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# Density altitude

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Why do we adjust pressure altitude by temperature to get density attitude? Doesn’t pressure altitude take into account QNH and doesn’t QNH take into account the temperature?

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QNH is the pressure. Temperature affects density with pressure remaining constant. If you heat up a pan of molasses the pressure at the bottom of the saucepan will remain constant but the mollasses will expand, become less dense and less viscous. Things that used to float in cold mollasses will now tend to sink. When the density of air decreases things that used to float, like an aeroplane, will now tend to fall and will need more power to stay afloat.

An everyday example of this is an Archimedes thermometer where the floaty things depend for their position on the density of the fluid which depends on the temperature.

An Altimeter with QNH correctly set will tell you how high you are. Density altitude tells you how well you will fly (if at all) and how long it will take you to take off (if you can).

Most performance charts will make take off and landing distance calculations easy for you. A high and hot day can bugger up your plans.

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Why do we adjust pressure altitude by temperature to get density attitude? Doesn’t pressure altitude take into account QNH and doesn’t QNH take into account the temperature?

Short answer - no It doesn’tLonger answer.

Yes it does but for the purpose of calculating a couple of more useful numbers ( pressure altitude and density altitude) we pretend it’s an independent variable.

QNH ( pressure) is affected by lots of stuff. Movement of air in horizontal direction due to gradients of pressure elsewhere , mass of the column of air above that layer of air. Yep local temp will expand the air causing some decrease in pressure. But The pressure from the mass of air above will be mostly the same because the air expands so it’s volume gets bigger but it’s mass is the same. So in effect the top of the atmosphere gets higher but mass stays the same. But yes some of it will expand outwards not all upwards so yes it will have some small and inconsistent effect. So Yes local temperature and temperature of air above will effect it a bit but in a complex way.

But the simple derived calculations by feeding it back into the equations (regardless of any effect it might have) provide usable numbers that give usable markers of It’s effect on flight characteristics.

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The day I did my rpc test was the hottest on record here for years, and after 4pm in the arvo at 1350’ amsl (density alt was close to 5000 feet) we took over 3 nm to reach 500 feet for our first turn,it was 45 degrees c. Other cooler days I have been at 500 before 1/4 the way down the runway. it can be suprising how big of a difference temp plays in performance when first starting out. Now I look a beaut day and if it’s too hot I don’t even bother thinking about flying.

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The day I did my rpc test was the hottest on record here for years, and after 4pm in the arvo at 1350’ amsl (density alt was close to 5000 feet) we took over 3 nm to reach 500 feet for our first turn,it was 45 degrees c. Other cooler days I have been at 500 before 1/4 the way down the runway. it can be suprising how big of a difference temp plays in performance when first starting out. Now I look a beaut day and if it’s too hot I don’t even bother thinking about flying.

Some aircraft have temperature limitations in the operating manuals and so not only would it be unpleasant it might actually be illegal to fly on days which exceed those temps.
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Was recently in Cusco, Peru - altitude 11,000 ft amsl. According to theory, that should mean it was -8 deg C. But, at 12 deg C, that equates to a density altitude of 13,600. No wonder the ground speed of the Airbus A321 looked rather fast! And that's lower than Bogota in Bolivia!

Back in my dim past, we operated into Keglsugl, PNG - alt 8400 ft amsl. If it wasn't for the significant slope on the runway, our normally aspirated C185 & C206 would never have made it. Pretty daunting when you look at 21 ins MP while standing on the brakes!

happy days,

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Thanks everyone. Very helpful. Am I right in understanding that pressure is the amount of air above and density is the space the air occupies?

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umm. Yeah. Sort of.

They are not the things themselves but would be the predominate factors in generating each of those.

Certainly enough knowledge in that principle when going for pilot exams.

Easy to overthink these things.

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umm. Yeah. Sort of.They are not the things themselves but would be the predominate factors in generating each of those.Certainly enough knowledge in that principle when going for pilot exams.

Easy to overthink these things.

Your last line pretty much says it all.....'easy to overthink these things'

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Thanks everyone. Very helpful. Am I right in understanding that pressure is the amount of air above and density is the space the air occupies?

I'm inclined to agree with Mr Jaba & Gravity here fil. . . not worth going DEEP into it unless you REALLY need to know, for a MET exam for instance. . .although I would change your term 'Space' for 'Volume' as air can be more or less dense within the same cubic metre. . . the Pressure in that same volume would ALSO depend upon whether your little 'Block' of air was at sea level, or up at 10K feet ! Get your head into a good reference guide and look up ISA International Standard Atmosphere. All the numbers are there if you REALLY want to go techy . . . .

Phil.

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Forgive me if this link is referenced elsewhere - Course: Fundamentals of Aviation Meteorology - this is a handy online course that covers Pressure and Density topics along with many other topics.

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"Density Altitude relates to your aircrafts and engines PERFORMANCE.   Even on a warm day at sea level you will be affected by it. It's more likely to surprise you when you fly from an aerodrome well above sea level or try to climb over a Mountain range en route. All Lift formula's have  a symbol for air density in them. Density is a measure of the actual weight of the air molecules  in a given volume  of air.. It affect the prop the wings and the engines ability   to do their jobs.. Mass airflow  (not volume airflow) gives power.. I don't see this as  a Met subject.   Nev.

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"Density Altitude relates to your aircrafts and engines PERFORMANCE.   Even on a warm day at sea level you will be affected by it. It's more likely to surprise you when you fly from an aerodrome well above sea level or try to climb over a Mountain range en route. All Lift formula's have  a symbol for air density in them. Density is a measure of the actual weight of the air molecules  in a given volume  of air.. It affect the prop the wings and the engines ability   to do their jobs.. Mass airflow  (not volume airflow) gives power.. I don't see this as  a Met subject.   Nev.

It’s not a Met subject; it is part of Performance and Operations, which seems to be missing from the RAA syllabus.

Take off with a full load on the coast at day break, fly inland where the temperature has been rising, take off again in hot weather and if the strip is short, you’ll know all about density altitude. The calculation is relatively simple to tell you how much runway will be required, to quite an accurate length, or whether you’ll actually get off by the end of the runway. If you can’t it’s just a matter of checking the temp as the day cools, as long as you have the requisite daylight available.

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Or just go anyway and end up in the fence.

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Density altitude is simply a way of describing the density of air under current conditions. It takes into account the two factors affecting density, namely:

- pressure, which is affected by elevation and changes in the atmosphere (high/Low pressure patterns as you’d see in weather reports).

- temperature.

After correcting for pressure and temperature we express the result as a density altitude, which corresponds to the density of air in a standard atmosphere. Aircraft performance is then calculated in terms of this standard atmosphere.

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This video might help too?

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