Cold Wx Altimeter Correction???

[FL420]

There is some useful information here 3.1-Barometric Altimeter and Radio Altimeter

http://www.flightsafety.org/alar_resources.html

This is a great resource. Thanks for the link.

 

[FL420]

It's not a QNE/QNH transition level issue. It has to do with the possiblility that an airplane with the correct (for the pressure) QNH might hit trees on approach because the altimiter was not adjusted for extremely low temps.

In CONUS, I agree. But not everyone on this board flies only in CONUS. In Europe or South America where local TL's are as low as FL030, it could be a problem. The upper atmosphere is more homogeneous than down low where you can get wide deviations from ISA with the passage of warm and cold fronts.

 

[FoxHunter]

Just to add to the discussion there is also altimeter error caused by high winds.​

http://dcaa.slv.dk:8000/icaodocs/Doc%208168/ Text starts on page 242, tables pg 245



Part
​

111 ​

- Section I. Chapter 4 111-1-4-5​

4.5​

MOUNTAINOUS TERRAIN ​

- TERMINAL AREAS​

4.5.1 The combination of strong winds and mountainous terrain can cause local changes in atmospheric pressure
due to the Bernoulli effect. This occurs particularly when the wind direction is across mountain crests or ridges. It is not
possible to make an exact calculation, but theoretical studies

​

(CFD Norway, Report 109.1989) have indicated altimeter
errors as shown in Tables 111-1-4-4 and 111-1-4-5. Although States may provide guidance, it is up to the pilot-incommand
to evaluate whether the combination of terrain, wind strength and direction are such as to make a correction
for wind necessary.
4.5.2 Corrections for wind speed should be applied in addition to the standard corrections for pressure and
temperature, and ATC should be advised.



​

 

[paulsalem]

Already required by the military. Often a significant factor up at RDR and MIB.

Airman's Information Manual, paragraph 7-2
http://www.faa.gov/ATPubs/AIM/Chap7/aim0702.html

Just scroll all the way to the bottom.

I don't understand why the table asks for hight above airport, instead of MSL altitude. Could someone explain this?

I got into a discussion about the cold weather altimeter errors today with a CFII student of mine. He says that the altimeter setting you get from ATC or AWOS/ASOS is corrected for non standard tempature, so that when you're flying an apporach you don't hit somthing, and that is why the table in the AIM refers to altitudes above the airport, not MSL.

Thanks

 

[A Squared]

I don't understand why the table asks for hight above airport, instead of MSL altitude. Could someone explain this?

I got into a discussion about the cold weather altimeter errors today with a CFII student of mine. He says that the altimeter setting you get from ATC or AWOS/ASOS is corrected for non standard tempature, so that when you're flying an apporach you don't hit somthing, and that is why the table in the AIM refers to altitudes above the airport, not MSL.

Thanks

It's because the amount you are off due to cold temps is a function of how high you are above the reporting station (usually the airport, but not always), not above sea level. This ties into what your student said. Yes the altimeter setting is corrected for non-standard pressure and temp *at the station*. but that is only meaninful at the station.

When an altimeter setting is computed, whatever the pressure and temp is at the station, the setting is figured so that an altimeter adjusted to that setting will read the correct elevation *at the station*. If you're above the station, at non-standard temps, your altimeter will read incorrectly, even if though the correct setting is used. The reason for this is that the inner workings of the altimeter are constructed so that it "calculates" altitude from pressure based on a standard atmosphere. In a standard altitude a 200' difference in altitude is approximately a pressure difference of 0.2 inches of mercury. But on a very cold day, there might be 0.2 in.hg difference every 150 feet of altitude. SO a correctly set altimeter, if it senses a pressure that is 0.2 in.hg.less that the pressure at the reporting station, it will display an altitude 200' above the station, when in reality you are only 150 feet above it. As you get closer to the altitude od the station the error decreases, until when you are on the ground, the altimter reads correctly.

So, your student is right, but not really. the altimeter setting corrects for non-standard pressure and temp *at the altitude of the observing equipment*, but the higher you are above that, the more your altimeter will be off from true altitude.

 

[paulsalem]

A squared, thanks for the info. Do you know of a document that states this?

I had no clue that the altimiter setting was corrected for temp.

 

[Donsa320]

A squared, thanks for the info. Do you know of a document that states this?

I had no clue that the altimiter setting was corrected for temp.

I do not think there is any correction for temperature. And the altimeter does not "calculate" anything. It is a simple clockwork mechanism that changes the hands by 1,000 ft for every 1 inch of Hg pressure change. Every time. There is a bi-metallic yoke in the case that does bias the reading but its purpose is to compensate for the local temperature right there in the panel. How much bias....no one seems to know. I asked that question at a Terps meeting 40 years ago. I still do not have the answer.

DC

 

[paulsalem]

I think he and Asquared are right.

http://www.nwstc.noaa.gov/DATAACQ/d.ALGOR/d.PRES/PRESalgoProcess.html#

Check out number three.

They use tempature in the formula.

 

[Donsa320]

I think he and Asquared are right.

http://www.nwstc.noaa.gov/DATAACQ/d.ALGOR/d.PRES/PRESalgoProcess.html#

Check out number three.

They use tempature in the formula.

Aren't you confusing Station Pressure with altimeter setting. In any event the transmitted altimeter setting figure will cause your altimeter to read the field elevation when that is where it is on the airport and nothing is corrected for the non-standard atmosphere above, if it is non-standard. IMHO.

DC

 

[sky37d]

Thanks,
With winter firmly upon us, this is very useful information.

 

[paulsalem]

I sent an email to the NOAA ASOS people asking the same question.

I'll post once I get a response.

What company makes the ASOS?

 

[A Squared]

And the altimeter does not "calculate" anything.

Well, actually, yes it does. It measures pressure and computes and displays altitude (actually, an estimate of altitude) through it’s mechanism. An altimeter is in fact an analog mechanical computer, although that may be a little abstract for some, but it is true nonetheless.

It is a simple clockwork mechanism that changes the hands by 1,000 ft for every 1 inch of Hg pressure change. Every time.

You are correct that it is a relatively simple clockwork mechanism, you are absolutely incorrect that it changes the hands 1000 ft for every 1 in. Hg pressure change. Pull out a chart of the IACO standard atmosphere. At sea level, 1000 ft is 1.06 in Hg. The difference between 19,000 ft and 20,000 ft. is 0.59 in. Hg. That is a *LONG* way from 1 inch each and every thousand feet. Even in the idealized standard atmosphere, pressure is a non-linear function of altitude

I had no clue that the altimiter setting was corrected for temp.

I should probably clarify what I meant by altimeter settings being corrected for temperature. True, they don’t take a pressure reading then apply a factor to it based on temp, however an altimeter setting, by it’s nature takes into account the effects of non-standard temp. Atmospheric pressure ant any point will be affected by the pressure patterns ant temperature, and possibly other factors. So if you compute an altimeter setting for a location based on the measured atmospheric pressure at that location, the altimeter setting will account for any factor which affects the pressure, including the temperature.

I think he and Asquared are right.

http://www.nwstc.noaa.gov/DATAACQ/d.ALGOR/d.PRES/PRESalgoProcess.html#

Check out number three.

They use tempature in the formula.

What they are doing in step three is adjusting for the altitude difference between the pressure sensor and surface of the field, and that altitude correction includes a term for temp.

 

[afres bum]

just another site with info on the subject....


http://math.isu.edu/~wolperj/cold.html

 

[Donsa320]

Well, actually, yes it does. It measures pressure and computes and displays altitude (actually, an estimate of altitude) through it’s mechanism. An altimeter is in fact an analog mechanical computer, although that may be a little abstract for some, but it is true nonetheless.


You are correct that it is a relatively simple clockwork mechanism, you are absolutely incorrect that it changes the hands 1000 ft for every 1 in. Hg pressure change. Pull out a chart of the IACO standard atmosphere. At sea level, 1000 ft is 1.06 in Hg. The difference between 19,000 ft and 20,000 ft. is 0.59 in. Hg. That is a *LONG* way from 1 inch each and every thousand feet. Even in the idealized standard atmosphere, pressure is a non-linear function of altitude



I should probably clarify what I meant by altimeter settings being corrected for temperature. True, they don’t take a pressure reading then apply a factor to it based on temp, however an altimeter setting, by it’s nature takes into account the effects of non-standard temp. Atmospheric pressure ant any point will be affected by the pressure patterns ant temperature, and possibly other factors. So if you compute an altimeter setting for a location based on the measured atmospheric pressure at that location, the altimeter setting will account for any factor which affects the pressure, including the temperature.




What they are doing in step three is adjusting for the altitude difference between the pressure sensor and surface of the field, and that altitude correction includes a term for temp.

I hate to argue with you, but using the anticeptic language that you do, you are correct in a way but: a mechanical barometric altimeter does not account for any non-linear pressure or temperature lapse rate as far as I know.
The electric altimeters that get a signal from the air data computers are corrected for some errors but if you compare them with the mechanical standby altimeter while in flight at say M.80 at FL350 you see only a couple of hundred feet difference between them. One's actual altitude may not be anywhere near 35,000ft even if the local altimeter on the ground below is 29.92". One would have to consult the relevant constant pressure chart to see at what height above the standard datum plane the equivelant pressure level is. Who does that anymore?

But our discussion here I thought was for appoach error.

The FAA does not expect us to make the cold temperature correction. Should we? I don't know. If you look at high, cold airport IAP charts for say Jackson Hole WY, and look out at the procedure turn area where the Required Obstruction Clearance is 1,000 ft I think you will find that the actual clearance is more like 2,000 ft. There is nothing in TERPS that I can find that would cause the procedure specialist who designed the procedure to build in that significant extra margin but they seem to be doing it.

It is time that the Feds clarify these things IMHO.

Best, DC

 

[Lead Sled]

This is a great thread. You guys who are flying with FMSes need to be aware that some of them can correct for low temperatures. About the only time I've ever used the feature was up in Canada and Alaska. If you've got questions, get out your manuals and do a little research.

'Sled