Utterly Confused

uwochris

Flightinfo's sexiest user
Joined
Dec 21, 2001
Posts
381
Total Time
2500+
Hey guys,

I am very confused with a few concepts, and I hope some people can help clarify the situation for me and explain some issues in a very simple manner. The first deals with the relationship between air pressure, density and flight performance, while the second concern deals with pressure altitude.

First off, I read in my flight manual that air density is dependant upon air temperature and air pressure. I realize that warmer air is less dense than cool air- no problem here; however, does air density increase as air pressure increases, or is it the other way around- air pressure increases as air density increases? Does it even make a difference how the relationship is stated (ie. higher pressure causes higher density VERSUS higher density causes higher pressure). I know the gas law is stated in terms of pressure.

Now... how does all this affect flight performance? I get confused because I know that different air pressures can affect the instruments, but can the pressure differentiation directly affect flight performance? ...... this is what I am assuming- higher air pressure causes the air density to increase because more molecules are getting compressed. This increase in air density (caused by a higher air pressure) increases performance because of the extra force exerted during downwash and by the extra thrust created by the engine. Am I right? Does a higher air pressure (ie. 30.00' versus 28.00') give better aircraft performance by affecting air density, or am I completely mistaken? Perhaps air pressure and performance aren't even directly related??

Now on to my other concern... pressure altitude. I just hope someone can provide a very simple, easy to understand definition of what pressure altitude is. I know that it is "indicated altitude corrected for non-standard pressure." The problem for me is that I always assumed that indicated altitude already compensated for non-standard pressure (ie. when you use the altimeter setting, aren't you directly correcting your altitude for non-standard pressures? Another explanation I received is that PA is "the altitude the aircraft THINKS it is flying at, assuming standard temperature. Is this correct? If so, I assume that a higher air pressure gives a lower pressure altitude, and thus, implies better performance (because flight performance diminishes with increases in elevation).

Well, I really appreciate all the opinions and truly hope someone can help clarify these issues for me.

Thanks a lot.
 

Timebuilder

Entrepreneur
Joined
Nov 25, 2001
Posts
4,625
Total Time
1634
Actually, the airplane thinks it is at density altitude, which is pressure altitude corrected for nonstandard temperature.

Any change in one of the variables, ie: pressure, temperature, or elevation, will affect the airplane's performance; how well the engine runs, how much air the prop accelerates, and how much lift the airfoils create.

A decrease in altitude, a decrease in temperature, or an increase in barometric pressure will all increase performance. When the variables move in the opposite direction, performance decreases. When two or more variables change in opposing ways, you can end up with no loss or gain. It's all a matter of current conditions when you are asking about air density.

The more time you spend with these concepts, the more they will tend to make sense.

I hope I was able to help make them clear.
 

Cardinal

Of The Kremlin
Joined
Nov 25, 2001
Posts
2,308
Total Time
K's
Bring it back down to the molecular level. Density is a measure of particles in a given space. Heat is really an expression of the energy level of a bunch of molecules. In hot air the molecules are physically moving faster. Thus hot molecules, contained in a theoretical balloon, will stretch it out more as they literally bounce off each other and into the walls of the balloon. To attain a given pressure you can either use a whole bunch of cold molecules, (very dense) or use a few very hot molecules (much less dense but equal pressure).
 

tarp

Well-known member
Joined
Jan 24, 2002
Posts
539
Total Time
Lots
"Pressure Altitude" is what you read on the altimeter when it is set to 29.92. It is a theoretical number unless you are flying on a "standard" day (i.e. that the sea level pressure and temperature are standard). For example, you are sitting on the ramp at Atlantic City, NJ (Bader field) and the pressure today is 29.92 and the temperature is 59F. Your altimeter should read nine feet if is calibrated correctly. At this one point in time, the indicated, pressure, density and true altitudes are all equal.

The idea of 29.92 or Pressure altitude is to provide a "standard datum plane" or point of reference. We use it at altitudes above FL180 because at that high in the atmosphere, it is just unreasonable to find a number that would represent all of the pressure eddies in the air.

"True altitude" is where the airplane is.

"Indicated altitude" is where the altimeter says you are for the settings you have dialed in. If you have set in the proper pressure in the kohlsman window and you are near standard temperature with a properly calibrated altimeter, then this will be near your actual or "true" altitude.

"Density altitude" is pressure altitude corrected for temperature. It is also theoretical and is used for airplane performance.

So you are fighting concepts of reality and theory. "True" is real, but almost unmeasurable because you are using an instrument that is adjusted for pressure and temperature and precision is almost unattainable. "Indicated" is real because it is what your altimeter "thinks" the altitude is and is read by you, the pilot. "Pressure" is theoretical unless fate steps in and creates a truly "standard" day and "density" is theoretical and calculated from Pressure.

This is simplified FAA version (as most of my answers are) - do we need to dig deeper?
 

uwochris

Flightinfo's sexiest user
Joined
Dec 21, 2001
Posts
381
Total Time
2500+
First off, thank you everyone for the responses.

After reading from my Transport Canada Weather Manual, it mentions that air density is directly related to pressure and inversely related to temperature. Thus, I suppose a very high pressure day (say 31.00" versus 27.00") would give better aircraft performance because of the effect the pressure has on air density (assuming temperature is the same for a 31.00 versus a 27.00 day).

I'm also starting to grasp the concept of "pressure altitude" (although very slowly). I was told that in simplest terms, it is the distance from the 29.92 standard reference datum. The importance of this figure still boggles my mind though... Why should I want to know the distance I am from the reference line?? (maybe the reference line implies good performance- the farther below the better, the farther above, the worse... is this a good way to think of it?... also, I know that PA does not consider non-standard temperatures; that's what density altitude is for).

I know PA is defined as your indicated altitude corrected for non-standard pressure. I get confused here because I thought using the altimeter setting compensates for non-standard pressure...????

I just hope this will one day all click in! Perhaps I'm trying to go too much in depth??
 

Timebuilder

Entrepreneur
Joined
Nov 25, 2001
Posts
4,625
Total Time
1634
We need various standards from which to begin calculations, and an assumed sea level pressure of 29.92in hg is only one example. We need to "start somewhere".

Also, when we fly above 18,000 feet msl, we all set our kollsman windows of our altimeters to 29.92, and fly a pressure altitude. This keeps all of the aircraft in a given area separated vertically, since we are no longer concerned about what the actual altimeter setting might be. Our true altitude will change as we move from one geographic area to another (due the differences in actual barometric pressure), but we will rise and fall together as we climb and descend slowly to maintain the indicated pressure altitudes on the faces of our altimeters.

For private pilot purposes, all that you really need to know is:

1) pressure altitude is what your altimeter indicates when 29.92 is set in the window.

2) 29.92 is a standard value used for performance calculations. Many performance charts list the pressure altitudes along the left side of a chart, and various temperature possibilities are shown along the top. The values at the point of intersection are things like takeoff or landing distances, based on the density altitude which could also have been shown (but wasn't) in that rectangle on the chart.

3) you are expected to be able to "interpolate", or find values that are not directly shown on the chart.

>> Thus, I suppose a very high pressure day (say 31.00" versus 27.00") would give better aircraft performance because of the effect the pressure has on air density (assuming temperature is the same for a 31.00 versus a 27.00 day).

Yes. Other things remaining equal, an increase in pressure makes air more dense, as the molecules are forced to be "closer together", literally providing "more air" in the same space or volume.

You're getting it.
 
Last edited:

tarp

Well-known member
Joined
Jan 24, 2002
Posts
539
Total Time
Lots
You said -

"I know PA is defined as your indicated altitude corrected for non-standard pressure. I get confused here because I thought using the altimeter setting compensates for non-standard pressure...????"

**PA = Indicated Altitude corrected for non-standard pressure.

This a true but conceptually hard statement. Let's turn it around.

**Indicated Altitude = Pressure Altitude plus or minus the non-standard pressure setting.

Your initial textbook statement is saying this. Pressure Altitude is what your altimeter would show if the Kohlsman was set to 29.92. Let's say you are at 3,000 ft and the altimeter setting for a station directly below you is 30.92" You have 30.92 in the Kohlsman and you are pretty close to 3,000ft. If you dialed in 29.92" or a standard pressure day value, your altimeter would read 1,000 feet lower or 2,000ft (Remember the adage "High to Low look out below"). So your true altitude was 3,000 feet MSL, your indicated (when set to 30.92") was 3,000 feet MSL but your pressure altitude was 2,000 feet (remember this is a theoretical number).

So your next question is performance - does an airplane perform better at 2,000 feet or 3,000 ft. The answer is 2,000ft or closer to sea level - the pressure altitude was LOWER than your actual altitude and therefore even though you are flying at 3,000 feet, your airplane's wings and engine feel like they are at 2,000 feet on such a nice day! If the day was of extremely low pressure (say the station barometric pressure was 28.92"), now the plane would behave like it was at 4,000 feet - the engine would be more sluggish and the wings would not lift as well.

I think you are asking for affirmation -

Airplanes perform better on Cold, High Pressure Days. Cold High pressure air has dense molecules and we would say that Density Altitude is "low" because again we suppose that airplanes perform better close to the ground.

Airplanes suffer performance on Warm, Low Pressure Days. Warm, low pressure air has relatively sparse molecules and we would say that Density Altitude is "high".

By the way, it only took me about ten years of flying to get a hold on these concepts so take your time and study lots of different textbooks. I highly recommend the new Jeppesen Instrument Commercial Manual. I think Trevor Thoms book is pretty good. I even like the FAA Weather book but only after you have a firm grasp of the concepts then go back and say "oh yeah, why didn't I get that the first time through!"

Smile.
 

Salty Dog

2 bells, no stinger
Joined
Dec 4, 2001
Posts
152
Total Time
600+
To expand on what somebody mentioned earlier, it's all about stacking bb's (air molecules).

BB's and physics review: The atmosphere is made up of all the bb's covering the planet. Way up high, they're spread apart pretty far, but as you get closer to the earth's surface, the weight of all the bb's above them squash them closer together. Thus, air is "more dense" - there are more bb's per cubic inch - near the surface.

Now this envelope of bb's is not uniform. Some large general areas are a little more compressed than others due to movement of large air masses. So as you fly from one area to the next, you adjust the kohlsman window of your altimeter to reflect the changes in ambient air pressure. The altitude you read from the altimeter after you adjust it for ambient pressure is "pressure altitude".

Within these areas of varying ambient pressure, an increase in temperature will cause the bb's to vibrate more, causing them to spread out - decreasing density (and pressure) as well. Density altitude is the term applied to air at a pressure AND temperature other than standard. My cheat sheet reads:
P Alt = Indicated Alt @ 29.92
P Alt = True Alt @ Std Press + Std Temp
P Alt = Density Alt @ Std Temp

As for performance...

Engine (not airplane!) performance is enhanced at higher density because you are packing more bb's into the cylinder. After you squirt gasoline on them and spark it, they try to expand to a proportionately larger size. That's the whole theory behind turbocharging and supercharging - using a fan or gears to push more bb's into a cylinder to get more power out of it. Higher air density (due to lower alt, colder air, or higher ambient pressure) is like free turbocharging for your engine!

AIRPLANE performance is actually BETTER at higher altitudes because you have to force the airframe through fewer bb's. If you look in the POH for a Cessna 172, you'll see that at a given power setting (65%) your true airspeed is higher at greater altitudes. (121 mph @ sea level and 138 at 8,000 ft) As they say, you "true out" at higher altitudes for the same power/fuel burn.

The bb theory has helped me understand all of this junk. For instance, that 65% power takes a higher rpm setting at the higher altitude, why is that? It's all in the bb's. Power is a measure of work over time. For each minute of flight at a given RPM setting, the fewer bb's filling each cylinder at 8,000 ft means you'll get less power out of your engine. BUT, you can get the same number of bb's through the engine that you would at sea level if you ran it a little faster for that minute. Check the charts - 2400 RPM gives you 63% power at 2500 ft, but it takes 2500 RPM to get the same 63% at 7500 ft.

It applies to the pitot tube as well. Why is indicated air speed slower at high altitudes? You got it - fewer bb's getting packed into the pitot tube up high vs. down low while the airframe moves through the air at the same speed.

Hope this wasn't too long-winded to be useful. Best of luck!
 

onetaco

Active member
Joined
Mar 1, 2002
Posts
31
Total Time
1100
Pressure lapse rate

Good info from all:

Now to muddy the waters some more...

uwochris said:

The problem for me is that I always assumed that indicated altitude already compensated for non-standard pressure (ie. when you use the altimeter setting, aren't you directly correcting your altitude for non-standard pressures?
Yes, you are adjusting your altimeter for non-standard pressure with a local altimeter setting. But the adjustment is reported to you AS IF YOU WERE AT SEA LEVEL. You probably are NOT at sea level. Much like temperature has a lapse rate, pressure has a lapse rate too. Pressure decreases with increased altitude, but at different rates depending on whether it's hot, cold, dry, humid, etc.

Jepp makes reference to this in their Inst. Comm. Manual p. 2-19 & 2-20 in a discussin of True Altitude.

Good Luck,
onetaco
 

Timebuilder

Entrepreneur
Joined
Nov 25, 2001
Posts
4,625
Total Time
1634
Salty-

>>AIRPLANE performance is actually BETTER at higher altitudes because you have to force the airframe through fewer bb's. If you look in the POH for a Cessna 172, you'll see that at a given power setting (65%) your true airspeed is higher at greater altitudes. (121 mph @ sea level and 138 at 8,000 ft) As they say, you "true out" at higher altitudes for the same power/fuel burn.<<

If you are judging airplane performance by the amount of true airspeed for a given power setting, you are correct. However, the emphasis on "performance" during pilot training has more to do with usable runway for operations, such as takeoff and landing distance with various winds and 50 foot obstacles, which is better at higher pressure, and lower altitudes. I think this is the basis of the question, referring to a training scenario where pressure and density altitude are the concepts being discussed.
 

uwochris

Flightinfo's sexiest user
Joined
Dec 21, 2001
Posts
381
Total Time
2500+
Makes lots of sense now! Thanks a lot!!

This is a new way that I like to interpret Pressure Altitude (PA)... Assuming temperature and all other variables (other than barometric pressure) are held constant, PA is the altitude the aircraft thinks it's flying at, or it would be flying at on a standard day.

So, if the elevation outside was 3000 ft and the pressure was 29.92 (let's assume 15 degrees C for ease) the aircraft would think it's at 3000 ft. If there was any other pressure outside and the other variables all stayed the same, the aircraft would think it's at a different altitude. For example, if it was 30.92, because of the increased pressure/density and effect this has on performance, the plane would think it's at 2000 ft. This of course is assuming all other variables are being held constant, as PA only factors in non-standard pressure. Higher than standard pressure reduces your PA and implies better performance than would be expected on a standard day at the same elevation.

Also, on another forum, I was told that Indicated altitude is PA corrected for non-standard pressure, and NOT the other way around. Perhaps this is why I was so confused with the definition.

Have I gotten it??? Please say yes!!!!
 

Timebuilder

Entrepreneur
Joined
Nov 25, 2001
Posts
4,625
Total Time
1634
If the pressure suddenly increased, the aircraft would perform better, IF you remained at the same true altitude, but chances are you wouldn't remain at that altitude. You would make a slight climb.

Here's why. You said that you have a barometric pressure of 29.92, and a temperature of 15C, for a standard day. Your airplane is at 3000 MSL, indicating "3000" on the face of the altimeter, your "indicated altitude".

Now, you start flying toward an area of higher pressure. Unless you compensate for this by resetting your altimeter to the local pressure, which is higher, your altimeter will slowly, imperceptibly, begin to register a loss in altitude as the pressure increases, and you will slowly correct for it. You are in a VERY SLOW climb! If you did compensate by dialing up an AWOS station and putting in the "new" pressure setting, which would be higher, it would move your altimeter's hands to indicate a slightly HIGHER altitude, and you would make a slight descent to make the altimeter say"3000" once again.

The pressure went from low to high. "Low to high, clear the sky".

The same thing happens when you fly through a change in temperature. Training aircraft don't have altimeters that compensate for changes in temperature, so your true altitude on a cold winter's day will be lower than on a hot summer's day.

The other half of this rule is "High to low, look out below".

When you are confronted with these questions on the written test, you have to read them very carefully, since their construction is a little awkward. You must first understand what is being asked and then choose your answer.

Did any of that make a light go on for you?
 

uwochris

Flightinfo's sexiest user
Joined
Dec 21, 2001
Posts
381
Total Time
2500+
Thanks for that last response Timebuilder. That certainly does clear things up for me :)

Thanks again to everyone who helped me out here!!
 

L.A.pilot

Active member
Joined
Dec 16, 2001
Posts
27
Total Time
5000
Sentient Aircraft

Hey guys,

Airplanes don't think. The computers on board execute sequenced instructions, and they do not think either. They could care less about what altitude they may or may not be flying at, since they do not have feelings or emotions, either.

Pressure Altitude:

The International Standard Atmosphere (ISA) was established by scientists using measurements from around the globe. The standard includes variations of pressure, temperature, and density with altitude. These values represent an average; in reality the atmosphere varies greatly.

Pressure altitude is the altitude defined in the International Standard Altmosphere that corresponds to the actual ambient air pressure. The altimeter actually measures pressure, and is calibrated to indicate pressure altitude corrected for non-standard sea-level pressure (SLP), as set in the Kollsman window. An altimeter setting is issued by a ground station computing the actual SLP. When this is set in your altimeter, it will give you a height above that reference datum (MSL). When set to 29.92, the altimeter indicates height above standard SLP, or pressure altitude.

Why 29.92"?

The ISA sea-level pressure is 29.92" Hg (meaning that pressure of the atmosphere at sea-level is equivalent to that of a column of mercury 29.92" high). You may learn that pressure decreases 1" per 1000 ft. It is important to note that this is a very rough approximation that is not valid for higher altitudes, as the variation of pressure vs. altitude is non-linear. Indeed, if that were the case, you would encounter negative pressures above 29,920 ft (a physical impossibility). I had to laugh when I saw one of Jeppesen's latest text books containing a chart with negative pressures because somebody had applied the 1"/1000' lapse rate all the way up.

Density altitude is similar to pressure altitude: it is the ISA altitude that corresponds to the actual air density. Aerodynamically, airplane performance will be constant at a given density. However, engine performance varies with different combinations of pressure and temperature. Therefore, modern airplane flight manuals do not use density altitude, but rather components temperature and pressure (expressed as pressure altitude).

The standard ISA temperature at 3000' is 9°C, not 15°C.
The standard is 15°C at sea level and decreases about 2° per 1000', up to 36,000'. If the temperature were 15°C at 3000', it would be 6° above standard, resulting in a higher density altitude-- less performance than standard.

So if an airplane could think, assuming it was familiar with the International Standard Atmosphere, the airframe would think it was at density altitude, and the engine would think that it is really too loud and hot in here and I've got a headache.
 
Last edited:
Top