Cold Wx Altimeter Correction???

[paulsalem]

An update from NOAA

"Thank you for contacting the National Weather Service. I am forwarding your question to our aviation service expert, Michael Graf. You should hear back from Mike in a few days."

 

[A Squared]

I hate to argue with you,

THat's OK, I like to argue, I don't take it personally, I hope you don't either.

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.

I agree that they don't account for non-standard temp lapse rates. I didn't intend to imply that. They do however account for the non-linear pressure lapse rate. If they didn't, the altitudes would be so far off as to be useless. If, as you say, an altimeter merely displayed a change of 1000 ft of altitude for each change of 1"Hg in pressure, that altimter would display 16,000 ft MSL when you climbed to a pressure of 13.93 "Hg. The problem comes when you consider that 13.92"Hg is found at 19,694 ft MSL in the ISA. So, do you really think that all altimeters have a built in 3000 ft error in the mid-teens on a standard day? That if your altimeter reads 16,000 ft, you're really at almost 20,000 ft?

From personal experience, I have flown around Denali (summit elevation 20320' MSL) with my (mechanical) altimeter reading 15,000 ft. I can assure you, I was not 2000 ft from the summit, which I would have been if altimeters merely displayed altitude as a linear function of pressure at 1000' per inch of mercury.

Unless you are willing to beleive that a mechanical altimeter is in error by 3500 ft at 16,000 on a standard day (and I do not) you would have to agree that mecnhanical altimeters do in fact correct for the non-linear pressure lapse rate of the ISA. How exactly this is accomplished, I don't know. That would probably be a question for an instrument technician. If I had to guess, I'd guess that the expansion of the aneriod is a logarithmic function of pressure, as many natural phenonenon are logarithmic. The relationship of altitude and pressure, for example, is logarithmic. If my guess is correct, that would simplify the necessary mechanism for displaying the altitude.

But our discussion here I thought was for appoach error.

It is, but somtimes discussions wander into other related topics. That is thier nature.

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

Agree with all that. I'm fairly up on the TERPS and I don't know of anything which would require the design of an appraoch to include any more than 1000' clearence in mountainous terrain. (enroute, of course requires 2000') It is my undestanding that in CAnada and other places, a temperature correction for an MDA is required, here in the US it's usually not even mentioned.

 

[Donsa320]

THat's OK, I like to argue, I don't take it personally, I hope you don't either.




I agree that they don't account for non-standard temp lapse rates. I didn't intend to imply that. They do however account for the non-linear pressure lapse rate. If they didn't, the altitudes would be so far off as to be useless. If, as you say, an altimeter merely displayed a change of 1000 ft of altitude for each change of 1"Hg in pressure, that altimter would display 16,000 ft MSL when you climbed to a pressure of 13.93 "Hg. The problem comes when you consider that 13.92"Hg is found at 19,694 ft MSL in the ISA. So, do you really think that all altimeters have a built in 3000 ft error in the mid-teens on a standard day? That if your altimeter reads 16,000 ft, you're really at almost 20,000 ft?

From personal experience, I have flown around Denali (summit elevation 20320' MSL) with my (mechanical) altimeter reading 15,000 ft. I can assure you, I was not 2000 ft from the summit, which I would have been if altimeters merely displayed altitude as a linear function of pressure at 1000' per inch of mercury.

Unless you are willing to beleive that a mechanical altimeter is in error by 3500 ft at 16,000 on a standard day (and I do not) you would have to agree that mecnhanical altimeters do in fact correct for the non-linear pressure lapse rate of the ISA. How exactly this is accomplished, I don't know. That would probably be a question for an instrument technician. If I had to guess, I'd guess that the expansion of the aneriod is a logarithmic function of pressure, as many natural phenonenon are logarithmic. The relationship of altitude and pressure, for example, is logarithmic. If my guess is correct, that would simplify the necessary mechanism for displaying the altitude.



It is, but somtimes discussions wander into other related topics. That is thier nature.



Agree with all that. I'm fairly up on the TERPS and I don't know of anything which would require the design of an appraoch to include any more than 1000' clearence in mountainous terrain. (enroute, of course requires 2000') It is my undestanding that in CAnada and other places, a temperature correction for an MDA is required, here in the US it's usually not even mentioned.

I see your point but in looking at a simple baro altimeter I'd be surprised if it were as you say. To get two aneroids to make the same nonlinear correction let alone thousands of them stretches my imagination.

We have an instrument repair shop here at MKE. I will make a point of stopping in and seeing what I can learn.

The next step might be finding the technical order for altimeters and see what it might say. I was taught years ago in the USAF that it was simple clockwork. Stay tuned.

DC

 

[paulsalem]

FROM NOAA

Paul:

The answer to your question is yes the ASOS does calculating the
temperature into the barometric pressure setting announced on the
automated broadcast system.

The NWS ASOS is a digital barometer. The sensor internally calculates
sensor pressure and altimeter setting. The altimeter setting is
calculate for ASOS using the available five-minute temperature TR in an
exact algorithm.

From an aircraft stand-point. These days most aircraft are still using
aneroid wafer altimeters that are sensitive to change in temperature
during flight. The phrase "High to Low, Look out Below" is generally
applied moving from a high pressure system to a low pressure system, but
also can applied moving from warmer air to colder air. The altimeters in
most aircraft do not compensate for this issue, hence enroute it is up
to the pilot to determine the error. The Aeronautical Information
Manual (AIM) provides an ICAO Cold Temperature Error Table in section
7-2-3 Altimeter Errors to assist pilots in calculating this error in
flight.

Enroute, my experiences have lead me to conclude unless operating at low
absolute altitudes (true distance above the terrain) or in extreme
temperature changes the error to the altimeter is minor.

To error on the side of safety, if conditions are deteriorated enough,
that a pilot is depending on his altimeter for altitude above the ground
on landing, I would recommend consulting the ICAO Cold Temperature Error
Table prior to attempting the landing. In most cases, pilots in
deteriorated conditions are operating low to the ground using IFR
approaches. Adjustments to altimeter readings in IFR approaches would need to be made to Precision Approach Decision Heights and Non-precision approach MDAs. These are the two locations where pilots are operating
close to the ground. The problem with temperature errors is they cause
a ripple effect throughout the entire approach phase. It is in the best
interest of the pilot to consult the ICAO Cold Temperature Error Table,
if they are unsure if the barometric pressure on the field included
temperature in its calculations.

We hope this answers your question.

(Lisa Glikbarg)
Aviation Weather Services Program Staff

 

[A Squared]

I see your point but in looking at a simple baro altimeter I'd be surprised if it were as you say. To get two aneroids to make the same nonlinear correction let alone thousands of them stretches my imagination.

Yeah, I don't know how it's done. All I do know is that the relationship of pressure and altitude is quite non-linear, so the altimeter does *something* non-linear. whether it's in the expansion of the aneroid or it's in the mechanism, I don't know.

We have an instrument repair shop here at MKE. I will make a point of stopping in and seeing what I can learn.

The next step might be finding the technical order for altimeters and see what it might say. I was taught years ago in the USAF that it was simple clockwork. Stay tuned.

DC

Let me know what you find out.

 

[Donsa320]

Yeah, I don't know how it's done. All I do know is that the relationship of pressure and altitude is quite non-linear, so the altimeter does *something* non-linear. whether it's in the expansion of the aneroid or it's in the mechanism, I don't know.



Let me know what you find out.

The more I think about it the more I agree with you....the 1in Hg per 1,000 ft may only apply to Kollsman window adjustments whilst the aneroids are cleverly engineered to approximate the non linear atmosphere. That is the attitude I will take to the instrument shop.

DC

 

[CaptO'Brien]

In Canada i know that they add cold wx corrections to all heights AGL for the procedure turn, DH, MDA, DME arcs, and min sector altitudes. There all published in the "CAP GEN" book in an easy table that you match up the temp and height AGL above the reporting station and then add it to all appropriate altitudes. When enroute in Canada when cold wx corrections are applied you are required to add 1000 feet to the publised MEA and fly the next appropriate flight planned altitude. When on radar vectors ATC automatically adds the cold wx corrections in the altitudes assigned...anyhow just thought i would give my imput...but im still a low timmer and this is only what they do in Canada.

 

[Donsa320]

In Canada i know that they add cold wx corrections to all heights AGL for the procedure turn, DH, MDA, DME arcs, and min sector altitudes. There all published in the "CAP GEN" book in an easy table that you match up the temp and height AGL above the reporting station and then add it to all appropriate altitudes. When enroute in Canada when cold wx corrections are applied you are required to add 1000 feet to the publised MEA and fly the next appropriate flight planned altitude. When on radar vectors ATC automatically adds the cold wx corrections in the altitudes assigned...anyhow just thought i would give my imput...but im still a low timmer and this is only what they do in Canada.

Thanks for the input. I suspect it will take a tragic loss of someone important here in the U.S. before anything is done. Something like the famous Senator Cuttings accident in MO in the 1930's. That got us the Air Weather service and rules about the alternates and so forth. TWA hitting Mt Weather got approach clearances fixed for the most part.

DC