Density Altitude

[jspeed87]

I have a question about take-off distance calculations when density altitude is a factor. Let's say for example, it's a hot and humid day and the airport elevation is 100ft. The density altitude is about 2300ft. When trying to figure out the aircraft's take-off distance using the charts, do you use pressure altitude or density altitude? I always used the take-off distance altitude based on what the density altitude was for that day because that's the altitude the aircraft think's it's at. I just want to make sure I haven't been doing wrong all this time.

 

[gotWXdagain]

What does the chart say to use?

 

[killbilly]

All of the (ridiculous) performance charts for the Arrow II I've been flying are predicated on DA. You have to calculate that first before you can run any other performance numbers.


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[mf_eli92]

 

[JeppUpdater]

What does the chart say to use?

Exactly.

Such as this 172 POH that specifically says pressure altitude.

 

[jspeed87]

Exactly.

Such as this 172 POH that specifically says pressure altitude.

Yes, so using the 172 POH, if the density alititude was 2300ft, would you use the 2000ft pressure altitude or what ever pressure altitude was that day?

 

[Roger Roger]

Yes, so using the 172 POH, if the density alititude was 2300ft, would you use the 2000ft pressure altitude or what ever pressure altitude was that day?

Um...you would not bother calculating density altitude and would use the pressure altitude and row and the temperature column appropriate for conditions. Not to be an • but what stage of training are you at? This is pretty simple stuff and the instructions are literally right on the chart.

 

[MidlifeFlyer]

If you figure out the density altitude in advance of going to the chart, there is no big problem in using that number as the pressure altitude number a bypassing the temperature, especially if you use a buffer. I've been doing that for years, including 20 years in Colorado.

 

[jspeed87]

Um...you would not bother calculating density altitude and would use the pressure altitude and row and the temperature column appropriate for conditions. Not to be an *I don't have the education to emote without using a curse word* but what stage of training are you at? This is pretty simple stuff and the instructions are literally right on the chart.

Commercial. I have done most of my flying at sea level. I always thought density altitude was used for take-off performance calculations. I just didn't understand why the POH mentions pressure altitude when density altitude is what could get you in trouble.

 

[Roger Roger]

Commercial. I have done most of my flying at sea level. I always thought density altitude was used for take-off performance calculations. I just didn't understand why the POH mentions pressure altitude when density altitude is what could get you in trouble.

Umm...because every chart I've ever worked with has a temperature input. That accounts for density altitude.

 

[jspeed87]

So one could either... 1: find pressure altitude(29.92 - current altimeter settting x 1000 add to field elevation) then use that PA and current temperature for the POH chart.
2: Find density altitude using Foreflight or with an E6B, and use the DA for calculating take-off distance with the poh.
Both methods are ok?

 

[Roger Roger]

So one could either... 1: find pressure altitude(29.92 - current altimeter settting x 1000 add to field elevation) then use that PA for the POH chart.
2: Find density altitude using Foreflight or with an E6B, and use the DA for calculating take-off distance with the poh.
Both methods are ok?

Sure, but you have to understand exactly what you're doing and TBH the way you're talking about this and the questions you're asking make me wonder if you really do.

 

[jspeed87]

Sure, but you have to understand exactly what you're doing and TBH the way you're talking about this and the questions you're asking make me wonder if you really do.

I just need some clarification. I know that DA is pressure altitude corrected for non standard temps. I know that it effects performance all around. For example KCDC airport: elevation 5621ft, altimeter setting: 30.23, OAT: 86F, I get a density altitude of 8280. The pressure altitude would be(29.92 - 30.23 x 1000 + 5621) 5311ft. I get two different take-off lengths using those numbers in the POH. Since I did most of my training in Florida, this has never been a issue for until now. Sorry for the noobish questions.

 

[n156499000]

So one could either... 1: find pressure altitude(29.92 - current altimeter settting x 1000 add to field elevation) then use that PA and current temperature for the POH chart.

Using the C172 POH posted earlier, that's the way to do it. Determine the pressure altitude, go up or down the pressure altitude column until you get to your calculated pressure altitude, then move your finger over to the right on the chart to the column with the current temperature and that will give you your takeoff distance. The chart corrects for temperature and determines the density altitude for you and takes in to account the necessary change in performance.

2: Find density altitude using Foreflight or with an E6B, and use the DA for calculating take-off distance with the poh.

But at what temperature are you reading the takeoff data for off the chart? Converting to density altitude you would need to then determine the ISA standard temperature then read the chart by selecting the density altitude in the pressure altitude column then move right to the temperature column corresponding to the ISA temperature to read your numbers.

Both methods are ok?

If the manufacturer gives you charts/tables/graphs that ask you to input either the pressure altitude or the density altitude, then yes. If, like Cessna, Piper, and Diamond (I assume other GA manufacturers as well but those are the ones I have experience in) do and only give you charts that use pressure altitude as an input, you can not use density altitude and get accurate information.

 

[n156499000]

Since I did most of my training in Florida, this has never been a issue for until now.

That's no excuse for not being able to properly calculate takeoff performance and read what is a pretty basic chart. Even at sea level it's possible to not have the performance to make it into or out of an airport. Your instructor should have made sure you knew how to properly read the takeoff and landing data charts and ensure you knew how to use them. If you are a commercial pilot (or even a private working on a commercial certificate) this kind of thing should be old hat. You have been doing it for EVERY flight just like the FARs say you are supposed to do, haven't you?

Sorry for the noobish questions.

There's never ANY reason to be sorry for asking how to properly do anything when it comes to operating an airplane. No matter if you are a brand new student pilot or a crusty old ATP counting days until retirement. However, there is a difference between a "my brain just locked up and I can't remember because my instructor just gave me 800+ pages worth of material to study and 5 days to learn it" question and a "wow, I'm really not prepared for this and I need to seek additional training" question. If it's the later, consider pausing your training if you are working on your commercial or going into the local FBO for a few hours visit with a CFI if you have your commercial, and take some time to get refreshed on how to do takeoff performance calculations. Especially operating out west in the mountains, it could very well save your life.

 

[jspeed87]

Thank you! So what is the point of knowing DA if all the performance charts use pressure altitude? I was taught on hot and humid days to use DA for takeoff distance. I understand now that it's the same thing just that the chart corrects for temperature.

 

[n156499000]

Because if you don't understand density altitude, how it affects your performance, and what causes it to change, you just might not be able to make it over that next mountain or clear the trees as the end of the runway. I've had quite a few students who were surprised that it took longer to climb than they planned or weren't able to climb to their planned altitude because they couldn't connect the dots between high density altitude and aircraft performance.

Raw density altitude calculations especially those made with standard E6Bs aren't exactly accurate. Pressure altitude and temperature are readily available and far more accurate, especially in the 1950s and 1960s when many of these manuals were written.

 

[MidlifeFlyer]

Thank you! So what is the point of knowing DA if all the performance charts use pressure altitude? I was taught on hot and humid days to use DA for takeoff distance. I understand now that it's the same thing just that the chart corrects for temperature.

There is a difference between:

1. Understanding that your aircraft's performance is based on density altitude. That's universal.
2. Understanding the way your aircraft manufacturer decided to get to that end result. That's dependent on the manufacturer.

Having a good understanding of the process also means understanding there are various ways to get there. And how the tools you have work.

• 5+5=10 but so do 7+3 and 4+6.
• Piper uses graphs;Cessna uses tables.
• A chart or table might calculate density altitude by asking you to input pressure altitude and temperature or it might ask you to calculate density altitude in advance and input that.
• On graphs with a standard temperature line, as discussed, one can to a certain degree make it easier by having density altitude in advance and bypassing the temperature input.
• AWOS gives you density altitude directly so if you regularly where it is a factor, you might even create your own chart of guidelines based on density altitude and make the process even quicker.

 

[jspeed87]

Thank you all. appreciate the insight.

 

[z987k]

Thank you! So what is the point of knowing DA if all the performance charts use pressure altitude? I was taught on hot and humid days to use DA for takeoff distance. I understand now that it's the same thing just that the chart corrects for temperature.

Density altitude is pressure altitude corrected for temperature. So when the chart asks for pressure altitude, and the very next thing is temperature(it will be), the chart is doing the DA work in that step.