The nerve to think my flash card is wrong: Density Altitude

[Keystoner]

Quoting exactly from my flash card:

If the outside air temperature is colder than standard temperature, then the altimeter reading will show LOWER THAN ACTUAL ALTITUDE.

I think it should read *higher* than actual altitude.

"High to low, look out below." This applies to temperature and pressure. If the temperature is low, you're actually lower than the altimeter is telling you, therefore look out below! The altimeter reading will show higher than actual altitude. Am I right?

 

[Tantalum]

High to low, look out below

 

[dmspilot]

I think it should read *higher* than actual altitude.

"High to low, look out below." This applies to temperature and pressure. If the temperature is low, you're actually lower than the altimeter is telling you, therefore look out below! The altimeter reading will show higher than actual altitude. Am I right?

You are correct.

 

[Keystoner]

Well, I sent a message to the company. No response yet. It's a little disconcerting as to the accuracy of the rest. How did this not get caught before going to the printers? Plus, I got this set of flash cards more than a year ago. No mention of errata on the website and there were no hits on Google about errors with this set.

 

[Baked Potato]

I'm not sure I understand. If it's colder than normal, then the air is denser than normal. Then would the altimeter indicate that you are lower than it would normally? Since air is denser at lower altitudes?

 

[Steamflyer]

Cold or low, look out below.

 

[crash7]

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[noahfong]

I'm not sure I understand. If it's colder than normal, then the air is denser than normal. Then would the altimeter indicate that you are lower than it would normally? Since air is denser at lower altitudes?

Baked Potato is correct. The card is CORRECT.

Think of it this way.... Flying from a hot to a cold, the altimeter encounters denser air (cold air is more dense than hot). Denser air is usually at lower altitudes. So, the altimeter will report a LOWER altitude. You will then fly lower to agree with your altimeter...even though you are at the correct altitude. Flying lower puts you closer to the ground. Therefore, "look out below".

 

[noahfong]

"High to low, look out below." This applies to temperature and pressure. If the temperature is low, you're actually lower than the altimeter is telling you, therefore look out below! The altimeter reading will show higher than actual altitude. Am I right?

No. Assuming you are trying to hold altitude, you will be flying based on what the altimeter is telling you. It will read lower and you will go lower. You have no other altitude reference.

Maybe your usual habit is to fly lower than what your altimeter says?

 

[Keystoner]

Baked Potato is correct. The card is CORRECT.

Think of it this way.... Flying from a hot to a cold, the altimeter encounters denser air (cold air is more dense than hot). Denser air is usually at lower altitudes. So, the altimeter will report a LOWER altitude. You will then fly lower to agree with your altimeter...even though you are at the correct altitude. Flying lower puts you closer to the ground. Therefore, "look out below".

The altimeter isn't affected by temperature. The 'given' of the question refers to temperature only, therefore, no regard is given to pressure.

 

[Baked Potato]

The altimeter isn't affected by temperature. The 'given' of the question refers to temperature only, therefore, no regard is given to pressure.

Do they expect us to disregard the correlation between temperature and density in these types of questions?

 

[Keystoner]

Do they expect us to disregard the correlation between temperature and density in these types of questions?

When I take a test, I answer based on what's given. If they give me temperature and pressure, I know how to plug and chug in the formulas for pressure and density altitude.

 

[Baked Potato]

When I take a test, I answer based on what's given. If they give me temperature and pressure, I know how to plug and chug in the formulas for pressure and density altitude.

But then based on your statement that the altimeter isn't affected by temperature, then in the OPs example, the altimeter reading would stay the same.

 

[Keystoner]

But then based on your statement that the altimeter isn't affected by temperature, then in the OPs example, the altimeter reading would stay the same.

Exactly, but I would be physically lower, therefore LOOK OUT BELOW!

 

[Baked Potato]

Exactly, but I would be physically lower, therefore LOOK OUT BELOW!

If your altimeter isn't affected, why would you be lower?

 

[Keystoner]

If your altimeter isn't affected, why would you be lower?

Altimeter error due to cold temperature--read about it.

 

[dmspilot]

Baked Potato is correct. The card is CORRECT.

Think of it this way.... Flying from a hot to a cold, the altimeter encounters denser air (cold air is more dense than hot). Denser air is usually at lower altitudes. So, the altimeter will report a LOWER altitude. You will then fly lower to agree with your altimeter...even though you are at the correct altitude. Flying lower puts you closer to the ground. Therefore, "look out below".

That's not how it works. By the way you just contradicted Baked Potato while saying he's right

 

[Keystoner]

If I'm right here, you may think I feel confident in the Knowledge Test I'm taking at the end of the month, but I'm far from it!

 

[Keystoner]

I have a practical question based on this discussion. Why am I setting the altimeter pre-flight based on the ATIS barometric pressure only? I know later in the checklist I'm verifying the field elevation on the altimeter to be within 75', but why don't we set the altimeter to the exact elevation of the field that so that the altimeter starts off already taking into account the temperature error?

 

[Tarheelpilot]

I have a practical question based on this discussion. Why am I setting the altimeter pre-flight based on the ATIS barometric pressure only? I know later in the checklist I'm verifying the field elevation on the altimeter to be within 75', but why don't we set the altimeter to the exact elevation of the field that so that the altimeter starts off already taking into account the temperature error?

Because you would be the only person doing that and everyone doing the same thing has value.

 

[Keystoner]

Because you would be the only person doing that and everyone doing the same thing has value.

I know--I'm supposed to bite my tongue at posts like this but what a complete waste of bandwidth.

 

[Tarheelpilot]

I know--I'm supposed to bite my tongue at posts like this but what a complete waste of bandwidth.

No reason to bite your tongue. It was an honest answer to your question.

edit to add:

Why do you think my input is so pointless?

 

[Keystoner]

No reason to bite your tongue. It was an honest answer to your question.

In that case, you didn't answer the question at all. I asked why do *we* do something and your answer is 'don't ask questions--just do what everybody does."

 

[Baked Potato]

Altimeter error due to cold temperature--read about it.

Ok, I read about it. But I do find it hard to believe that questions about temperature are supposed to be completely divorced from the associated changes in density that occur in the real world when those temperature changes are encountered.

Not that I also haven't encountered objectively nonsensical answers in my studies as well.

 

[Tarheelpilot]

In that case, you didn't answer the question at all. I asked why do *we* do something and your answer is 'don't ask questions--just do what everybody does."

Ahh

I see. I always just assumed it was to make things easier and standard. Everyone just uses baro. The minor differences from temperature only matter in limited cases doing IFR ops and there are procedures to accommodate. VFR it doesn’t matter. What’s more important is that we are all the same. Correction for temperature would make it complicated for no benefit and some small negatives.

 

[Patrick Bolte]

You park your airplane when the temp is 60F and the altimeter reads field elevation. You come back the next morning, but the temperature is now 30F. What will your altimeter read that morning?

 

[Keystoner]

Ahh

I see. I always just assumed it was to make things easier and standard. Everyone just uses baro. The minor differences from temperature only matter in limited cases doing IFR ops and there are procedures to accommodate. VFR it doesn’t matter. What’s more important is that we are all the same. Correction for temperature would make it complicated for no benefit and some small negatives.

Thank you.

 

[dmspilot]

In that case, you didn't answer the question at all. I asked why do *we* do something and your answer is 'don't ask questions--just do what everybody does."

The answer is literally exactly that. So that everyone is using the same altimeter setting. It wasn't a sarcastic answer.

 

[Capt. Geoffrey Thorpe]

I have a practical question based on this discussion. Why am I setting the altimeter pre-flight based on the ATIS barometric pressure only? I know later in the checklist I'm verifying the field elevation on the altimeter to be within 75', but why don't we set the altimeter to the exact elevation of the field that so that the altimeter starts off already taking into account the temperature error?

The ATIS barometric pressure should be the actual barometric pressure. It should give you the exact elevation of the field unless your altimeter (or the ATIS) is off a bit. Effects from temperature and high/low pressure are all lumped together.

Temperature error comes from the fact that the rate of change in pressure with altitude varies with temperature - the total height of the atmosphere changes. So, your altimeter reads a pressure altitude of 5000 feet, but on a cold day your actual geometric altitude is lower, on a hot day it is higher. Mr. Crash7 illustrated it perfectly.

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[Keystoner]

The answer is literally exactly that. So that everyone is using the same altimeter setting. It wasn't a sarcastic answer.

Oy.

 

[dmspilot]

The ATIS barometric pressure should be the actual barometric pressure. It should give you the exact elevation of the field unless your altimeter (or the ATIS) is off a bit.

Temperature error comes from the fact that the change in pressure with altitude varies with temperature - the total height of the atmosphere changes. So, your altimeter reads a pressure altitude of 5000 feet, but on a cold day your actual geometric altitude is lower, on a hot day it is higher. Mr. Crash7 illustrated it perfectly.

Never heard a barometric pressure on an ATIS. They usually only give altimeter setting. However, sea level pressure is usually available in the METAR in the remarks section.

 

[Kritchlow]

I have a practical question based on this discussion. Why am I setting the altimeter pre-flight based on the ATIS barometric pressure only? I know later in the checklist I'm verifying the field elevation on the altimeter to be within 75', but why don't we set the altimeter to the exact elevation of the field that so that the altimeter starts off already taking into account the temperature error?

Elevations at various points on large airports can vary significantly. It would be chaos.

 

[Keystoner]

You park your airplane when the temp is 60F and the altimeter reads field elevation. You come back the next morning, but the temperature is now 30F. What will your altimeter read that morning?

It's warmer, right? So the pressure changed, right? Or do you want me to assume it's the same as the night before? If I do assume that, and that could be the case because other things affect the atmospheric pressure, then the altimeter will still read the field elevation.

 

[Capt. Geoffrey Thorpe]

Never heard a barometric pressure on an ATIS. They usually only give altimeter setting. However, sea level pressure is usually available in the METAR in the remarks section.

Yes, I did not quite use the correct words - the ATIS gives you the setting that gives you the correct altitude for the field including any changes in what the theoretical sea level pressure would be if you dug a deep enough hole and any temperature effects.

 

[dmspilot]

I know later in the checklist I'm verifying the field elevation on the altimeter to be within 75', but why don't we set the altimeter to the exact elevation of the field that so that the altimeter starts off already taking into account the temperature error?

The altimeter setting already takes temperature into account. The 75' tolerance is for errors in your altimeter and static system and variation in airport elevation.

 

[Keystoner]

Yes, I did not quite use the correct words - the ATIS gives you the setting that gives you the correct altitude for the field including any changes in what the theoretical sea level pressure would be if you dug a deep enough hole and any temperature effects.

No, that was my bad. You were just responding to what I wrote.

 

[Tantalum]

The flashcard is missing some data points. The "look out below" thing implies that you're obeying your altimeter and holding your altitude. If you look at my first post up here the chart very nicely shows this, the altitude lines decrease with temperature

In the flashcard all it says is "if it gets colder the altimeter will X" which is a stupid thing to say because the reading won't change, assuming you're flying competently.

Maybe a better way to think about it would be, what happens if you set your altimeter to field elevation today, and then come back tomorrow when it is 20* colder out, will the reading by higher or lower than yesterday? In that case the reading will be higher than it was yesterday.

It can seem counter intuitive, but the altimeter isn't reading the density of the air

The plane "floating on a column of air" always helps me.. warm air expands and the plane is higher than what the altimeter says. Cold air shrinks and the plane is lower than it thinks.

 

[Keystoner]

In the flashcard all it says is "if it gets colder the altimeter will X" which is a stupid thing to say because the reading won't change, assuming you're flying competently.

I'm fine with the question as written. I'm not fine with the answer.

The reading won't change assuming you're flying competently. That's exactly right and the point of the question. What will change is your actual elevation above the ground.

 

[Keystoner]

Maybe a better way to think about it would be, what happens if you set your altimeter to field elevation today, and then come back tomorrow when it is 20* colder out, will the reading by higher or lower than yesterday? In that case the reading will be higher than it was yesterday.

Someone else asked this same question above. The altimeter reading is not affected by a change in temperature.

 

[Tantalum]

The altimeter reading is not affected by a change in temperature

That's not totally correct, the whole reason the plane ends up flying lower than actual is because you followed the altimeter down "look out below". If the plane had stayed on a fixed point in the sky while the temp dropped the altimeter will read higher as the temp goes down..

https://www.faa.gov/air_traffic/pub...0 feet,than the indicated barometric altitude.

But this is where flashcards and rote memorization has their limitations.. if I'm being honest.

The other question about why not everyone just setting their altimeters to field elevation is because airports (A) do have variation in their elevation and (B) if you fly somewhere else you are going to need to adjust your altimeter to whatever setting it is at that other field.. or along the way to keep seperation from planes. If you don't set it to the correct setting and verify that you are at plus or minus 75 feet you're not validating the accuracy of your altimeter. What if the setting is 29.87 but you need to set 30.02 to get to your field elevation. Then you fly somewhere and the next airport ATIS reports 29.83.. what would you do? Set it there and have an altimeter that is 200 feet off?