Let's talk curves (Topic of the Week)

[ceo_of_the_sofa]

TOPIC OF THE WEEK

Mmmk, ladies and gentlemen…
Well, since we had such a great turn-out, and a resulting discussion last week regarding why the cabin pressure is lower than the outside pressure (while in flight, non-pressurized airplane)…I figured I’d throw you guys (and gals) another bone to chew on (hopefully, we’ll turn this into a weekly series of “odd questions that CFIs don’t like to talk about”)

Anywho, here’s a preface to today’s question:

When flying from an area of warm temperature into an area of cold temperature, indicated altitude will be higher than true altitude. But, it is kind of counter-intuitive to me…Here’s why…If we flew into an area of colder air, wouldn’t automatically mean that the density of this air-mass is higher and thus the pressure ought to be lower? And shouldn’t the altimeter, upon registering high pressure, now indicate low altitude, and thus cause us to climb? Instead, the books tell me that although I may think I am high, I am actually low? What gives?

I thought about this long and hard, and here’s what I came up with….Please feel free to correct and add at this point!
When density is increased (cold air), the pressure levels become lower in height (because higher density of the cold air substitutes for the volume of air on top of a pressure level) And since the pressure level has become thinner, while we may be indicating a certain PSI number, this level is now narrower since all of the pressure levels have become compressed ??? So in reality, we’re flying a narrowing line of the same pressure, down to the ground (sort of) Makes sense? Right or wrong?

Thanks

 

[starvingcfi]

mmmmmm....cuuuuuurves.

i dated a girl with curves once.

 

[midlifeflyer]

> What gives?

You're right. If we fly into an area of colder air, it will be denser. But denser doesn't mean "lower pressure". It means "more compact". And more compact means lower pressure (the push of something that is expanding beyond whatever is holding it). Which is denser and takes up less space - the same number of molecules of water vapor or ice?

I had the same problem as you and consistently got this wrong on written tests. Finally, I came across this mental picture of the process:

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

Temperature rises, the air, like any gas, expands, so the column rises (and your airplane along with it). Temperature lowers, density increases, and the column of air shrinks. Same for pressure changes. Lower pressure moves upward, low pressure moves downward.

That pretty much takes care of the airplane. Sitting on that column, it 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.

The reverse is also true. Your altimeter reads 2000'. The pressure setting is correct, but it's an unusually cold day. You're riding lower than the standard day the altimeter is set for.

 

[ceo_of_the_sofa]

[BWhich is denser and takes up less space - the same number of molecules of water vapor or ice?[/B]

Well, that's how I came up with my explanation, too...The same amount of molecules acting on the altimeter, albeit in a more dense surrounding (narrower pressure level) results in a constant altitude read-out...So, in turn, we just fly this narrowing (increasing density) tunnel down, or an expanding tunnel on a hot day?
Anyone else? I am so regretful that I didn't take physics seriously in school...along with a few other subjects

 

[100LL... Again!]

You are absolutely correct.

The original incorrect explanation you gave sort of makes sense until you realize that it would not be possible to fly into anarea of air that is colder and more dense, just like it would not be possible to walk along a sidewalk into a wall of water. The two pressures would have equalized before you could get there.


The compressing of the pressure levels is what happens. This is also why FL180 is not usable if the local altimeter setting is below 29.92.

Think of it this way: Suppose you are at 18,000 ft. half the weight of the atmosphere is above you, half below you.

Now cool the air until it freezes into solid oxygen. What would the total thickness be? I don't know exactly, but 18,000 ft indicated altitude would have you pretty close to the houses.


----------------------------------
top of the atmosphere (hot)
---------------------------------


--------- 18,000'




_________________surface_________________________

 

[100LL... Again!]

You are absolutely correct.

The original incorrect explanation you gave sort of makes sense until you realize that it would not be possible to fly into anarea of air that is colder and more dense, just like it would not be possible to walk along a sidewalk into a wall of water. The two pressures would have equalized before you could get there.


The compressing of the pressure levels is what happens. This is also why FL180 is not usable if the local altimeter setting is below 29.92.

Think of it this way: Suppose you are at 18,000 ft. half the weight of the atmosphere is above you, half below you.

Now cool the air until it freezes into solid oxygen. What would the total thickness be? I don't know exactly, but 18,000 ft indicated altitude would have you pretty close to the houses.

 

[ceo_of_the_sofa]

Great responses! I just wasn't sure if higher density brought higher pressure, but the last example seems to be mirroring what I was thinking earlier (that the pressure level just narrows, due to higher density, yet remains the same in regards to pressure) thus bringing it (pressure level) closer to the ground.

 

[bobbysamd]

Pressure and altitude

Just remember the old Aviation Weather saw: Hot to Cold, High to Low, Watch out Below.

The colder ambient air temp is how I remember it, too. The cold compresses the air molecules. Just remember how the airplane seems to leap off the runway on a cold day with high pressure and does not do as well on a hot day at the same pressure. The heat spreads out the air molecules. Density altitude has something to do with it as well.

In line with this subject, I'll throw in a plug for taking physics in high school and/or college if you want to be a pilot. This subject is part of thermodynamics.

 

[ceo_of_the_sofa]

Re: Pressure and altitude

In line with this subject, I'll throw in a plug for taking physics in high school and/or college if you want to be a pilot. This subject is part of thermodynamics.

Where in tarnation were you 10 years ago?

 

[172driver]

There is a good diagram of this topic in the Instrument Flying Handbook along with temp. correction tables.