. It's surveyed at 6 MSL, but the GPS said I was swimming in the Bay at -30.

GPS presumes the earth is exactly ellipsoidal. That's not good enough for altimetry.

WAAS can improve that somewhat, but it's still not good enough, and your altimeter tells you where your precision instrument approach terminates.
Click to expand...

Sure the GPS has its own error budget, but it is independent of temperature. At a SL airport, the barometric altimeter should have a 20 foot scale error tolerance at 29.92 and the barometric scale error is allowed up to a 25 foot variance. Pilots normally consider a 75 foot variation from field elevation as a go-no-go value. At the airport elevation, the effect of temperature has already been accounted for in the barometer setting. As you go up in altitude, the variance due to uncompensated temperature swamps the other errors on days when the temperature is non standard. So that same altimeter that is exactly matching the airport elevation of 6 MSL and 30 C (86 F) will indicate 407 feet below the true altitude at an indicated 8000 feet. The GPS will have the same error budget, so if it matched the error you saw at field elevation, it would read 8407 - 37 feet = 8370 and be much closer to the true altitude.

It is true that the GPS is a calculation that uses a spheroid such as the WGS84 spheroid and that the earth is not a true spheroid. Over the continental US this error is on the order of -100 feet (-105 feet at Oakland Airport). When you read the GPS altitude from your GNS430W, it is corrected by a geoid database to account for local variations in gravity, so that the height it displays will match a true height above sea level.

Although the GPS altitude displayed by a GPS is corrected from WGS84 to a sea level value, the value broadcast by an ADS-B Out device is called Geometric Altitude because it is strictly the WGS84 value without any correction.

I indicated in my other post the 95% error values measured using the standard GPS position service and they are well within the altimetry limits of a barometric altimeter, and are generally much closer to the true altitude. WAAS is even better. But GPS altitude, in spite of its accuracy, is not suitable for vertical separation from other aircraft which depends not on a true altitude, but a pressure value.
 
So right now, the altimeter setting is 29.87 at KPAE, with a field elevation of 604' MSL. The outside air temperature is 19 deg C. That gives us a density altitude at field elevation of 1,257' MSL by my handy ea6b. So, semantics aside: pressure altitude is indicated altitude corrected for non standard barometric pressure and density altitude is pressure altitude corrected for non-standard temperature. GPS altitude is probably the most precise indication of true altitude available to us normal pilots. And, each of these measures of altitude have specific uses and one probably should not be substituted for the other without proper due diligence - right?
 
wilkersk said:
So right now, the altimeter setting is 29.87 at KPAE, with a field elevation of 604' MSL. The outside air temperature is 19 deg C. That gives us a density altitude at field elevation of 1,257' MSL by my handy ea6b. So, semantics aside: pressure altitude is indicated altitude corrected for non standard barometric pressure and density altitude is pressure altitude corrected for non-standard temperature. GPS altitude is probably the most precise indication of true altitude available to us normal pilots. And, each of these measures of altitude have specific uses and one probably should not be substituted for the other without proper due diligence - right?
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Yes. In general, WAAS-GPS is reliable and accurate when it comes to terrain avoidance, while the old pressure altimeter is most reliable for traffic separation. This is because even though the pressure altimeter can be grossly inaccurate (relative to true AGL height) at altitude on a non-standard temperature day, all traffic at your level will use the same kind altimeter with the same error, keeping everyone safe if they follow their assigned altitudes (or flight levels) and use the same altimeter setting.
 
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I distinctly remember seeing an altimeter in the "panel" at the old KBJC tower cab in the late 1980s and watching a controller record the ATIS after adjusting it to read field elevation (or field elevation plus tower cab height, whatever the pointy arrows someone had painted on the edge of the face were).

No double altimeter, no vibration table.
Were there altimeters made for tower cabs that didn't have the internal friction problems that mandate the use of a vibration device to make sure they weren't stuck during small pressure changes?

I'm sure the fancy new tower has something automated and digital. This was a long time ago...
 
The altimeter setting is corrected for non-standard temperature UP TO field elevation.

It can not correct for non-standard temperature ABOVE field elevation because it doesn't know how high above field elevation you are.
 
Larry in TN said:
The altimeter setting is corrected for non-standard temperature UP TO field elevation.
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Nope. Somewhere in my training material I have a chart of all the interactions between AWOS sensors and the AWOS reports. Altimeter setting only uses barometric pressure and the sensor elevation.

denverpilot said:
I'm sure the fancy new tower has something automated and digital. This was a long time ago...
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Allweather AWOS uses a pair of expensive and very accurate Honeywell pressure transducers. They are compared in software and if they closely agree, the one reporting the equivalent lower altitude is used. If they don't agree, the altimeter setting goes missing until someone can fix it.
 
Old Geek said:
Nope. Somewhere in my training material I have a chart of all the interactions between AWOS sensors and the AWOS reports. Altimeter setting only uses barometric pressure and the sensor elevation.
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If it didn't correct for non-standard temperature up to field elevation then the reading while on the ground at the reporting station would be wrong.

The AWOS doesn't need a temperature input to make the correction because it knows the sensor (field) elevation. That builds the correction for non-standard temperature into it's calculation of altimeter setting (QNH).

The reason why it is important to understand that the QNH includes the correction for non-standard temperature UP TO field elevation is so that you'll understand that you must correct for non-standard temperature between field elevation and your altitude. IOW, when you go into the cold weather altitude correction chart you use your height above the reporting station, not your height above sea-level. The altimeter setting has already corrected for non-standard temperature UP TO field elevation so all you have to do is apply the correction from field elevation up to your altitude.
 
Thanks Scottd, bookmarked and will watch shortly!
 
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