pressure, temperature, indicated altitude relationship?

[korben88]

I'm having a hard time with questions pertaining to what the indicated altitude would do when the temperature or pressure changes.

What is a good way to understand the relationship?

 

[SoCal RV Flyer]

For pressure change, remember: "From high to low, look out below"

As barometric pressure goes from a higher value to a lower one, the altimeter (without adjustment) will read higher than actual altitude, which can bring you dangerously close to terrain.

 

[GRG55]

For pressure change, remember: "From high to low, look out below"

As barometric pressure goes from a higher value to a lower one, the altimeter (without adjustment) will read higher than actual altitude, which can bring you dangerously close to terrain.

The relationship for temperature is when flying from warmer air into colder air the altimeter (without adjustment) will read higher than actual altitude. In other words you will be descending even though the altimeter will be reading a constant altitude.

 

[korben88]

So....

warm air = lower pressure = me being higher than I think.

Cold air = high pressure = lower than I think.

If I'm flying at 9000' indicated and the temperature drops but I don't adjust the altimeter then I would be flying at like 8500' true???

 

[bobmrg]

I'm having a hard time with questions pertaining to what the indicated altitude would do when the temperature or pressure changes.

What is a good way to understand the relationship?

Others have covered the bases, but why not just get flight following and ask the controller for an altimeter setting? You are supposed to check with a ground station within 100 nm of your position, but when you are on FF you will get one with every handoff between sectors.

Bob Gardner

 

[korben88]

Others have covered the bases, but why not just get flight following and ask the controller for an altimeter setting? You are supposed to check with a ground station within 100 nm of your position, but when you are on FF you will get one with every handoff between sectors.

Bob Gardner

Because I can't ask a controller for the answer when I'm taking the test.

 

[Cooter]

Warmer air is less dense - indicated altitude goes up.

Doesn't this contradict "Hot to Cold--Look out Below"?
Colder air will indicate a higher altitude, right?

 

[FastEddieB]

What always worked for me was to sketch out a side view with isobars drawn in.

Something like these:

High to low:

 

[Dan Thomas]

Doesn't this contradict "Hot to Cold--Look out Below"?
Colder air will indicate a higher altitude, right?

Colder air is denser and therefore heavier and so exerts more pressure on the altimeter, making it read lower than actual altitude. "High to low, look out below" applies to both temperature and barometric pressure.

The thing with temperature: Low temperature shrinks the air mass. The effect compresses the altimeter readings. If, say, the 30,000 feet of air above you is at its usual temperature, your altimeter will read about right as you climb or cruise or whatever. When that air gets much colder than usual, that 30,000 feet of it shrinks to something less, let's say 25,000 feet, so that the altimeter rises to what it thinks is 30,000 feet in only 25,000 feet, and if you believe it you will be lower than might be safe. The MOCA over higher terrain needs to be increased in cold weather to allow for that, as does MDA or DH, especially at higher airports.

 

[GRG55]

So....

warm air = lower pressure = me being higher than I think.

Cold air = high pressure = lower than I think.

If I'm flying at 9000' indicated and the temperature drops but I don't adjust the altimeter then I would be flying at like 8500' true???

Warm air doesn't equal lower pressure
Warm air = lower density; Cold air = higher density
Air density determines the rate at which the pressure changes with altitude.

When you set the altimeter on the ground (the adjustable numbers in the Kollsman window) at your point of departure it will read the actual ground elevation (or very close to it). If you do not have a local altimeter setting available (such as a remote air strip) then you should adjust the altimeter setting in the Kollsman window so the altimeter reads the actual ground elevation (or lake elevation if you are flying with James331 ).

The altimeter is a barometer; it senses changes in air pressure only. It is calibrated for a theoretical standard atmosphere (roughly one inch of mercury for every 1000 ft of altitude change). If the air below the airplane is colder then the actual altitude will be lower than the indicated altitude, because the air is denser than the standard atmosphere the altimeter is calibrated for. Conversely if the air below the airplane is warmer then the actual altitude will be higher than the indicated altitude.

The dangers of moving from a region of high pressure to low pressure, while maintaining a constant indicated altitude, were described in an earlier post. Same for the danger of flying from a region of warmer air into colder air.

 

[Cooter]

Colder air is denser and therefore heavier and so exerts more pressure on the altimeter, making it read lower than actual altitude. .

But it reads higher not lower when moving into colder air, otherwise hot to cold look out below wouldn't make sense. The answer just posted above is the best explanation.

 

[GRG55]

Colder air is denser and therefore heavier and so exerts more pressure on the altimeter, making it read lower than actual altitude...

In colder than standard air the altimeter (e.g. indicated altitude) will read higher than actual altitude.

 

[korben88]

Warm air doesn't equal lower pressure
Warm air = lower density; Cold air = higher density
Air density determines the rate at which the pressure changes with altitude.

When you set the altimeter on the ground (the adjustable numbers in the Kollsman window) at your point of departure it will read the actual ground elevation (or very close to it). If you do not have a local altimeter setting available (such as a remote air strip) then you should adjust the altimeter setting in the Kollsman window so the altimeter reads the actual ground elevation (or lake elevation if you are flying with James331 ).

The altimeter is a barometer; it senses changes in air pressure only. It is calibrated for a theoretical standard atmosphere (roughly one inch of mercury for every 1000 ft of altitude change). If the air below the airplane is colder then the actual altitude will be lower than the indicated altitude, because the air is denser than the standard atmosphere the altimeter is calibrated for. Conversely if the air below the airplane is warmer then the actual altitude will be higher than the indicated altitude.

The dangers of moving from a region of high pressure to low pressure, while maintaining a constant indicated altitude, were described in an earlier post. Same for the danger of flying from a region of warmer air into colder air.

now you've just confused me again.

 

[GRG55]

now you've just confused me again.

I suggest you post the specific questions you are having difficulty answering and let the group help you work through the logic to the answers to see if that works better.

 

[Cooter]

now you've just confused me again.

Re-read his explanation just above. That is the best answer because it is opposite what you would naturally be inclined to think. Warmer air is less dense, you already know that from aircraft performance charts. It is the rate of change and the column of air compressing that leads to the right answer. I think the aeronautical handbook has a thorough explanation.

 

[korben88]

I suggest you post the specific questions you are having difficulty answering and let the group help you work through the logic to the answers to see if that works better.

I'll do that the next time I come across them.
The main thing I don't like about this practice test that I'm using is that it tells me when I answer a question wrong but doesn't explain why.

 

[azure]

The picture that FastEddieB posted is the way I understand it. Your altimeter is nothing but a barometer, calibrated to translate pressure into altitude based on the way pressure decreases with altitude in a "standard" atmosphere. To the altimeter, lower pressure for whatever reason equals higher altitude. A column of air contracts when cooled and expands when warmed; also, the altimeter will read correctly close to the ground, in the vicinity of the airport for which the altimeter setting is valid. Therefore,

1. If you fly into a region where the sea level-corrected barometric pressure is lower than that at the airport where your altimeter setting was given, if the temperature profile is the same as in a standard atmosphere, your altimeter will read an altitude that is higher than the true altitude.

2. Similarly, if you fly into a region where the sea level-corrected barometric pressure is the same as at the airport where your altimeter setting was given but the temperature is colder than standard, the pressure will decrease more quickly with altitude than your altimeter assumes, and therefore the altimeter will read an altitude that is higher than true altitude.

3. Even in the vicinity of the airport where you got the altimeter setting, but at a higher altitude, if the temperature is colder than standard, the pressure will decrease more quickly with altitude than your altimeter assumes, and therefore the altimeter will read an altitude that is higher than true altitude.

If the barometric pressure is higher instead of lower, or the temperature is warmer than standard, the opposite is true.

 

[bobmrg]

Because I can't ask a controller for the answer when I'm taking the test.

My bad. ASA Test Prep books have explanations accompanying the questions and (in their opinion) the correct answer. I believe that Gleim does, too.

Bob

 

[flyingron]

Others have covered the bases, but why not just get flight following and ask the controller for an altimeter setting?

Asking ATC is not an option while taking your FAA Private and Instrument writtens which specifically ask about such things.

 

[FlySince9]

ba-dee-ba-dee-ba-dee....THAT'S ALL FOLKS!

 

[korben88]

My bad. ASA Test Prep books have explanations accompanying the questions and (in their opinion) the correct answer. I believe that Gleim does, too.

Bob

@bobmrg

No worries, the online practice test that I use doesn't have the explanations which is really the one thing that I don't like about it.

Btw. Where is the best place to get a copy of your book?

 

[bobmrg]

@bobmrg

No worries, the online practice test that I use doesn't have the explanations which is really the one thing that I don't like about it.

Btw. Where is the best place to get a copy of your book?

ASA publications are available at pilot supply shops nationwide (with some overseas retailers). Go to www.asa2fly.com and check out Products. My own books, which are a tiny subset of ASA's catalog, are available at the same places (did I mention Sporty's?) plus bookstores like Barnes and Noble.

Bob

 

[korben88]

Okay help me break down this one.

 

[Cooter]

Okay help me break down this one.View attachment 49551

B. Think of it as being on the ground and setting a lower altimeter setting than what was in there at start up.

 

[korben88]

So as pressure lowers the true altitude lowers?

 

[Cooter]

The altimeter senses the lower pressure and registers it as a climb. If you correct to hold the same indicated altitude, you will actually descend to a lower altitude.

 

[denverpilot]

Okay help me break down this one.View attachment 49551

The trick there is ...

You'd be flying lower BECAUSE the altimeter is showing higher than it should at that lower real altitude.

It's showing 3000 and you're really at something below 3000.

You flew from "expanded air" down the slope to "more compressed air". The altimeter still says you're at the altitude you were at in the expanded air.

Thus, it's showing higher than it should.

It's word games. They flipped it around to ask you what the altimeter is showing that's is making you fly at a lower true altitude, sitting there in the cockpit looking at it.

And it drives me nuts because FAA themselves say that gotcha questions like that are totally inappropriate in their own FOI for instructors writing test questions.

 

[azure]

Okay help me break down this one.

Remember: the altimeter is just a barometer with a scale that reads altitude instead of pressure. The Kollsman window is how you adjust it for changes in barometric (atmospheric) pressure. Atmospheric pressure decreases with altitude. So if the barometric pressure goes down without adjusting the Kollsman window, the altimeter reads a higher altitude than it should.

 

[GRG55]

Okay help me break down this one...

Let's try this step by step; apologies if I cover stuff you feel you already know well but I will try to emulate what might be a real life flight:

Assume the airport you plan to depart from is at sea level and happens to be in the area of high pressure described in your question.

The altimeter senses the change in pressure as you climb higher and uses that to display a change in altitude. However the altimeter cannot distinguish between a high pressure air mass and a low pressure air mass; it is calibrated to always indicate the same change in altitude for a given change in the pressure it senses. This is why the altitude shown by the altimeter is called indicated altitude, because it can vary from the aircraft's true altitude. High pressure air is more dense than lower pressure air, but the altimeter cannot tell the difference by itself. This is why the altimeter has an adjustable setting (the numbers in the Kollsman window) to allow the pilot to manually compensate for changes in local barometric pressure. On the ground, after you set the altimeter accurately for local barometric pressure, it will indicate the true altitude of the airport - in this case sea level or zero ft ASL.

Now imagine you take off and climb to a point directly above the airport where your altimeter is indicating 2000 ft ASL altitude. Your true altitude will also be close to 2000 ft ASL because you set the altimeter at the airport directly below just before you departed. If you now want to fly the airplane at a constant altitude along the coastline you will normally do this by referring to and maintaining a constant indicated 2000 ft ASL on your altimeter. As long as the local barometric pressure along your entire route remains the same as the departure airport (constant isobar) your true altitude will also stay constant, close to 2000 ft ASL.

Turning to the question, assume this same flight is made on a day when the local barometric pressure is not constant, but instead changes "from an area of high pressure to an area of lower pressure". The lower pressure area is air with less density ("lighter"). Your altimeter has no way of knowing this, or adjusting for it by itself. If you do not adjust the altimeter setting, and you maintain a constant indicated 2000 ft ASL on the altimeter, the airplane will be slowly descending, and your true altitude will become less than 2000 ft (hence the "From high to low, look out below" mnemonic). For the altimeter to read 2000 ft it needs the same air pressure as it had when the airplane was in the high pressure area directly above your departure airport, but as the airplane flies into the low pressure area you have to get closer to the ground to maintain that same air pressure in the less dense, "lighter" air.

The result is your true altitude above the sea level coastline you are following is becoming less than 2000 ft, even though the altimeter is still indicating 2000 ft ASL. So the answer is (B), the altimeter will indicate higher than the actual altitude above sea level.

 

[coloradobluesky]

Answer this question for me:
When you move the altimeter setting UP (like from 29" to 30") in the Kollsman window, which way does the elevation move in the altimeter?
Go out to the airplane and try it to make sure you got it right.
Then get back to me.

 

[exncsurfer]

Answer this question for me:
When you move the altimeter setting UP (like from 29" to 30") in the Kollsman window, which way does the elevation move in the altimeter?
Go out to the airplane and try it to make sure you got it right.
Then get back to me.

How is this helpful? I find it more confusing as an answer to the original question. The Kollsman window behaves opposite of what someone might expect. The pressure drops outside, your altimeter is erroneously displaying a higher altitude than actual. You get the current altimeter setting from ATC and lower your setting in the window, lowering your indicated altitude. Lower pressure outside gives you a higher indicated, lowering your setting in the altimeter lowers your indicated. Clear as mud.

 

[coloradobluesky]

It is counter intuitive. That was part of the point. But remember that, moving the altimeter setting up in the Kollsman window moves the elevation up in the altimeter.

Look, full understanding of this is a hard concept. Its as hard as solving a typical college physics question. If you don't understand it, yet, be relieved that you dont HAVE to understand it to be a pilot. Just remember high to low, look out below.

What you really need to remember while actually flying is to set your altimeter frequently, especially if flying through different climates and in the mountains. Typically the windward side of a mountain range has higher pressure than the leeward.

 

[coloradobluesky]

However, if you want to understand it, here we go.

Work through this:
1. Plane is at airport 1000' Elevation.
2. Sea Level Pressure is 30 inches of mercury
3. 30" is set in the Kollsman and the altitude reads 1000'
4. Storm comes in and Sea Level Pressure goes down to 29"
What does the elevation in the airplane's altimeter read? (approx rounded)

 

[coloradobluesky]

Use these facts to find the answer to the previous post:
1.The plane's altimeter goes down when the Kollsman is turned down.
2. The plane's altimeter goes up when the Kollsman value is turned up. (this is the one I remember so...)

3. The plane's altimeter goes down when the Absolute Pressure goes up (but the Kolsman stays the same)
4. The plane's altimeter indicated altitude goes up when the Absolute Pressure goes down.

The above are true statements I believe unless I've made a mistake. Don't just take my word for it, see if you can find a mistake. I can't so they must be right! (but I have been surprised and even devastated to find that I made a mistake)

 

[coloradobluesky]

So what happens is:

begin
#4 occurs /*resulting in altimeter indicated altitude of 2000 */
then
#1 occurs. /*altimeter indicated altitude now reads 1000 */
end

What seems counterintuitive is #3 and #4. Getting #3 and #4 wrong is where some are going wrong and thus getting the wrong answer.

The preceding paragraph must right becasue as a plane descends pressure increases and indicated altitude goes DOWN.

 

[coloradobluesky]

5. The altimeter's indicated altitude goes down as the plane flys to a lower elevation. This is due to higher Absolute Pressure.

The above #5 is a true statement.
And leads to this:
6. The altimeter's indicated altitude goes down as the absolute pressure goes up

ABSOLUTE PRESSURE makes the altimeter go up and down. ABSOLUTE PRESSURE is the pressure from the weight of the atmosphere above the point of measurement.

As you go down in altitude the altimeter reads lower due to higher absolute pressure! Thats how it works.

 

[coloradobluesky]

Kolsman Ind Altitude Sealevel Pressure Actual Elevation

How do I spread these words apart for a chart?
Theres a bug in this edit and preview system. When I preview it they ARE spread apart because I put spaces in them. But when I post them they arent. So how do I tab?

 

[denverpilot]

Kolsman Ind Altitude Sealevel Pressure Actual Elevation

How do I spread these words apart for a chart?
Theres a bug in this edit and preview system. When I preview it they ARE spread apart because I put spaces in them. But when I post them they arent. So how do I tab?

It's a web interface. Welcome to hell.