Density Altitude??

my understanding is that the human bodies response is based mainly on pressure altitude, because by the time any air gets to the point where it is saturated into the bloodstream it has either warmed or cooled to 98.6 degF or whatever your internal body temperature happens to be at that time.
 
Everything I've seen on the topic says "partial pressure", which is a combination of atmospheric pressure with the composition of the atmosphere (i.e. what percentage is oxygen, as opposed to nitrogen, water vapor, mosquito flatulence, etc).

So converting that into "pilot words", it would indeed suggest that the pertinent number is pretty close to "pressure altitude", your elevation adjusted for non-standard pressure (the altimeter setting), with a small adjustment made for varying humidity, I suppose.
-harry
 
As Tony said by the time the air comes in contact with the alveoli (gas exchange units of the lung) it is almost always 98.6°F and 100% humidity if I remember correctly what Dr. Bruce said.
 
Thanks for clearing this up. I'm at an airpark in AZ where the DA can soar when it's 115 and I wondered if the O2 that the body had available would drop.

If I'm lazy when the temp climbs then I can't blame the density altitude anymore:cryin:

But then again, my wife doesn't follow this board.:wink2:

Cheers:

Paul
N1431A
2AZ1
 
Areeda said:
As Tony said by the time the air comes in contact with the alveoli (gas exchange units of the lung) it is almost always 98.6°F and 100% humidity if I remember correctly what Dr. Bruce said.
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But a heated atmosphere is less dense than a cold one, so fewer oxygen molecules per cubic foot are reaching those lungs. That, I think, would affect blood oxygen levels even if the pressure was normal. Wouldn't it?

dan
 
why wouldn't the air density change as the temperature changes inside your body?
 
Dan Thomas said:
But a heated atmosphere is less dense than a cold one, so fewer oxygen molecules per cubic foot are reaching those lungs. That, I think, would affect blood oxygen levels even if the pressure was normal. Wouldn't it?

dan
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If its ALWAYS that temp and ALWAYS that humidity in the lungs, the only true variable is pressure altitude.
 
Areeda said:
As Tony said by the time the air comes in contact with the alveoli (gas exchange units of the lung) it is almost always 98.6°F and 100% humidity if I remember correctly what Dr. Bruce said.
Click to expand...
Actually, about 75% humidity when it hits the lungs then up from there.

Assuming one breathes through the nose it only takes about a quarter-second for temps to equalize. When the outside temps start approaching zero Fahrenheit the respiratory tract begins having difficulty equalizing but even when I walk around in -40 temps in the winter here I can assure everyone that my alveoli are not experiencing DAs in the negative numbers ;)
 
The human alveolus (the sac in the lung) is always at 37C 100% relative humidity, thus only Pressure altitude matters.
 
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Hi! I'm a pilot in training - and new to the forum.
What is the point of calculating density altitude over pressure altitude? Its not like were going to change our altimeter setting? Why do we do this?
 
Sky-Pidgeon said:
Hi! I'm a pilot in training - and new to the forum.
What is the point of calculating density altitude over pressure altitude? Its not like were going to change our altimeter setting? Why do we do this?
Click to expand...
There are aspects of aircraft performance that are dependent on the density of air molecules, i.e. the count of how many air molecules are in a given cube of space. For instance, the amount of oxygen available to burn in the engine's cylinders, and thus the power the engine can generate, drops as the air density drops. This is a concern as air pressure drops (e.g. as we climb), temperature rises, and humidity rises. Lift also varies with air density, so as density drops, the aircraft must fly faster to maintain the same lift, and that results in longer takeoff and landing distances.

Pressure altitude doesn't measure air density, it measures pressure. You can lower air density while simultaneously raising temperature, and the pressure would remain the same (and thus pressure altitude would remain the same), but an engine will have less oxygen available for combustion, and a wing fewer air molecules to generate lift.

In short, by referencing density altitude, we can consider the impact that not only reduced pressure but also increased temperature has on air density, and thus on aspects of aircraft performance that are dependent on density.
-harry
 
Hi! I'm a pilot in training - and new to the forum.
What is the point of calculating density altitude over pressure altitude? Its not like were going to change our altimeter setting? Why do we do this?

I'm at an airpark in Arizona. Our 2200' runway is at 1970'MSL. Right now it's 109 outside at 16% humidity. The density altitude is 6000. My airplane and engine will think it's at 6000 feet MSL.

If I load up my airplane with myself, wife, fuel, some baggage and head for a cooler place--(after all it's a 4 place plane and I only have 2 people and some baggage on board)--I'd get a nasty surprise when I hit the end of the strip while still in ground effect.

There have been countless accidents caused by not heeding the loss of performance caused by density altituse.

Cheers:

Paul
N1431A
2AZ1
www.indianhillsairpark.com
 
pmanton said:
Hi! I'm a pilot in training - and new to the forum.
What is the point of calculating density altitude over pressure altitude? Its not like were going to change our altimeter setting? Why do we do this?

I'm at an airpark in Arizona. Our 2200' runway is at 1970'MSL. Right now it's 109 outside at 16% humidity. The density altitude is 6000. My airplane and engine will think it's at 6000 feet MSL.
Click to expand...

And that's one reason I don't live in Phoenix anymore...I remember the first time I flew in Phoenix (it was January) and started to lean the engine and the CFI asked me why...just habit I explained, since I live at 5500 MSL.

If I load up my airplane with myself, wife, fuel, some baggage and head for a cooler place--(after all it's a 4 place plane and I only have 2 people and some baggage on board)--I'd get a nasty surprise when I hit the end of the strip while still in ground effect.

There have been countless accidents caused by not heeding the loss of performance caused by density altituse.

Cheers:

Paul
N1431A
2AZ1
www.indianhillsairpark.com
Click to expand...
Mostly when flatlanders load up a C172 SP (180 HP, service ceiling 14K) with 4 people & baggage & full fuel that is just fine at sea level but when you get up to the hills, it's time to call CAP for SAR.

For example, I was going to Salida for lunch today (other things intruded & kept me away). My airport is 5500, the flight over the short hills is at 12.5. But with temps of 12C at 12K (according to winds aloft) that means a DA of 15K. With a service ceiling of 16K in a 40 year old aircraft, I'm not sure I wanted to try that. The slightly longer way around (only 15 min more) keeps me at 10.5K or DA of 12.5K, no problem.

Now consider the flatlander going to Steamboat Springs over Corona Pass, probably at 14.5K....DA is 17.9K

Welcome to Colorado and watch out for Cumulo Granite!
 
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