Use of Density Altitude

T

turbojet28

Well-Known Member
Hey everyone,

Just sitting around pondering as I study for my PPL, I realize that density altitude gives a general idea of performance increase/decrease, but what is it's practical value in determining actual performance? All performance charts that I have seen (for Cessna's at least) have had performance based on pressure altitude and then another correction column for non-standard temps. What would even be the use of calculating density altitude then? Are the performance sections in the POHs of other makes of aircraft based on density altitude? Thanks!


--turbojet28
 
There are many POHs and AFMs that use density altitude in their performance charts rather than pressure altitude and temperatures. Remember, density altitude is pressure altitude corrected for non-standard temperature. So, regardless of what type of chart it has, you ultimately have to know you density altitude.
 
I thought “Density altitude” was a new show about Paris Hilton going to flight school.
grin.gif
 
I don't have an of my POH or AFMs around right now but I can't think of any with charts that don't consider temp for you.
 
[ QUOTE ]
I don't have an of my POH or AFMs around right now but I can't think of any with charts that don't consider temp for you.

[/ QUOTE ]I have one for a Cherokee 180G. The performance charts are all based on density altitude.

In the Cessnas, although many of the charts take temperature into consideration, the cruise charts tend to have a "standard temperature" column and I generally find it easier to use density altitude (especially since AWOS gives it to you) and the equivalent standard temperature column than extrapolate.

Using density altitude also helps with those places that are off the chart. For example, the CE-172S only gives take off distance to 8000' pressure latitude. So what do I do at Leadville (9927 msl) By converting the chart to density altitudes, I can get the numbers I'm looking for (or at least find out if I'm really off the chart). For example, that 8000' pressure altitude column has takeoff numbers for 40° C. That's 12,400 feet density altitude. So when it's a balmy 20°C at Leadville and the AWOS tells me that the density altitude is 12,600, I at least have something to go on.
 
[ QUOTE ]
I don't have an of my POH or AFMs around right now but I can't think of any with charts that don't consider temp for you.

[/ QUOTE ]

I just read somewhere, and I can't remember where right now, that older AFM/POH's before about the mid 1970's would typically use density altitude in the charts. You then had to separately figure out your density altitude before using that chart. Newer airplane manuals often just list pressure altitudes with a correction for non-standard temperature. Same result either way.
 
I always use it to get a 'feel' for the approach / landing when I get the AWOS inflight; if you know how to figure it out you can get a good estimate in your head, and if you're famaliar with the book's numbers you'll know about how much runway you'll use.
 
You must log in or register to reply here.
Back
Top