Hi All Again,

Here is a another question that could get you and still bugs me.

To me when I consider density altitude, I am giving consideration to the number of air particles that "fit" within a give volume of air. So when I link the words "high density" vs "low density" I naturally think of the number of air particles being greater in the former than the latter.

However it seems I am in error for thinking this way, as on a CASA exam question they refer to a "high density altitude" when instead it seems it would be better to refer to the statement as a High Altitude (density has nothing to do with it) or a "low density altitude" as a Low Altitude (once again the word density it seems is improperly used here).

Any thoughs on why CASA or other regulatory authorities persist in using this terminology would be appreciated, unless of course I am missing something.

Dear Brook,

I think that they mean (confusingly) that the ambient conditions (high altitude and temperature) produce a situation that is tantamount to a higher altitude equivalent...which has of course low density in terms of the number of atoms of gases present per given volume. Was that what you were saying?

G

Hi Brook,

First off, just to clarify something: Density is not the number of molecules in a volume but rather the mass of the molecules in the volume.

If you compress air, the density goes up. However the density is not higher simply because there are more molecules per unit volume but rather because the increased number of molecules per unit volume actually increases the mass.

If you replace some of the heavier air molecules with molecules that weigh less, you would actually decrease the density even though the number of molecules was unchanged.

That's why humid air is less dense than dry air: water vapour molecules weigh less than some of the molecules found in dry air. More water vapour in the mix, less mass for the same molecule count per volume and therefore less density.

CASA is correct in using the term density altitude since it is density altitude that affects the performance of an aircraft and it's aerodynamic properties. Density altitude is simply the altitude in the theoretical standard atmosphere (ISA) where the density of the air is the same as the density of the air you're considering.

For example, consider an aircraft flying at an "altitude" of 2000 feet. It is 35^oC on the ground and there's a low pressure system about. The aircraft will have appreciably poorer performance because it is actually flying in air that has the same density as air at about 4500ft in the standard atmosphere. In other words its "altitude" is 2000ft but its "density altitude" is more like 4500ft and it's the density altitude that determines its performance.

That's why density altitude is so important - and often very different from your actual physical altitude above sea level.

Take a look at the sections on ISA and density altitude in the book and review your Performance sections in the PPL book relating to P-Charts.

If the ambient conditions of the day have a higher density altitude then it will be higher altitude in ISA where the density of the air is the same as the ambient conditions. The aircraft will have poorer performance.

If the ambient conditions of the day have a lower density altitude then it will be a lower altitude in ISA where the density of the air is the same as the ambient conditions. The aircraft will have better performance.

Cheers,

Rich

Thanks G and Rich for those clarifications!

It would still appear to me however that when CASA say High Density Altitude they actually mean ANY DENSITY at a High Altitude. Would I be right in saying this?

Compared to Low Density Altitude when they mean ANY Density albeit at a Low Altitude.

Can you relate?

Brook

The density altitude is the altitude in ISA (not in the real world) where the air is as dense as the air where you are. Your actual height above sea level is irrelevent. It's all about the density.

So, no, when they say "High Density Altitude" they don't mean any density at high altitude they are simply saying the air is not very dense as if it were air at high altitude in ISA.

When they say "Low Density Altitude" they don't mean any density at low altitude they mean air is quite dense as if it were air at a low altitude in ISA.

In practical terms, if you read "high density altitude" replace it in your mind with "thin, less dense air" - and therefore poorer performance than in air with a lower density altitude.

If you read "low density altitude" replace that in your mind with "thick, more dense air" - and therefore better performance than in air with a higher density alitude.

Cheers,

Rich

Remember that density altitude is not a measure of altitude - it is a measure of density. 10,000 feet density altitude means that the air being considered has the same density as the air at 10,000 feet in the standard atmosphere. The higher the density altitude the lower the density.

By the way, you can't blame CASA for using the term density altitude. It is accepted internationally as a standard way of expressing density; not only in aviation but in other areas of science.

Thanks Bob and Rich,

Appreciate the correction. I spoke with another ground instructor on this and it confused the heck out of him too, which is probably what got me on my high horse.

I thought he did a good job of explaining when he said High (Density Altitude) and Low (Density Altitude) on the whiteboard. We had both concurred on the subject of air "density" when in fact as Richard pointed out its not really the density of molocules rather the MASS of the molocules, an important point that I was definitely missing.

And Bob you are right, it was inappropriate to discredit CASA for that one (OK they get a break this time as it is indeed an International term (hence my take on using the words "other regulatory authorities) and used in Science. Apologies if my slant was to agressive, I was simply trying to crush a walnut with a hammer to see what was inside

PS: Thanks for your help too Pannier, I just found a great bit of information (ok it was only Wikipedia) on air density after the above posts and it was very interesting in its brief consideration of a new variable - VAPOUR PRESSURE! (different from STATIC, DYNAMIC or EXAM pressure but as Bob rightly said "The subject of Aerodynamics could well occupy years of study at the Tertiary Level".

Wikiquote below: (quite interesting)

The addition of water vapor to air (making the air humid) reduces the density of the air, which may at first appear contrary to logic.

This occurs because the molecular mass of water (18 g/mol) is less than the molecular mass of dry air (around 29 g/mol). For any gas, at a given temperature and pressure, the number of molecules present is constant for a particular volume (see Avogadro's Law). So when water molecules (vapor) are added to a given volume of air, the dry air molecules must decrease by the same number, to keep the pressure or temperature from increasing. Hence the mass per unit volume of the gas (its density) decreases.

The density of humid air may be calculated as a mixture of ideal gases. In this case, the partial pressure of water vapor is known as the vapor pressure. Using this method, error in the density calculation is less than 0.2% in the range of −10 °C to 50 °C

Yeah, air hunidity isn't a big factor when it comes to density and performance. The biggies are temperature and pressure - but especially temperature.