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If a flight manual (for a naturally aspirated piston engine plane) gives a figure for a best climb rate at standard temperature and pressure, is there a way to adjust that figure for a different density altitude?

(I'm thinking there must be, but I can't find such a formula anywhere. Assume the same speed, weight, etc.)

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There are several tools which can do what you want. Some electronic flight computers will perform the calculation, and here is a mechanical one which will: https://www.mountainflying.com/products/apr_304

Ed Williams has a graph, which is almost linear, showing the impact on BHP with differing DENALT. See page 6, http://edwilliams.org/smxgigpdf/mfly2.pdf

Essentially ROC is a nearly linear function of excess HP, so you could use the slope of Ed's chart to predict your climb rate at varying DENALT.

Addendum 1: Approximate figures I use for NA aircraft are:

Time Fuel Distance +10% for each 10C

TO roll +10% for each 1000ft DENALT

ROC -10% for each 2000ft DENALT (a conservative number, as 7% is probably more accurate)

ROC -10% for each 20C (also conservative, as 7% is probably closer)

These are just generalized performance figures I use, and I do not supply their derivation, etc. Check them against the POH / AFM or manufacturer test data for your aircraft.

Addendum: This article, while it disagrees with my generalized TO roll estimation, may provide some supplemental understanding: https://www.flyingmag.com/technique/tip-week/calculating-density-altitude-pencil

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  • $\begingroup$ Can you explain why the answer isn't to use the density altitude with the standard table? For instance, if my true altitude is 5kft but the density altitude is 10kft, doesn't that mean the 10kft (at STP) performance figures should be correct as-is? I thought that was the entire point of the calculation. $\endgroup$ – StephenS Jan 3 at 6:16
  • $\begingroup$ @StephenS perhaps the question has some ambiguity. I interpreted that OP had data points for STP and wanted to adjust for different DENALT. While I didn't give an equation, I provided ways the data could be interpolated. Perhaps the question could be clarified. I would be happy to correct or modify the answer accordingly. Not all POH / AFM include performance charts. $\endgroup$ – mongo Jan 3 at 14:31
  • $\begingroup$ mongo thanks you interpreted my question correctly. @StephenS indeed this POH doesn't have that performance table, just a single figure for STP at sea level :( $\endgroup$ – snoopy Jan 6 at 22:27
  • $\begingroup$ Does my answer adequately address your question? $\endgroup$ – mongo Jan 7 at 2:47
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That depends on what the POH for your specific aircraft includes. For example the POH for the PA-28 Warrior has a correction chart for calculating just that. In this case you need to use pressure altitude and local temperature (which combine to density altitude anyway).

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Its important to use the POH for the specific aircraft as things like engine modifications, installed propellors and other changes to the airframe can greatly affect the numbers. These changes may only be reflected in the POH for the airframe its self and a copy found elsewhere may be inaccurate. Correction charts and your POH may also include pertinent information about operational limitations during a climb as well.

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