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

Maybe there was something about atmospheric pressure lapse rate vs. temperature...

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Keystoner said:
The altimeter isn't affected by temperature. The 'given' of the question refers to temperature only, therefore, no regard is given to pressure.
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You are right. I stand corrected. Thank you. Good luck.
 
Keystoner said:
The altimeter isn't affected by temperature. The 'given' of the question refers to temperature only, therefore, no regard is given to pressure.
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Wait… the altimeter isn’t affected by temperature??
 
Tantalum said:
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..
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The key being that the airplane is in flight, not on the ground. Your previous post
Tantalum said:
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.
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Implies that the airplane is on the ground when you take these readings. Temperature errors don’t occur on the ground, because the altimeter (or equivalent instrument) from which the altimeter setting comes is at that same place, so there won’t be an error from one to the other. If the altimeter setting is 29.87 both days, the altimeter will read the same both days, regardless of temperature.

But the farther you get away from the field elevation at which the altimeter setting was obtained, the greater the temperature error will become.
 
Man, I was convinced the look out below thing was wrong until I participated in a thread on it a few years ago. I finally figured out a way to think of it that made sense to me, but now I forget it. Lol. If only I could find the thread.
 
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I found it.
Salty said:
I think I've finally figured out a way to make sense of this. Following is a few randomish statements that help me work this out.

At sea level the altimeter won't change with the temperature because the weight of the air above sea level isn't changing.
On top of the mountain (or up in an airplane) it will change, because denser air means less of it is above you (less weight above you)
To extend the previous two statements, the higher you are the larger the error from this effect. This is shown in the pictures above in that the lines are less steep when lower.
Putting an altimeter in the freezer when on top of the mountain won't change it's reading because it's not the temperature of the air it's reading that matters, it's the change in configuration of the column of air that it's in that matters, and putting in a freezer isn't changing that.


And it's that last statement that is the root of my confusion. I want to use the air being sampled to explain the error, and you simply can't. You have to look at the bigger picture of the atmosphere and how the temperature affects it. Temperature isn't effecting the altimeter at all, it's changing the atmosphere in a way that makes the altimeter inaccurate.
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here are the images referred to, but probably will make more sense to read the other thread

Sport Pilot said:
View attachment 78830 View attachment 78829 View attachment 78828
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One of the few times I wish I didn’t open a thread here!

temp doesn’t matter except when it does

flying incompetently changes readings

etc

Problem solved: I’m going to look out below at all times
 
An altimeter setting is corrected for non-standard temperature AT REPORTING FIELD ELEVATION. The correction for non-std temperature from sea-level to the reporting stations field elevation has already been applied. That allows the altimeter to display the correct altitude when on the ground at that field.

As your altitude increases above field elevation, the amount of error increases. We have to correct for that error, from field elevation to our altitude, when the temperature differs from standard enough to become significant.

When above reporting field elevation...

If temperature is BELOW standard, the altimeter will indicate a HIGHER altitude than actual and you will be LOWER than you think you are.

If temperature is ABOVE standard, the altimeter will indicate a LOWER altitude than actual and you will be HIGHER than you think you are.


An unattended airplane sitting on the ramp will see its indicated altitude INCREASE as ambient temperature DECREASES.
 
Years ago this was always just stated as “cold or low look at below”

low temps or low barometric pressure
 
It's irritating not to have an acknowledgment of my inquiry from this company. I'm not asking for my money back.
 
Salty said:
Man, I was convinced the look out below thing was wrong until I participated in a thread on it a few years ago. I finally figured out a way to think of it that made sense to me, but now I forget it. Lol. If only I could find the thread.
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Perhaps the "column of air" analysis?

Basically, you are riding on a column of air. The column is closed on the sides (like a test tube). If the temperature decreases, the air in the column becomes more dense, so it contracts, bringing the airplane lower. Opposite for increase heating.

Same for changes in pressure. High pressure expands outward; low pressure contracts inward.

Corrected
 
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Steamflyer said:
Years ago this was always just stated as “cold or low look at below”

low temps or low barometric pressure
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The "high to low" mnemonic has been around forever in one form or another. But, as usual with mnemonics, they have very little to do (I'm being kind) with understanding.
 
midlifeflyer said:
The "high to low" mnemonic has been around forever in one form or another. But, as usual with mnemonics, they have very little to do (I'm being kind) with understanding.
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Neither does this flashcard OP is talking about. I like flash cards, but, without even considering the fact that it's wrong, this is not a very good one. Book learning is a combination of understanding and memorization. This flashcard, even if it had the right answer, doesn't help understanding, and doesn't help memorization either because you would only be memorizing the answer to a very specifically worded question rather than a general principle. OP should name the guilty publisher of this flawed training product.
 
dmspilot said:
OP should name the guilty publisher of this flawed training product.
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I was really hesitant to do this. Mind if I give them more time to acknowledge my question?

The flashcard doesn't just have the questions; there is some theory before the questions part.
 
Keystoner said:
I was really hesitant to do this. Mind if I give them more time to acknowledge my question?
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Sorry...didn't mean to call you out like that. It is your choice.
 
Larry in TN said:
An unattended airplane sitting on the ramp will see its indicated altitude INCREASE as ambient temperature DECREASES.
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Unless this effect is working from another principal, doesn't it depend on the elevation of the ramp? If the ramp is at sea level, would it still change?
 
noahfong said:
Unless this effect is working from another principal, doesn't it depend on the elevation of the ramp? If the ramp is at sea level, would it still change?
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Without mentioning pressure, I don't think Larry is correct making that blanket statement. Temperature causes error with the altimeter--that's the extent temperature has on an altimeter--but it is not reading anything from temperature changes alone.
 
noahfong said:
Unless this effect is working from another principal, doesn't it depend on the elevation of the ramp? If the ramp is at sea level, would it still change?
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There's nothing magic about sea level. It's no different than any other elevation.
 
Larry in TN said:
There's nothing magic about sea level. It's no different than any other elevation.
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From what I'm hearing, it has to do with the height of the column of air above the altimeter. Temperature changes the height, therefore the number of molecules above it, therefore the weight/pressure on the altimeter. The example was with the altimeter at altitude. But if it is at sea level, all of the air molecules remain above the altimeter regardless of temperature. Therefore, the weight/pressure is the same. Isn't it? If not, what is the explanation?
 
noahfong said:
But if it is at sea level, all of the air molecules remain above the altimeter regardless of temperature. Therefore, the weight/pressure is the same. Isn't it? If not, what is the explanation?
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Whenever you are on the ground, regardless of field elevation, all of the air molecules remain about your altimeter. Below you is only ground.

Consider an airplane on the ground at L06, Furnace Creek Airport in Death Valley, CA. L06's elevation is -210 MSL. Is that 210' more than all air molecules above the airplane?

A barometric altimeter assumes a standard pressure lapse rate as altitude changes. When temperature is either above, or below, standard, the lapse rate is either more, or less, than standard and the indicated altitude will diverge from true altitude as you climb, or descend, from the reporting station's field elevation.
 
Larry in TN said:
Whenever you are on the ground, regardless of field elevation, all of the air molecules remain about your altimeter. Below you is only ground.

Consider an airplane on the ground at L06, Furnace Creek Airport in Death Valley, CA. L06's elevation is -210 MSL. Is that 210' more than all air molecules above the airplane?

A barometric altimeter assumes a standard pressure lapse rate as altitude changes. When temperature is either above, or below, standard, the lapse rate is either more, or less, than standard and the indicated altitude will diverge from true altitude as you climb, or descend, from the reporting station's field elevation.
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I'm not following. I'm talking about the scenario you spoke about earlier where an airplane is sitting on the ground and the temperature changes. You said if the ambient temperature decreases, the altimeter will indicate an increase in altitude. Is this also what you're talking about? I'm just trying to understand the science of how this happens.
 
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midlifeflyer said:
If the temperature decreases, the air in the column becomes less dense, so it contracts, bringing the airplane lower. Opposite for increase heating..
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Think you have your density backwards. It does contract when colder, more molecules in the same space. More dense. Unless I'm missing something in your test tube.
 
Hang 4 said:
Think you have your density backwards. It does contract when colder, more molecules in the same space. More dense. Unless I'm missing something in your test tube.
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The airplane is at the top of the air in the test tube.
 
Hang 4 said:
Think you have your density backwards. It does contract when colder, more molecules in the same space. More dense. Unless I'm missing something in your test tube.
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Oops. You are right. Fixing.

More or less.
 
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Salty said:
I found it.


here are the images referred to, but probably will make more sense to read the other thread
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Thank you for posting these diagrams! Cleared up any confusion for me.
 
Keystoner said:
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?
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In my opinion that is the correct answer for the FAA knowledge test.

70% is a pass but you will need to go over your incorrect answers with your CFI before you can take your practical test.
 
Vance Breese said:
In my opinion that is the correct answer for the FAA knowledge test.

70% is a pass but you will need to go over your incorrect answers with your CFI before you can take your practical test.
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I took the test last week. 88%—same as my CFI.
 
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Keystoner said:
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?
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I set the altimeter to ATIS because I want the people who are approaching the airport to use ATIS to set their altimeter so we are all reading the same. The pressure may have changed since the ATIS came out and since they are flying they have no way to set their altimeter to field elevation.

I don't use the altimeter to tell where the ground is, I use it to tell me where the traffic is.

I find the ground visually and by feel.

I recommend you spend some ground time with your CFI to work out your misconceptions.

The ground portion of your practical test may focus on the areas where you were weak on your knowledge test.
 
It wasn't the flash cards. It was the 105 hours in 5 weeks studying the Gleim prep book. I was studying at the same library, same floor, same tables, I studied at 30 years ago when I was in college.
 
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