Takeoff Performance

[JJ Campbell]

Please see the attached Takeoff Performance chart from the Tecnam P92 Eaglet POH.

Suppose, I am sitting at home and can't simply walk out to the AC to set the altimeter to 29.92 to get the Pressure Altitude. But, I know from calling the airport's AWOS-3 what the temperature and density altitude are (as well as the current altimeter setting). Can I use the attached chart to figure out the takeoff distance? If yes, how?

Thanks in advance.
jj

[JJay]

You can follow the same logic as they show in the example, the only thing you have to do is convert Density Altitude to Pressure Altitude.

Pressure Altitude = Density Altitude - (120 * (OAT - ISA))

OAT = outside air temperature (in Celsius)
ISA = standard temperature (in Celsius).

We think of standard temperature as 15 degrees Celsius, but that is only at sea level. From an AOPA page:

It decreases about 2 degrees C (or 3.5 degrees F) per 1,000 feet of altitude above sea level. The standard temperature at 7,000 feet msl, therefore, is only 1 degree C (or 34 degrees F).

[JJ Campbell]

Since the field elevation at my airport is 14 feet, using 15 degrees Celsius should work fine?

[JJay]

Yes - indeed.

[JJ Campbell]

Cool, thank you!

[FastEddieB]

Looking at your chart, it has 3 panels.

It looks to me that the lefthand one is taking Pressure Altitude and OAT to generate Density Altitude that moves laterally to the second panel.

I think since you know the Density Altitude already, you probably don't need that left panel at all.

What's confusing is the left panel's vertical axis is unlabled. I guess it doesn't have to be, but could be calibrated as Density altitude. If it was, that would be a lot clearer.

I'm thinking you could plot your Density Altitude along the ISA line, since the temp and pressure is already accounted for in the Density Altitude you get from AWOS.

But 'tis a puzzler, and I'm none to sure about it - let me think about it a bit more.

Edited to add: I thought the lack of a vertical scale for the left panel was odd. But I just did a search of Takeoff Performance Charts, and although they are de facto using temp and pressure to carry a Density Altitude to the next panel, virtually none I came across was so labeled. I guess I was remembering the charts dedicated to determine Density Altitude, like this one, similar to your lefthand panel but with the vertical axis marked:



Good news is the panel on the right can help with what to do with a non-standard altimeter setting.

[JJ Campbell]

Thanks FastEddie,

If I understand what you are saying correctly, then I can use the nicely labeled ISA+20° line on the left panel. Find the density altitude from the AWOS on that line and then go straight across to the center panel. Yes?
jj

[FastEddieB]

I think you can just use the ISA line, since the +20 is accounted for in the DA provided by AWOS.

But I’d like to hear other’s take on this.

Professor?

[JJ Campbell]

I think you can just use the ISA line, since the +20 is accounted for in the DA provided by AWOS.

It took me a second to see what you meant. I was enamored by having the density altitude labels so close to the ISA+20 line. But, now I see using the center ISA line to plot the density altitude point is actually easier.

[drseti]

Professor?

Yes, young Padawan?

I think you're pretty much on target, Eddie.

[JJ Campbell]

I talked to my CFI about the Eaglet Performance Chart. She agreed that the leftmost panel was converting pressure altitude to density altitude. But, she disagreed that I could start from this panel with density altitude and then move to the center panel. She said that it was not possible to pinpoint density altitude on the left panel since there are no labels or grid lines for it.

When I tried to claim here is density altitude x (using some of the supplied labels), she said that is pressure altitude. Looking at it, I had to agree with her.

So, thinks me, I can use the PA to DA formula to generate a bunch of potential points to see where they hit the center panel and come up with a DA grid for the left panel. However, faced with the small scale of the chart and its woeful lack of granularity, I gave up.

Do you guys agree that I am wasting my time tilting at windmills? (Even if I claim it is all for science? )
jj

[JJay]

I wouldn't call it "tilting at windmills". I applaud your dedication to understanding the performance of your aircraft to that extent. Far too many pilots just shrug and say "well, I usually get off the ground in xxx feet, so I should be good".

Part of me thinks that the low granularity of the charts is due to the fact that the performance figures put out by the manufacturer are not all that precise. But using the charts is far better than not using them. Good for you.

[FastEddieB]

I wouldn't call it "tilting at windmills".

Nor would I.

These questions help exercise brain cells, and bring new insights as to what the charts are actually doing “behind the scenes”.

Anyway, I’m still thinking you can use DA on the ISA line. The logic is...

You’re at Denver. It’s 90°. DA is roughly 8,400’. So if you did have a Standard Day, it would be equivalent to being at 8,400’ in those ISA conditions, right? So why not just find 8,400’ on the ISA line and carry that to the right on the chart?

I think. I remain open to having flaws in my reasoning pointed out.

[TimTaylor]

http://imageserver.fltplan.com/Pressure ... USER=PILOT

Use this to determine pressure altitude, then use your chart.

[JJ Campbell]

Eddie & Tim,
I called my AWOS and used Tim's suggested tool and did it Eddie's way and (allowing for granularity and a -142 PA) the results were darn close. I think we found the magic!
Thanks guys.
jj