The performance of aircraft deteriorates in thin air - in "hot-and-high" conditions. Wings have less air for lift, and engines have less air for thrust.

But is there any difference between hot and high? Do aircraft perform differently in low-but-hot conditions and cold-but-high conditions? Or is the air density all that matters, with temperature and pressure being irrelevant for performance at a given air density?

Brett_Henderson wrote on Dec 11^th, 2006 at 7:35am:


Temperature and pressure are two separate variables. Air density and therefore density altitude is a single variable. Yes, it is a function of both temperature and pressure. But obviously there are many different combinations of temperature and pressure that give the exact same density.

For example, Phoenix in Arizona is at actual altitude of about 300 msl. If the temperature at Phoenix is 52 Celsius, which does happen sometimes, (ISA+39), then the air density is comparable to air density in ISA conditions at about 1800 msl.

Does it mean that a plane at 1800 msl and ISA conditions (+5 Celsius) performs exactly the same as a plane at 300 msl and +52 Celsius, since the air density is equal?

Air density, temperature, and pressure are all idealy related using the formula P=dRT (and many derevations of it), where P is the pressure of the air, d is the density of the air, T is the (absolute) temperature, and R is the gas constant of air (287.0 J/kg*K), so you can now plug in various values of P (in pascals), d (in kg/m³), and T (in Kelvins), and see how other values are affected. 

As for a useful answer, see what everybody else wrote...

Mobius wrote on Dec 11^th, 2006 at 9:20am:


Hey, now... leave Pascal and Kelvin out of this... they are dead, and won't be able to help you make that go/no-go decision at that short-runway airport on a hot, humid day...and that is what understanding density altitude is all about- not spouting formulae and numbers, and dropping names of famous physicists.

Although air density is NOT dependent solely on altitude MSL (pressure altitude), that does not mean pressure alt. is irrelevant... it's ALL relevant. At the highest airport in the world, it is generally quite chilly and dry, but it's too damn high, pressure-alt. wise for many airplanes to operate there.


Conversely, if the air is hot- and humid- enough, it can create a situation where you will see dramatic reduction in performance (mostly engine performance, which makes all the difference on takeoff)... even at a sea-level airport. I may be wrong, but I think at Death Valley, which is a few hundred feet below MSL, it can get so hellishly hot that you coould have a problem climbing out over the rim, if you pick the wrong route for that.

What typically makes the critical difference in such a sea-level or near-sea-level scenario is not so much that the density altitude is so high that the airplane cannot develop what appears to be full power, but that it is just high enough so that the unwary pilot who's trying to get a heavily-loaded plane out of a short strip with obstructions finds himself rolling off the end of the runway.

That is, if he's lucky... sadly, things are often just marginal enough in such cases for the plane to become airborne but fail to clear trees, etc., or the pilot stalls trying to horse it up when it just won't climb.

Happens at mountain strips, too, but it can bite you even at sea level, if conditions are right (or wrong, depending on your POV).

And often a pilot will have sufficient runway, etc. but he forgets to lean the mixture for best power before takeoff... ("geez; why's it only putting out 2,000 rpm...that's odd... OMG!! TREE!!" )

 Add weak winds (typical in real sauna-like conditions) and it's even more likely to happen.

Mobius wrote on Dec 12^th, 2006 at 2:12pm:


There ya go... a little anecdotal data...we'll make a non-engineer out of you yet.

Now, why don't you explain how there's "less air" in humid air... that's usually not so critical, but I love driving people crazy insisting that no matter how hard might be raining, it's water vapor in air that spoils performance (aside from rain spoiling lift on some very-high-aspect wings)...

I love it when these discussions get more practical and less formulatic (almost as good as the plural for formula, hu ? )

The main runway at KOSU is just over 5000ft. As at most airports, when you're flying closed traffic they want you at 500agl before turning crosswind. On a hot, summer day (using 27L) you can be a full runway-length past the departure end before you're at 500agl... and lucky to be at pattern altitude by the time you turn downwind. On a brisk, winter day, you can be at pattern altitude before Bethel Road (red arrow). Those residents hate summer 

Brett_Henderson wrote on Dec 12^th, 2006 at 9:02pm:


I bet it's more like 2310 or 2320 lbs, assuming you have full fuel, and all the twinkies you can fit without going over 2300 lbs.  So being wet limits aircraft performance by increasing the weight, meaning you have to decrease your twinkie load, and 10 or 20 pounds is 800 or 900 twinkies or so (give or take a few ), so you should wait until it's dry out to fly, so you can carry as many twinkies as possible.

My brain is mush.............

Did you really?

Oh well, I guess studying for the last nine hours has actually made me partially illiterate, maybe I need a break....


The twinkie example still holds true though...

Mobius wrote on Dec 12^th, 2006 at 8:02pm:


Hmmmm... it stands to reason that a cohesive stream of water could force an airplane down (say, if you flew under a waterfall or something), but I dunno about even very heavy rain... never heard of such a thing happening, and planes have been flying in very heavy rain for ages. In that scenario, what usually gets 'em is downdrafts.

Brett_Henderson wrote on Dec 14^th, 2006 at 9:17pm:


You may be onto something with the paint analogy...
but then again, you may be on something pondering this in the first place...

I am actually curious as to what the actual amount would be, but I think (just guessing) that a typical aircraft paint would be heavier per unit of volume than water. I know water is way dense (the oxygen atom is a real fatso), but I dunno... the pigments are probably pretty heavy with oxygen and metals. The enamel would weigh less than water, especially after it dries, but my gut feeling is that paint weighs more.

And water tends to bead up and drip off an airplane, even a dirty one. Hell, probably beads up more on a dirty plane, what with all the oil, etc. so we're not really talking about a nice even coating of water.

But ... eh, I'm bored. How about the "can a plane take off from a conveyor belt" debate?    

And the beading is what I meant by surface tension. I buncch big beads is probably larger, volumetrically, than a very thin coat of paint.. And let's not forget water accumulations where surfaces meet rather sharply..

Even if it IS a whole 20 lbs (more likely 5-10), it's not worth worrying about...

ALTHOUGH.. there is that theoretical weight where you get into trouble.. and plus/minus ten pound when you're near it, might make a difference..

(how bored am I ?)