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Density Altitude

Old 10-30-2008, 07:37 PM
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Default Density Altitude

I am trying to differentiate between pressure altitude, the effects of temperature on pressure, and density altitude. This is for my CFI checkride which is coming up in a few weeks.

Pressure altitude is the altitude above the standard plane which would have a pressure of 29.92 inHg. Increases in temperature raise the pressure. As a result, a higher pressure would indicate a LOWER altitude on the altimeter (since higher pressure is found near the surface). Conversely, a decrease in temperature would lower pressure, resulting in a higher pressure indicated on the altimeter. This is general knowledge that a higher temperature gives higher pressure reading and on page 6-2 of the PHAK.

However, we start talking about Density altitude, which is pressure altitude corrected for non standard temperatures. In the PHAK, it says when the temperature is higher, the altitude will be higher than pressure altitude. However, in the paragraph above didn't I just prove the if the temperature is higher, the altitude will be LOWER?
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Old 10-30-2008, 10:20 PM
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High to low, look out below always seemed obvious to me in regards to pressure. I had to do a little digging to understand that in regards to temperature alone.

If you are flying along at an indicated altitude of 5000 feet and you were to abruptly transition from an area of 10C to 2C you would be flying at lower true altitude in the colder region.

The reason is because colder air is more dense than warmer air.

The following illustration should clear it up. Notice these areas have the same station pressure.

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Old 10-30-2008, 10:57 PM
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high to low, and low to high works for both pressure and temp, but the effects of temp on actual height above sea level or standard datum plane, is very small compared to the 1" of HG per 1000ft.

To answer the original question though, density altitude IS NOT A HEIGHT REFERENCE! Its an indication of aircraft performance. Engines and Lift work better in colder/denser air. So on a hot day, your 172 wont climb as fast as it would on a cold day. The actual value of DA is in FT yes, but it is comparing how your AC would perform against a standard day or whatever.

EX. Density altitude is 3000ft at an airport at sea level. That means your performance on the ground will be as if you were already at 3000ft.

Clear as mud?
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Old 10-31-2008, 04:47 AM
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I had the same problem understanding the PHAK at times as well. Read it very carefully, an INCREASE in temperature causes a DECREASE in Air Density which results in an INCREASE in Density altitude. The opposite holds true as well.
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Old 11-02-2008, 02:52 PM
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Yeah density altitude can be confusing sometimes but as said before it is only to determine how your A/C is/will perform based off of the current temperature and pressure. Pretty much it's just an information number for how the A/C will behave. Can't say I've ever had tower or anyone else ask me my DA in flight. So don't sweat it.

If it helps think of it like this... If your A/C's ceiling is 15000', but your density altitude puts you 2000' above actual altitude then your A/C will actually top out at 13000'. Just numbers I pulled out of thin air but I hope it helps illustrate a performance aspect of DA.
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Old 11-13-2008, 09:27 PM
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Thanks all for the input. I finally studied it for a while and got the idea and was able to teach it.

Oral exam this saturday morning! Haven't been getting much sleep this week...
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Old 11-14-2008, 11:13 AM
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GOOD LUCK ON THE RIDE! Thank god I will never have to take an initial CFI ride ever, ever again!!! If you get it done SAT, I suggest drinking heavily, to the point where you actually let everything you just crammed go right out your ear!
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Old 11-15-2008, 05:16 PM
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Thanks... I passed the oral today (not entirely sure how). I have the flight on wednesday. We'll see how that goes.
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Old 11-15-2008, 07:26 PM
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One way I explain it is from a physics perspective which for some people is over their head but if you can get them to visualize it helps. An altimeter is a measure of the amount of air molecules. Obviously the lower you are the more air molecules there are.

Pressure (static pressure) being the force those molecules exert when they are not moving. In other words if I take the student's hand place a book on it, then I am increasing the "pressure" by placing another book on top of his hand. Or if you can explain a manometer (just a U-shaped tube with one end closed) and why we use "inches" that works really well too.

As the the pressure increases, as in when you fly lower, decreasing your altitude, the static pressure of the air pushes the liquid (mercury, Hg) at Pa down. This by the way is what a static port measures. The higher the pressure the farther the mercury gets pushed (H) and that is why 30.92 inches Hg is a higher pressure than 29.92 inHg. This is the length in inches from the opening of the tube. In other words a lower altitude. If I set the altimeter on 0 and watch as the altimeter setting from the ATIS increases then the altimeter will indicate a negative altitude. Specifically about 1,000' for every inch of Hg. We all know this the question is whether or not we understand that and can get a student to understand that.

So when I say pressure altitude is the altimeter corrected for non standard pressure, I mean I'm trying to find the difference in altitude as if the altimeter is 29.92inHg. Why? so we can have a reference point when we throw temperature into the mix to calculate density altitude.

Remember temperature is a measure of molecule's kinetic energy. If I say your kid or brother or whoever is really hyperactive and is bouncing off the walls then they have a lot of kinetic energy, therefore a high temperature. If I decide I need to fill an auditorium full of kids, I can fit a lot more of them if they are calm and not moving around. If those kids were air molecules then they would be "cold air."

However, if those kids are really hyperactive and running around, I would not be about to get as many of them into the auditorium and therefore they will not be as densely packed. If they were air molecules then they would be "hot air," and therefore not be as dense, as in less of them, as in they would indicate a higher altitude. This would mean less air molecules to produce lift, be used for combustion, etc meaning worse performance.

Therefore density altitude is the altimeter corrected for non-standard temperature. Increase the temperature above standard, decrease the density, and therefore you will have a higher density altitude, less air molecules. Standard temp being 15C at sea level, 13 at 1000,' 11 at 2000' etc; 2C per 1000.'

Now after all that you can explain some of the other factors that affect density. i.e humidity (less air molecules, less performance, especially means less air to expand during combustion), and high elevation; less air molecules, less lift and performance, who knew?

Anyway, hope this helps, if you can get a student to visualize this then they will learn it.

Last edited by flyandive; 11-15-2008 at 07:33 PM.
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Old 12-09-2009, 07:16 PM
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Default Hot to Cold look out below

I read this forum but still am a little uncertain why the statement "Hot to cold look out below" is true. It seems contrary to what you would expect. I don't think the PHAK explains it very well or at least I am not getting it. There was an illustration previously posted from the PHAK above but I would like to make sure my interpretation of the illustration is correct.

Is it because in colder air, the pressure decreases with altitude more abruptly (faster rate) resulting in a "tighter stack" of pressure change? Therefore when encountering colder air, you will descend to stay at the same indicated altitude? That makes sense to me but I am not sure if it is correct.

The PHAK's explanation is that this occurs because cold air is more dense but I am not sure that really explains it.
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