Density Altitude

azaviator08

New Member
An accident happened recently to some people I knew related to density altitude. It made me start thinking a lot about density altitude and performance charts. I understand that we need to figure out pressure altitude in order to figure out takeoff and landing distances. Now when we are at high density altitude airports, do we use pressure altitude or density altitude. I am just wondering how density altitude comes into play with our performance charts. For example we are at X airport with a density altitude of 6,000 feet. How would we figure out the takeoff and landing distances at that airport. I understand pressure altitude and density altitude im just wondering how this would correspond in our performance calculations.
 

fish314

Well-Known Member
Depends on your aircraft. Check the POH.

In the aircraft that I fly, there is a correction for temperature and another correction for pressure altitude. Now, as you probably already know, correcting for temperature and then correcting for pressure is the equivalent of correcting for density.

In general, as density altitude goes up (air density goes down) performance of the engine decreases, and the wing produces less lift for the same TRUE airspeed.

These two things combine to cause take off and landing distance to increase, climb out performance to decrease, etc., etc.

But to figure out exactly how much, you need to consult the charts for your aircraft. There will probably be a correction factor of some kind on the chart.
 

Baronman

Well-Known Member
Your performance charts (in most GA aircraft) will account for higher than normal field elevations and different temperatures. I don't know that they always account for non-standard outside pressure.

You could account for pressure altitude on your own. You could either run out to the aircraft and set the altimeter to 29.92'' and see what the altimeter reads or you could do the simple calculation yourself.

If it's lower than standard pressure (bad performance), let's say 29.82'', when you put 29.92'' in the altimeter, it will "think" the plane is higher..in this case 100' higher. So (at sea level) when looking at your performance charts, you would use 100' as your field elevation (due to the non-standard pressure).

Then, from there most of the charts account for temperature on the graphs.

Sometimes people get confused with the pressure calculation I showed above. Just remember that if it's low pressure outside, the plane will "think" that it's at a higher elevation so you have to add. If you want to be conservative just add 1000' to whatever your field elevation is, that would account for an altimeter so low that maybe only a hurricane could cause such low pressure.
 

sdfcvoh

This is my Custom Title
If you really want to be safe, use the current DA as the base altitude for all of your performance figures. I've taken off from Tucson (2500') with DA very near 7500'. Believe me, the airplane behaves as if it is at 7500' because in all actuality, it is.

We fly in the atmosphere surrounding the airplane, and DA is a very real number to be aware of. Be safe out there!
 

Blackhawk

Well-Known Member
Almost all performance charts are DA charts, although you enter them using PA. You enter the chart at PA (NOT airfield elevation), then move over to a temperature. Since DA is PA adjusted for non-standard temperature... you then are computing performance based upon DA.
Also, remember that 15 degrees is standard temperature at SEA LEVEL ONLY. At 4000', for example, standard temperature would be 7 degrees, not 15.
The following in order affect DA:
1. Temperature.
2. Pressure.
3. Humidity.
Only once have I seen #3 have a major impact on performance (Kuwait City in the summer).
Finally, when computing performance, be sure to use maximum temperature for your flight period, not current. There may be a huge difference.
 

killbilly

Vocals, Lyrics, Triangle, Washboard, Kittens
Curious...

Was this the flight with the CFI, his student, and the student's wife? The one where they'd had some engine issues on landing at the field and had to tow it over to the fueling station? Story about it in one of the Arizona papers - I think it was a Cardinal - 177B.

If it's the same flight I am thinking about, I was studying that one closely with another CFI. At the time, only the preliminary report had been issued.
 

jynxyjoe

The Kickin' Chicken!
With Cessna's I always converted field elevation directly to DA and then used the standard temp column for performance stuff.

Other airplanes had the skwiggly line thing.
 

azaviator08

New Member
Yes that is the accident that i am talking about. Very sad :( By talking to other CFI's the only conclusion that we could come up with would be density altitude. But, we are not sure.
 

killbilly

Vocals, Lyrics, Triangle, Washboard, Kittens
Yes that is the accident that i am talking about. Very sad :( By talking to other CFI's the only conclusion that we could come up with would be density altitude. But, we are not sure.
I remember talking with my CFI about that accident. We both looked over the report. He seemed to think, in the absence of other information, coupled with the engine dying when they flew in, that DA and engine leaning might have been factors.

But, we both agreed that any further speculation would be useless until the NTSB filed the final report.

You mentioned that you knew them. I'm sorry for your loss - I read the articles about that crash and they were very poignant.
 

Blackhawk

Well-Known Member
With Cessna's I always converted field elevation directly to DA and then used the standard temp column for performance stuff.

Other airplanes had the skwiggly line thing.
If you look at the Cessna charts, they say to enter at PA, not DA. You are essentially figuring performance for DA, however, when you read across to the current temperature. Older POHs instruct you to adjust your results for non-standard temperatures.
 

MidlifeFlyer

Well-Known Member
If you look at the Cessna charts, they say to enter at PA, not DA.
They do. He's suggesting a shortcut in which you calculate DA first and then enter at that altitude and don't adjust for non-standard temperature.

I think if you run the calculations out both ways, there are some differences in the result, but they're not that significant.
Older POHs instruct you to adjust your results for non-standard temperatures.
Similarly, the shortcut here would be to figure out DA first and not adjust the results.
 

Blackhawk

Well-Known Member
They do. He's suggesting a shortcut in which you calculate DA first and then enter at that altitude and don't adjust for non-standard temperature.

I think if you run the calculations out both ways, there are some differences in the result, but they're not that significant.
Similarly, the shortcut here would be to figure out DA first and not adjust the results.
That's fine... but then you must remember what standard temperature is for your altitude. I've seen many pilots, who when asked, reply that standard temperature for my airport is 15 degrees C. The airport is at 4100', so standard temperature is about 7 degrees C, not 15. By entering at PA and reading across to the current temperature (on newer charts), you get predicted performance. Also, this teaches a bad habit that pilots might try to transfer to other charts where it could give bad information. If you read the chart, do what it tells you to do, make adjustments that it tells you to make, ensure you are using the configuration specified in the chart, you won't be wrong. Start taking short cuts and you may mess it up when faced with a non-standard situation.
 
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