Understanding pressure!

[newbie]

So I'm a brand new student to flying. Yesterday I entered into discussion with my instructor about pressure systems and how they affect altimeter settings. My main area of confusion is how temperature plays in the equation. It is my understanding that as temperature rises air molecules spread apart resulting in less density. Less air density equals less pressure resulting in a higher altimeter reading. With that said, if you are flying from an area of low temp into an area of high temp, your altimeter would be reading warmer (less dense) air placing your altimeter at a higher than accurate altitude. This goes against the "low to high clear the sky". If the altimeter were left uncorrected, you would actually be flying lower to maintain altitude per your altimeter. Can anyone help identify where I am going wrong?

 

[Geico266]

First, and foremost welcome to POA.

You have probably hit on one of the most confusing issues to new pilots, understanding air pressure, altimeters, and density altitude.

There will be "experts" along shortly to give you a much more detailed explanation so you can understand the dynamics to get your PPL and appease your instructor.

What I find most interesting is the difference the outdated, old school mechanical altimeter readings as compared to the laser accurate WAAS GPS readings. The two never seem to be the same, but clearly the GPS readings are more accurate and are not affected by air pressure or temperature. The altitude is computed by distance and triangulation from the GPS satellites orbiting the earth. Some day altimeters will be replaced by GPS readings so we are all on the same page and a lot less confusion.

In land surveying GPS readings are accurate to within 1/32" of an inch or less and that includes altitude (terrain levels about MSL)

Where is Henning when you need him!

 

[sballmer]

It is my understanding that as temperature rises air molecules spread apart resulting in less density.

Correct.

Less air density equals less pressure resulting in a higher altimeter reading.

That's we're you stumbled. In the atmosphere pressure is a measure of how many molecules (or how much air mass) is above you. Therefore, with a reduction in air density, the distance between two different pressure levels increases. Thus, if an altimeter is set to field elevation on the ground, it will read lower than actual altitude on a hotter than standard day, and higher than actual altitude on a closer than standard day.

 

[flyingmoose]

The Aviation Weather book has a whole chapter on this subject. Chapter 3. You can look it up right on google books. It is a good read.

 

[PPC1052]

The ideal gas law models the behavior of gas in response to various changes. It is expressed as PV = nRT, where P=pressure, V = volumn, n = number of moles (a measure of the number of molecules of the gas), R = Avagodro's number, a constant, and T = temperature measured in Kelvin. This allows you to calculate any particular variable if you know the other data points, and allows you to understand the relationship in changes in one variable to another. For example, solving for Volume, V = (nRT)/P, meaning, V is inversely proportional to P. In simple terms, holding all other variables constant, as the volume of a body of gas goes up, the pressure goes down. Or, Solving for T, T=(PV)/(nR). So, holding all other variable equal, as T goes up, so must P. But note the assumption that all other variables remain constant. That means that the body of gas must remain the same size for you to draw that conclusion.

So, to your first point, if you heat up the air molecules (assuming no change in the number of air molecules), the molecules will tend to spread appart (increasing Volume, and decreasing density) only if you are holding the pressure constant. But if you are holding pressure constant, then you aren't increasing pressure, are you? (No, this isn't a trick question.)

I was going to add further description of what this means in the real world atmosphere as you fly, but Scott's last paragraphs nailed it. No need to repeat.

 

[MAKG1]

PPC, go a little beyond the ideal gas law. For what we're considering, adiabatic behaviors are appropriate. None of density, temperature, and pressure are held constant.

The one exception is that we all try to maintain altitude based on the altimeter. If you do that (without changing the setting), it really is constant pressure. You'll need to climb or descend in true (geometric) altitude as the temperature changes to do that. But if pressure altitude is constant, so is pressure.

 

[denverpilot]

What I find most interesting is the difference the outdated, old school mechanical altimeter readings as compared to the laser accurate WAAS GPS readings. The two never seem to be the same, but clearly the GPS readings are more accurate and are not affected by air pressure or temperature.

GPS measures height above a mathematical ellipsoid, not the Earth.

http://www.colorado.edu/geography/gcraft/notes/gps/gps.html

More details there than you ever wanted to know. Dated but solid basics.

You mentioned survey work. In survey work the receiver must be left in a single location for a period of time (not hugely long) to average out errors in satellite ephemera since their orbits are not perfect. There's correction data for this included in the signal but they "wobble" for lack of a better term.

Peruse this person's directory at a school for some fun...

http://www.utdallas.edu/~aiken/GPSCLASS/

Particularly...

Errors and math... From an older U.S. Army document on using GPS for Survey work...
http://www.utdallas.edu/~aiken/GPSCLASS/ch5.pdf

Differential math... (Basically WAAS is just a very large differential GPS system...)
http://www.utdallas.edu/~aiken/GPSCLASS/ch6.pdf

These talk about the ellipsoid... And how it was chosen...
http://www.utdallas.edu/~aiken/GPSCLASS/datum1.pdf
http://www.utdallas.edu/~aiken/GPSCLASS/datum2.pdf
http://www.utdallas.edu/~aiken/GPSCLASS/datum3.pdf
http://www.utdallas.edu/~aiken/GPSCLASS/datum4.pdf

Or you can just take the class test... To see what ya know...

http://www.utdallas.edu/~aiken/GPSCLASS/geos5422exam.doc

This one (saved the best for last) has the easiest to read general coverage of the various issues...

http://www.utdallas.edu/~aiken/GPSCLASS/GPSBasics_en.pdf

None of the above talks about WAAS directly. That one is left for homework.

 

[Geico266]

Exactly.

No air pressure, no temp deviations, no settings. It is calculated by strangulation .... and that other thingy you mentioned.

MUCH more accurate and reliable over mechanical altimeters.


Why is it, even when you adjust altimeters to the hilt, they do not match the accuracey and readings of GPS?

 

[PPC1052]

PPC, go a little beyond the ideal gas law. For what we're considering, adiabatic behaviors are appropriate. None of density, temperature, and pressure are held constant.

Hence the statement about Scott's post.

 

[AuntPeggy]

So I'm a brand new student to flying. Yesterday I entered into discussion with my instructor about pressure systems and how they affect altimeter settings. My main area of confusion is how temperature plays in the equation. It is my understanding that as temperature rises air molecules spread apart resulting in less density. Less air density equals less pressure resulting in a higher altimeter reading. With that said, if you are flying from an area of low temp into an area of high temp, your altimeter would be reading warmer (less dense) air placing your altimeter at a higher than accurate altitude. This goes against the "low to high clear the sky". If the altimeter were left uncorrected, you would actually be flying lower to maintain altitude per your altimeter. Can anyone help identify where I am going wrong?

Welcome to PoA. We always enjoy talking to and encouraging new pilots, as you can see. There are a huge number of us with great answers to questions, as well as opinions of various sorts.

You lucked out with your question, because ScottD is an expert on weather (atmospheric pressure) and he gave you a very detailed explanation. Thanks, Scott. Others have pointed you to some great reading on the subject.

Anyway, keep up the good work, learning and asking. We'll be here as a resource for you.

Tell us a little bit about yourself. I'm a fat old lady who loves to fly and talk to newbies.

 

[jnmeade]

When we start out flying, we often think of the altimeter in terms of how high we are above the airport or the tower or the mountain. Another important feature is knowing what altitude we are at in relationship to other airplanes. That is why, hopefully, all airplanes in a given area are expected to be using the local altimeter setting. It is also why when you get to FL180 everyone sets their altimeter to 29.92. As you can guess, sometimes there is an altitude band that ATC will not clear you to to prevent airplanes at 17,500 and airplanes at FL180 from being too close to each other.
So, altimeter settings relate to separation -from terrain and other aircraft. That's why this whole question comes up.
One way to see this is to ask your CFI to go over with you the situation where you'd fly VFR from St. Louis to Kansas City at 6,500 on a day when there is a large pressure change between the cities and you meet IFR traffic flying from Kansas City St. Louis at 6,000'. Factor in the possibiltiy that both you and the other pilot are maybe 100' or even 200' off your altitude. Could be Ouch.