Calculating Density Altitude

MattXJ7

MattXJ7

Well-Known Member
Hello JC,

My private checkride is coming up and the examiner has asked me to come ready with some performance data...

One of the requirements is to "Determine density altitude at your highest cruising altitude."

Seems a little difficult seeing as though I don't know the temperature/pressure setting at 11,500MSL along my route. Maybe i'm missing something though, any help is appreciated! :beer:

Thanks
 
Your highest cruising altitude is 11500 MSL on your cross country? That's intense.

I guess you could use a winds and temp aloft chart to get the temp enroute and it shouldn't be to hard to find a altimeter setting to find pressure alt.
 
Maybe go with ISA until you can get data for the time of flight.

I imagine the examiner would be more interested in seeing how you derived the DA rather than actual DA for one flight.
 
I would just find out the pressure altitude on the ground and then apply whatever conversion factor you have to the altitude you'll be flying - that would be your pressure altitude aloft...

Then I would use the standard lapse rate (2 degrees celsius per 1,000') to find out my estimated temperature aloft (unless there was a PIREP that gave the temperature for your altitude)....

Now you have a pressure altitude and temperature to find out your DA - either use the chart or formula method to figure it out.
 
For your cruise altitude, take this formula

Pressure Altitude + (Current Temp - Standard Temp)*120 = DA

So in this case you may have to do a bit of extrapolating to determine your current temp at 11,500 the morning of the check ride from the winds aloft.

Assuming 2 deg C per 1,000' of altitude, your standard temp at 11,500 should be -8c.

A lot of times when asking questions such as these they are looking to see if you understand how to calculate DA as well as if you can apply it to the aircraft performance. It also often leads into questions of higher DA = what with regards to climb performance, takeoff distances, and what you would do if your calculated numbers indicated that you couldn't actually climb that high or your rate of climb would not be sufficient to reach 11,500'. They may also ask what you would do to increase takeoff performance if you found that your performance figures indicate marginal room for error (ie: takeoff in early morning or late night, reduce weight, take less fuel, etc.) Depending on where your at in the country, on a really hot day in the southwest desert region this can be difficult to impossible in reality.
 
Easy, either get a DUAT report or use the standard laspe rate. (2 degrees C for every 1,000 foot. Or 4.4 degrees F for every 1,000 foot.
 
Ayork62493 said:
Easy, either get a DUAT report or use the standard laspe rate. (2 degrees C for every 1,000 foot. Or 4.4 degrees F for every 1,000 foot.
Click to expand...

.______. if he is using standard lapse rate for calculating Density altitude all he is actually calculating is pressure altitude.
 
LV-ARG said:
.______. if he is using standard lapse rate for calculating Density altitude all he is actually calculating is pressure altitude.
Click to expand...

I didn't intend to tell him how to calculate pressure altitude, all I wanted to give him were the lapse rates. If you know pressure altitude, you can find density altitude easily. Hell, I very rarely see altimeter settings any higher than 3020 or 2970 where I live. So figure you've got 300 feet either way. If you use a standard E6B that's hardly even noticable...
 
Ayork62493 said:
I didn't intend to tell him how to calculate pressure altitude, all I wanted to give him were the lapse rates. If you know pressure altitude, you can find density altitude easily. Hell, I very rarely see altimeter settings any higher than 3020 or 2970 where I live. So figure you've got 300 feet either way. If you use a standard E6B that's hardly even noticable...
Click to expand...

my bad. sry about the smart ass coment
 
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