Finding density altitude for other than surface elevation

[UnAnswerd]

In the Cherokee 140 performance charts, TAS is displayed as being dependent on density altitude. Obviously, density altitude can only be found by knowing the pressure altitude, and the ambient temperature. But when planning a x-country flight, there is apparently no way to determine the actual density altitude for a given cruising altitude. You can get temperatures aloft, but without knowing the pressure altitude, you cannot compute density altitude. Also, it seems like using an altimeter setting to estimate pressure altitudes aloft would be a bad idea. Or would it????

Just wondering...if you cannot determine the DA for your cruising altitude, you cannot determine the TAS, and subsequently cannot really determine ground-speed, ETE, or fuel expenditures. You might have to take a guess, and then fail a checkride miserably...

Obviously I'm missing something here. Any information would be greatly appreciated.

 

[midlifeflyer]

Also, it seems like using an altimeter setting to estimate pressure altitudes aloft would be a bad idea. Or would it????

Um. Why estimate? Why not put 29.92 in the Kollsman window and read the pressure altitude off the dial?

 

[GravityHater]

Um. Why estimate? Why not put 29.92 in the Kollsman window and read the pressure altitude off the dial?

I think he is talking about flight planning. T or F, UnAnswered?

If so, then he wants to know DA at altitude, hours away.
I don't think you can reliably know that to the accuracy you want. BUT I think you could reasonably guess.... and not fail a thing. If you are off 1000', by what % does TAS or GPH change? Probably not a whole lot.
Show the DE you would chose the more conservative of two choices on the chart and I think that would be fine.

 

[UnAnswerd]

I think he is talking about flight planning. T or F, UnAnswered?

If so, then he wants to know DA at altitude, hours away.
I don't think you can reliably know that to the accuracy you want. BUT I think you could reasonably guess.... and not fail a thing. If you are off 1000', by what % does TAS or GPH change? Probably not a whole lot.
Show the DE you would chose the more conservative of two choices on the chart and I think that would be fine.

Yeah, I was talking about flight planning. Indeed, the percentages do not change very much on the performance chart. At 2000', the TAS at 75% power is around 119MPH. At 3000' it's 121MPH. Always using the more conservative estimate seems like a good policy....

Thanks

 

[Vector4fun]

Method #2 ( Just as a guestimate )

Figure the DA at the surface, then add the planned altitude above ground.

Example, if the field elevation at the departure airport was 4000', and DA for departure was 7000', and you planned to cruise at 8500' MSL, then at 7000 + 4500 = 11500'.

And believe me, the typical Cherokee 140 won't get to 8500' MSL in the summertime over El Paso; unless it's in a heck of a thermal. I worked lots of loaded singles that couldn't climb to MEA and had to cancel IFR.

 

[Immelman]

Do the math -- not only does pressure altitude vary 1:1 with density alt, but the change it makes is in the noise, compared to temperature. Pressure altitude varies by hundreds of feet, typically. Just assume standard pressure and go with it; the variance in temperature is far more significant, and more succeptible to forecast error to boot.

 

[A Squared]

Unanswered,

How would you do figure the density altitude in flight?

I'm guessing that you'd turn the knob on hte altimeter until the you had 29.92 in the kollsman window, read the pressure altitude, read the OAT and plug those into your chart, E6b, or whatever and get the density altitude, right?

OK, what's different with flight planning?

You have the temp from the Forecast.

You have the altimeter settig for that point in your route.

The only thing you don't have in front of you is the altimeter so you can twiddle the knob to get the pressure altitude. You can simulate that with arithmetic.

let's say your planned altitude is 7,000' MSL, the altimeter setting at that point in your trip is 30.44. Ss you are probable aware, an inch of mercury is approximately equal to 1000' of altitude, not exactly, but more than close enough for any flight planning purposes. the difference between the indicated altitude and the pressure altitude will be (29.92-30.44)x1000, or -520' So at an indicated altitude of 7000' msl your pressure altitude will be 6480' MSL. Grab your forcast temp and comute Density Altitude.

 

[aucfi]

UnAnswered, just curious. Are the rest of your performance charts done with pressure altitude? Ive never flown a 140 before.

au

 

[UnAnswerd]

et's say your planned altitude is 7,000' MSL, the altimeter setting at that point in your trip is 30.44. Ss you are probable aware, an inch of mercury is approximately equal to 1000' of altitude, not exactly, but more than close enough for any flight planning purposes. the difference between the indicated altitude and the pressure altitude will be (29.92-30.44)x1000, or -520' So at an indicated altitude of 7000' msl your pressure altitude will be 6480' MSL. Grab your forcast temp and comute Density Altitude.

Thanks for the information. I knew you could use that type of procedure to find DA on the surface, but wasn't sure if it could be used to find DA thousands of feet up. Again, thanks for the help.

 

[UnAnswerd]

UnAnswered, just curious. Are the rest of your performance charts done with pressure altitude? Ive never flown a 140 before.

The rest of the charts are done with "standard altitude". To be honest, I don't recall ever seeing this term in a textbook. This is probably the same as pressure altitude, no???

 

[A Squared]

The rest of the charts are done with "standard altitude". To be honest, I don't recall ever seeing this term in a textbook. This is probably the same as pressure altitude, no???

Honestly, I'm not sure. I'm speculating that they are refering to an altitude in the standard atmosphere. ie pressure altitude and temp at standard for that altitude.

 

[100LL... Again!]

You do not need to calculate the DA in cruise.


Look at your cruise perf charts. They will usually account for pressure altitude and temperature. When you use the correct temperature in the table or graph, THAT IS the DA calculation. Any attempts to "calculate" DA then use it in the graph will result in the WRONG values. The chart or table does the DA calculation FOR you.

DA is not a performance number in reality. It is a conceptual number so that pilots can understand how their aircraft will perform RELATIVE to a normal day.

If you want to know your pressure altitude for your planned cruise altitude, you could get very close this way:


Subtract your altimeter setting from 29.92 (the result can be positive or negative), and multiply by 100 to get your correction for pressure altitude in cruise.

If the difference is positive, add that to your planned cruise altitude to get pressure altitude.

If the difference is negative, subtract it from your planned cruise altitude to get your cruise PA.



For destination, do a landing performance calculation using the hottest temp you think could occur at your ETA. That is the best the airlines can do, so that is the best you can do. No one can forecast temperatures to the exact degree, so err on the safe side.


Does this make sense? Do you see why you do not need to make a seperate DA calculation for cruise?

 

[aucfi]

Subtract your altimeter setting from 29.92 (the result can be positive or negative), and multiply by 100 to get your correction for pressure altitude in cruise.

multiply by 1000