Density Altitude, etc...

[spudskier]

I'm working on my private, I'm doing so well, I understand almost everything.... everything exept Density Altitude.

I can compute pressure altitude, but for some reason I can't grasp density altitude and what affects it. Is there a simple expanation or a good web site where I can go??

Thanks in advance!

~Josh

[Tinstaafl]

Aircraft performance is affected by air density. Air density is affected by temperature, pressure & content (ie what gases are in the atmosphere). So that a/c performance can measured & compared an international standard atmosphere (ISA) has been defined.

This ISA means that measurements can be compared from moment to moment, day to day & equipment to equipment against a constant benchmark.

The ISA is defined as having a certain pressure, temperature & density at sea level as well as a rate at which these things change with altitude eg pressure at ~30' per HPa, temp. at 2 deg C per 1000'.

Since a change with altitude is part of the ISA, there must be a level from which the altitude is measured: 1013.2 HPa.

When you determine Pressure Altitude you are finding the altitude in ISA at which your current pressure corresponds, all based on the idea that 1013 HPa = 0' altitude.

Temperature also has an effect on density (& performance), so if temperature is different from that specified in ISA a correction must be made to find an ISA altitude equivalent.

Once this ISA equivalent altitude is known then performance charts can be written using altitude as a known variable.

At S.L. the temperature in ISA is defined as 15 deg C. At 1000' it would be 13 deg C, 2000' would be 11 deg C and so on.

Air becomes less dense as the temperature increases. If the temperature was 1 deg warmer than specified in ISA the density of this atmosphere would be the equivalent of being 120' higher in ISA.

For example, at S.L. if the temp. was 16 deg C, the temperature is 1 deg warmer than specified for that level in the ISA. You would experience an atmospheric density the same as if you had gained 120' altitude instead.

This 120' would be your density altitude ie you are experiencing a density that matches the density at 120' in the ISA.

Another example.

1000' pressure alt., temp 20 deg C.

In ISA the temp should be 13 deg however it is 7 deg warmer. The air will be less dense by an amount that corresponds to gaining 120' per degree warmer in the ISA.

7 x 120 = 840'.

At 1000' Pa. you would have to be at 1840' in the ISA to experience the same density. An a/c will perform as if it is at 1840'.

[OzExpat]

spudskier ... if you're accustomed to walking around at sea level, try going to the top of a really high mountain. Something above 5000 feet will do nicely for the purpose of this exercise. The first thing you'll find, when you get there, is that you'll have a bit of trouble with your breathing... probably considerably more if you've run all the way to the top, but that's not part of this consideration.

You got on a ski lift or a helicopter ride or something, so you're suddenly around 5100 feet above sea level. Okay? Now then, start walking around there, just as you would do at sea level. It won't take long before you find that you're trying to gulp air to breathe.

That's coz there's less of it up there. Your body needs to work harder to pull in the equivalent amount of air that you're accustomed to at sea level.

Now think in terms of the average aero (piston) engine. You start the engine at some sea level airport, take off and fly to some other airport around 5100 feet above sea level. Shut the engine down (after landing, o' course!), then try to restart it the same way you did at sea level.

The chances are REAL good that the engine will have more fuel than air for starting. It might start for you anyway, coz engines are robust critters, but more than likely it won't. You'll need to use a mixture setting that is somewhat less than full rich.

How much less than full rich? Well, at first it's likely to be a bit of trial and error but what you're trying to do is ensure that the fuel to air ratio is the same as it was at sea level. Now, here's the thing. You ain't at sea level ... the amount by which you ain't there can be worked out by applying all that good stuff that me mate Tinny just told you about, because there is less air available to the engine - so you won't get a proper ignition.

On different days at the same elevation, the air density can be different. Indeed, at different times of the SAME day, it can be different. Why? Changes in temperature and pressure.

The very reason we all go thru what Tinny has just explained is, among other things, that the fuel to air ratio in an aero engine is referenced to a thing called the international standard atmosphere, or ISA, for short. When you make the calculations you've just read about, by plugging in the temperature and pressure difference from ISA, you'll find out how high up in the atmosphere the engine "thinks" it is.

So those calculations are intended to find an equivalent altitude in the international standard atmosphere.

Air density affects aircraft performance in more ways than just engine performance. Think about this... you've found out that there's less air for you to breathe at that higher altitude. So how much difference in lift will your wings experience? You'll find that a longer take-off run is needed to build up enough speed to generate enough lift to get you off the ground. You always need more take-off distance at a high elevation airport than at a sea level airport, in the same basic conditions.

This is all pretty sketchy, of course, but I hope it helps you to understand at least a bit about the problems associated with air density. I'm sure that someone else will want to talk about its effect on True Airspeed.

[MightyGem]

Or more simply: Density Altitude = Pressure Altitude +/- 120 * T.
Where T is the difference between what the OAT is at your altitude and what it should be in a standard atmosphere.

[411A]

Not only engines have a problem.
Recently completed a flight to KLXV (Leadville Colorado), field elevation 9,927msl, highest in North America.
Altho the engines on my aeroplane are turbocharged (and can develop rated power to 16,000 feet), takeoff/climb performance suffers due to lower air density...the wing and propellors are not as efficient at these higher altitudes, especially with higher than standard temperatures.

[spudskier]

Thank you so much for your help!! It's makes a lot more sense. I think this coupled with the books I have should do it!!

Thank you!!

[mustafagander]

Density altitude is not just a performance problem for take off.

Think about a normal jet landing.

At a sea level airport, IAS = TAS (well, close enough) and in nil w/c = G/S. Your engines develop significant reverse thrust and your aerodynamic drag devices are efficient - flaps, spoilers. Don't forget landing climb and approach climb requirements.

Now lets land at Tehran, 7000ft AMSL. IAS is way below TAS meaning G/S is higher than IAS in nil wind (by a hell of a lot!!!). Forget about drag devices and engine reverse - thin air, remember. Now look at the brake temps!!! Landing and approach climb become marginal. An often forgotten factor is that the a/c feels "sloppy" at high D/A, so your accuracy is less when you really need to hit the aiming point on speed.