Altimeter and temperature...can't wrap my head around it

[Doug F]

I'm prepping for my FAA Sport Pilot test by doing lots of practice tests. I like to think I understand the physics of temperature and pressure but I get pretty much every question related to these two items and the altimeter wrong. I've tried searching for help but the sites I find just confuse me more. The specific question is attached as a screenshot from the King Air training system.

What I think should happen: If temp goes up, air density goes down (molecules move faster and are further apart) which is why on hot days you need more runway to take off. It seems to me that if the temp goes up, the altimeter reading should also go up since it's a pressure measuring device and should be seeing a lower atmospheric pressure and display a higher altitude. WRONG. Per the attached screenshot, as temps go up, so does pressure??? If I have a closed system (tire, for instance) this makes sense since the air has no place to go and increased molecular motion results in higher pressure. I am thinking of the atmosphere as an essentially unbounded/unconstrained system so as the air expands it moves out in all directions resulting in lower pressure.

I don't get the mechanics of what's going on. Does anybody have a simple explanation for what happens and why? Is my assumption that the atmosphere is unbounded the problem? It IS a closed system much like a tire and acts the same way?0

Once I finally wrap my head around this one, I'll post another question about the relationship between air pressure and the altimeter. That one seems backward to me too...

Cannot wait to be done with the test...then I can start perceverating on the check-ride, specifically the oral...

 

[FORANE]

You are correct in the assumption that the universe is not a closed system such as a tire.
Look up gay lusacs law and boyles law.

 

[midlifeflyer]

Not a technical explanation, but a visual picture of the concept some have found helpful.

Your airplane is riding on top of a column of air that is enclosed on the sides, but can expand up and down.

As temperature rises, the air, like any gas, expands, so the column rises. Temperature lowers, density increases, and the column of air shrinks. Same for pressure changes. High pressure moves upward, low pressure moves downward.

Sitting on that column, the airplane moves higher with increases in temperature or pressure and moves lower with decreases. Problem is the altimeter doesn't change unless you adjust it (for pressure anyway)

So, you're altimeter reads 2000'. The pressure setting is correct, but it's an unusually hot day. You're riding higher than the standard day that the altimeter is set for. Unusually cold, the air is more compressed and you're riding lower than the altimeter is set.

 

[Van Johnston]

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What I think should happen: If temp goes up, air density goes down (molecules move faster and are further apart) which is why on hot days you need more runway to take off. It seems to me that if the temp goes up, the altimeter reading should also go up since it's a pressure measuring device and should be seeing a lower atmospheric pressure and display a higher altitude.

I think you have the correct basic understanding. It looks like the phrase 'pressure level' is what is throwing you off. It does me too. Hopefully others can come along and explain that term so it makes sense in this context.

 

[Doug F]

You are correct in the assumption that the universe is not a closed system such as a tire.
Look up gay lusacs law and boyles law.

But aren't those both for closed systems? If the atmosphere is open, then the specific relationships are invalid. If the laws are valid for the atmosphere, then it acts as a closed system, and everything begins to make sense. Temp goes up, pressure goes up (same as my tires and as if I descend to a lower altitude) and the altimeter (which can't tell if I'm moving up/down or if the air is more dense) reads too low.

Side note: in a life long ago and far away, I was a chemist and we called it 'Charles Law'. We were worried about not blowing glassware all to hell when we mixed and heated stuff. Off to check out MidLifeFlyer's explaination.

 

[Doug F]

Not a technical explanation, but a visual picture of the concept some have found helpful.

Your airplane is riding on top of a column of air that is enclosed on the sides, but can expand up and down.

As temperature rises, the air, like any gas, expands, so the column rises. Temperature lowers, density increases, and the column of air shrinks. Same for pressure changes. High pressure moves upward, low pressure moves downward.

Sitting on that column, the airplane moves higher with increases in temperature or pressure and moves lower with decreases. Problem is the altimeter doesn't change unless you adjust it (for pressure anyway)

So, you're altimeter reads 2000'. The pressure setting is correct, but it's an unusually hot day. You're riding higher than the standard day that the altimeter is set for. Unusually cold, the air is more compressed and you're riding lower than the altimeter is set.

So the model is a cylinder with a movable piston and the plane rides on the piston rather than either a fully open system or a fully bounded one (tire). I can buy into that model but...
Gahh...if I look at the test explanation, it says temp goes up, pressure goes up, indicated altitude goes down which means the system acts as if it's a closed system of a fixed volume.
Side note, I find the possible answer set confusing. 'Higher temperatures expand the pressure levels'...what the heck does that even mean?

 

[FORANE]

But aren't those both for closed systems? If the atmosphere is open, then the specific relationships are invalid. If the laws are valid for the atmosphere, then it acts as a closed system, and everything begins to make sense. Temp goes up, pressure goes up (same as my tires and as if I descend to a lower altitude) and the altimeter (which can't tell if I'm moving up/down or if the air is more dense) reads too low.

Side note: in a life long ago and far away, I was a chemist and we called it 'Charles Law'. We were worried about not blowing glassware all to hell when we mixed and heated stuff. Off to check out MidLifeFlyer's explaination.

Yes, they apply to a fixed amount of gas in a closed system. The constants change depending on the law. Charles law is related also. We were taught to remember which was constant with this triangle:

C
V T
B P G

In the corners of the triangle are C,B & G representing Charles, Gay Lusacs or Boyles law.
Opposite C,B, or G is what remains constant in their respective laws. For example, Charles law involves constant pressure, Gay Lusacs law involves constant volume, etc. We were taught the mnemonic Can those girls possibly be v....

In the atmosphere, you do not have a closed system so a higher temperature will not increase pressure like a car tire. Instead of increasing pressure, the molecules will spread out leaving fewer molecules above you weighing down your air column thus less pressure on the altimeter. There is also fewer molecules of oxygen for the engine to burn, fewer molecules for the prop to bite, fewer molecules for the wing to gain lift, etc.

 

[midlifeflyer]

So the model is a cylinder with a movable piston and the plane rides on the piston rather than either a fully open system or a fully bounded one (tire). I can buy into that model but...
Gahh...if I look at the test explanation, it says temp goes up, pressure goes up, indicated altitude goes down which means the system acts as if it's a closed system of a fixed volume.
Side note, I find the possible answer set confusing. 'Higher temperatures expand the pressure levels'...what the heck does that even mean?

Part of the problem is that, for many, it's not particularly intuitive. We are dealing with a theoretical model Combine that with questions assuming a fixed altimeter setting, and it can become incomprehensible. I don't like the "expand the pressure levels" answer choice either, but I think they use the term "pressure level" in part because that is what you fly in the flight levels, with the altimeter set at 29.92.

All it really means is, using my column analogy, higher temperatures expand the column of air. You are higher than your altimeter reads. On a standard day, with your local altimeter set correctly and reading, 20,000' you are at 20,000'. With temperatures higher than standard, you will be higher than the 20,000' indicated.

Practical? There are a few. The overall "high to low" concept is the reason folks hear "FL180 is not available" on a low pressure days. With the altimeter set at 29.92 and showing 18,000' (that's what FL180 is), there is the potential for non-flight level traffic at 17,000' (using the local altimeter setting, not 29.92) to have insufficient separation. Another is flying in mountainous terrain where, even with the correct altimeter setting, a cold day reduces your "indicated" terrain clearance.

 

[Doug F]

Just got a PM from D. Tuuri
'The altimeter stays the same--if you're airborne. Because you fly in order to hold altitude. However, your "true altitude" increases as you need to ascend in order to maintain altitude.'

I think this may have broken the logjam in my brain. I'm thinking about the problem as if I'm sitting still at some given altitude (like on the ground) rather than what would happen if I was actually flying and following my (uncorrected) altimeter. Two different scenarios (I think) because in the first case, my actual altitude is not changing and in the second it is.

Soooo....do I have this right?
A. IF I'm sitting on the ground at an airport and the temp goes up, air density will go down, air pressure will go down, and my indicated altitude will go up.
B. IF I'm in the air at 3500' indicated and the temp goes up, air density will go down, air pressure will go down, the altimeter will begin to go up but I'll follow it to maintain the indicated 3500' and the altimeter will read lower than my actual altitude. If I had an AGL altimeter in addition to a barometric one and followed the AGL altimeter, I'd see my barometric one do the same thing (indicated go up) as if I were on the ground.
The physics of what's going on with the air pressure are the same but the effect of moving/not moving changes what I see on the altimeter.

 

[midlifeflyer]

BTW, you can answer the test question with very little understanding beyond the common "high to low" mnemonic since B&C are obviously wrong.

 

[Doug F]

BTW, you can answer the test question with very little understanding beyond the common "high to low" mnemonic since B&C are obviously wrong.

You don't know me very well do you? ;-) If they were obviously wrong, I would not get the answer wrong even though I know the 'high/low' thing...I get myself wrapped around the axle with very little effort!

 

[midlifeflyer]

You don't know me very well do you? ;-) If they were obviously wrong, I would not get the answer wrong even though I know the 'high/low' thing...I get myself wrapped around the axle with very little effort!

LOL! It's the bane of those who, although they can get by with very little understanding, choose not to Believe me, I ran into the same problem when I was trying to figure it out came across many others who also do - trying to think it through instead of applying basic rote, but not having as solid base for doing so..

Here's what I mean by very little understanding: The question is about temperature changes. "High to Low" tells us to "look out below." That just means if we move into an area with lower temperatures, our altimeter will lie to us. We will actually be lower than indicated or, said in the opposite way, our indicated altitude with be higher than actual. So we'd better watch out for the terrain like all the simplistic learning drawings show.

• Higher temperature = actual higher than indicated or its reverse, indicated is lower than actual;
• Lower temperature = actual lower than indicated or its reverse, indicated is higher than actual.

That's the mantra without really having to understand it.

Here's two of the three answers (with emphasis):

B. Higher temperatures expand the pressure levels and the indicated altitude is higher than true altitude. Nope. The mnemonic says higher temperatures mean indicated is lower than actual. The answer says the opposite
C. Lower temperatures lower the pressure levels and the indicated altitude is lower than true altitude. Again, no, the answer is the opposite of the rote knowledge.

You don't even have to know the right answer.

Folks know I'm not a fan of mnemonics precisely because they don't really require understanding. But sometimes, including the knowledge test, ignorance is indeed bliss

 

[Doug F]

LOL!
Folks know I'm not a fan of mnemonics precisely because they don't really require understanding. But sometimes, including the knowledge test, ignorance is indeed bliss

Pretty much, I want to understand the mechanisms because I think that makes you better able to do <insert task here>. At this point, I'm good with rote just to get through the test.
The reality for me is that I never intend to go more than a couple hundred miles from my home base and only in really good weather. I'm close to a bunch of mountains but I'll go with a CFI before I attempt them so, until then, I'm flying over flat country. I really don't even need instruments for the kind of flying I plan to do so it's all about passing the test and moving on.
OTOH, this one bugs the hell out of me because I think my background should make it easy for me to derive the affects.
OTOOH, I've been away from actual physics and chemistry for over 30 years so I need some help knocking the rust off!

 

[Doug F]

Part of the problem is that, for many, it's not particularly intuitive. We are dealing with a theoretical model Combine that with questions assuming a fixed altimeter setting, and it can become incomprehensible. I don't like the "expand the pressure levels" answer choice either, but I think they use the term "pressure level" in part because that is what you fly in the flight levels, with the altimeter set at 29.92.

All it really means is, using my column analogy, higher temperatures expand the column of air. You are higher than your altimeter reads. On a standard day, with your local altimeter set correctly and reading, 20,000' you are at 20,000'. With temperatures higher than standard, you will be higher than the 20,000' indicated.

Practical? There are a few. The overall "high to low" concept is the reason folks hear "FL180 is not available" on a low pressure days. With the altimeter set at 29.92 and showing 18,000' (that's what FL180 is), there is the potential for non-flight level traffic at 17,000' (using the local altimeter setting, not 29.92) to have insufficient separation. Another is flying in mountainous terrain where, even with the correct altimeter setting, a cold day reduces your "indicated" terrain clearance.

Ahhhh....that kind of makes sense too. Thanks.

So my world is that of sport pilot and one who's not likely to get more than 5K AGL and more likely only 3K. I'll mostly be flying over flat ground, daytime, VFR. Mountain flying is 'sometime in the future' and only after I spend time with a CFI.

I spent too many years trying to parse out the exact meaning of FDA regs so I tend to pick apart questions down to the comma. The FAA regs are so damn precise that I kind of expect the questions on the test to be equally precise. I'm finding that they're not and some of them require you infer the meaning of the question, or the 'best possible wrong' answer and it's driving me nuts.

 

[Skyrys62]

I'm like you and want to understand it fully, but the older I get the more dense I become, so the more I think about it the higher my temp goes, and the higher the pressure (in my head and blood vessels)
and so I just stick with: high to low, look out below

However, here is the best info I've found to explain it....

Pressure, density and temperature are related (approximately) through the ideal gas equation. In the general form it is PV=nRTPV=nRT

Where PP is pressure, VV is volume, nn is amount, TT is temperature and RR is ideal gas constant. If you have an enclosed container filled with air, volume (VV) and amount (nn) are the same, so pressure increases proportionally to temperature.

In free atmosphere, however, the pressure is determined by the weight of the air above and thus mostly fixed, so by heating the air it increases volume instead.

To get to density, we divide the equation by volume and arrive at:

P=ρRTP=ρRT

Where ρρ is the density (and handwave the switch from amount to mass, hiding the gas-specific conversion factor in the gas constant). The outside pressure is constant, so the density actually decreases as temperature increases.

Practical effect of this is that since engine power depends on amount of air it can draw in the fixed volume of the cylinders performance is worse when it's warmer.

Now it remains to be explained what governs the open air pressure. The pressure at any given point is caused by the weight of the air above it. Because from the above at constant temperature the density is proportional to pressure, the full equation is differential.

ΔP∼ρΔhΔP∼ρΔh

In words the change of pressure is equal to difference in height times density.

The pressure at ground level is affected by weather systems in complex ways. But since colder air is denser, it means that when it's cold the pressure will decrease faster with altitude than when it's hot. Now the altimeter really measures pressure and it only has adjustment for sea-level pressure, but not for temperature. So when you set your altimeter on the ground and climb 1000 feet, you'll be more than 1000 feet above ground when it's hot because the pressure decreases slowly and less than 1000 feet above ground when it's cold. Some procedures even have minimal temperature because of this.

 

[Doug F]

the full equation is differential.

ΔP∼ρΔhΔP∼ρΔh

Now you're dragging me back to calculus and physics..."Assume a table with an infinite number of legs..."

This discussion is great, scares the crap out of me, and reminds me of the song 'Does anybody know what time it is' where the aviation version is 'does anybody really know where they are in 3-D space'.

Thank you for the reply. I'll spend some time trying to internalize it.

 

[midlifeflyer]

I spent too many years trying to parse out the exact meaning of FDA regs so I tend to pick apart questions down to the comma.

I can't speak for the FDA regs but the FAA ones are a piece of cake compared to the banking regs with at least 4 different agencies issuing applicable ones..

 

[Doug F]

I can't speak for the FDA regs but the FAA ones are a piece of cake compared to the banking regs with at least 4 different agencies issuing applicable ones..

Well sure. There FDA is only responsible for life and health. Banking is all about money...MUCH more important.

 

[midlifeflyer]

Well sure. There FDA is only responsible for life and health. Banking is all about money...MUCH more important.

LOL. No drugs for the FDA test without money

But really, complexity of regulation has little to do with the importance of the activity. I think of it as FDA being more (obviously not exclusively) about scientific testing and verification using objective criteria. Banking is almost about politics with the various agencies engaging in turf wars among themselves - oh and I forgot, turf wars with the regulatory authorities of 50 states.

 

[Bradley W]

How did they make sure altimeter was correct on the ancient Cessna 172? I think they only had 1 decimal point?

 

[Doug F]

How did they make sure altimeter was correct on the ancient Cessna 172? I think they only had 1 decimal point?

I think they hung a string marked at increments out of the cockpit.

 

[SkyHog]

The way I remember it: it’s hotter closer to the ground, where air density is higher. As you go up, it gets colder and air density decrease.

Doesn’t help with the “why,” but it will help you pass he test better than “high to low, look out below!”

 

[Hippike]

Look for videos on YT about altitude/pressure/temperature. There are some great ones that explain the relationship better than any other official text (search for Gian Luca Noia videos). I'm a visual gal, I can learn things a lot easier if I see them vs. reading a ton about their workings.
Good luck on your test!

 

[exncsurfer]

I'm prepping for my FAA Sport Pilot test by doing lots of practice tests. I like to think I understand the physics of temperature and pressure but I get pretty much every question related to these two items and the altimeter wrong. I've tried searching for help but the sites I find just confuse me more. The specific question is attached as a screenshot from the King Air training system.

What I think should happen: If temp goes up, air density goes down (molecules move faster and are further apart) which is why on hot days you need more runway to take off. It seems to me that if the temp goes up, the altimeter reading should also go up since it's a pressure measuring device and should be seeing a lower atmospheric pressure and display a higher altitude. WRONG. Per the attached screenshot, as temps go up, so does pressure??? If I have a closed system (tire, for instance) this makes sense since the air has no place to go and increased molecular motion results in higher pressure. I am thinking of the atmosphere as an essentially unbounded/unconstrained system so as the air expands it moves out in all directions resulting in lower pressure.

I don't get the mechanics of what's going on. Does anybody have a simple explanation for what happens and why? Is my assumption that the atmosphere is unbounded the problem? It IS a closed system much like a tire and acts the same way?0

Once I finally wrap my head around this one, I'll post another question about the relationship between air pressure and the altimeter. That one seems backward to me too...

Cannot wait to be done with the test...then I can start perceverating on the check-ride, specifically the oral...

The memory aid that helped me the most with these test questions was 'high to low, look out below'. So, based on how they ask it, you figure out which answer applies that concept. If they say higher temperatures(implies: low to high) causes your altimeter to read higher than actual(implies danger of flying into terrain, aka look out below), that has to be wrong . If they say lowering temperatures(implies: high to low) causes your altimeter to read lower(implies no danger of flying into terrain), that has to be wrong.

Same memory aid works for pressure changes also, high to low, look out below. You fly from high pressure, into low pressure, without adjusting your altimeter, you'll be indicating higher than actual, which is dangerous close to the ground in low visibility.

That worked for me, now I have to stop thinking about it before I confuse myself. And hopefully I said it correctly.

pressure:



temperature:


your test question:


The opposite saying to that is Low to high, clear blue sky(or something like that). != Look Out Below would be 'clear blue sky'. Answer A matches that 'saying'. B says low to high look out below, which is wrong. C says High to low, clear blue sky.

 

[Dan Thomas]

Standard day, the altimeter tells the truth because it's built to show a given altitude for a given pressure.

The next day, a hot day, it will under-read. The air has expanded, which means it expands in all directions, including upward. You are flying at 2000'ASL true but the altimeter is telling you that you are lower than that because there is more atmosphere above you than there was yesterday, and the extra weight above you means more pressure on your altimeter. The molecules that were at 2000' yesterday are now up at 2300' or whatever. They're adding to the pressure.

A week later it's cold and the altimeter is lying again. The atmosphere has contracted and now there's less of it above you at 2000' true than there was before. Less weight above you, less pressure on the altimeter, so it's over-reading. Dangerous for the IFR pilot if he doesn't make some corrections for a non-precision IMC approach in cold weather. He'll be lower than the altimeter says.

Don't need a lot of physics to understand that.

 

[labbadabba]

The way I remember it: it’s hotter closer to the ground, where air density is higher. As you go up, it gets colder and air density decrease.

That would really screw me up...

 

[SkyHog]

That would really screw me up...

It wouldn’t if you lived somewhere that density altitude matters. For us, it’s burned into our brains

 

[Ravioli]

Do what you need for the test... obviously... and then stay on Flight Following where every time you change sectors you get an altimeter setting.

Yes, it's required to pass the written. But afterwards...

 

[labbadabba]

It wouldn’t if you lived somewhere that density altitude matters. For us, it’s burned into our brains

He said: "As you go up, it gets colder and air density decrease." I see what he did as a memory device but as far as what is actually happening it's actually the opposite of what he is describing. That's why I said it would screw me up.