# For a fixed density altitude, how does climb performance depend on pressure altitude?

Below is the climb performance from the 1978 Cessna 152 POH, which I've plotted as climb rate versus density altitude. It shows that higher pressure altitude gives decreased performance even when the density altitude is approximately the same. It also gives a different climb speed for each pressure altitude, so it isn't exactly comparing apples to apples. But it identifies this as the best climb speed so it still claims that performance is degraded with higher pressure altitude independently of density altitude.

Is this trend accurate? If so, how and why does pressure altitude affect climb performance for a fixed density altitude?

• Because the loss of climb performance at the higher PA overwhelms the gain in performance from the lower temperatures associated with the unchanging density altitude? Guessing, but that seems consistent with your data.
– Ralph J
Nov 12, 2023 at 23:17
• What type of aircraft does this POH come from? Nov 13, 2023 at 0:53
• @RobertDiGiovanni Whoops. It's from a Cessna 152. Nov 13, 2023 at 3:43
• The differences in climb rate for the same density altitude may be due to differences in pitch and airspeed (slower is more Vx like). Note that increasing density altitude lowers Vy. You can almost use that chart to determine DA from PA and climb performance! Nov 13, 2023 at 13:22
• @ThomasPerry The x-axis here is density altitude. For instance, at 5000 feet pressure altitude the climb rate varies from a little over 400 to more than 500 at essentially the same density altitude as the pressure altitude varies from 2000 feet to 8000 ft. I know that performance is degraded at higher density altitude- I'm asking why it is also degraded at higher pressure altitude/lower temperature for a fixed density altitude. Nov 17, 2023 at 21:43

Perhaps it depends on how you read the graph. If indicated airspeed is held constant, then the graph shows lower climb rate for higher density altitude.

Just stick with one airspeed reading it.

It appears that the listed KIAS is best for that pressure altitude, with the corresponding effect of higher density altitude on climb rate.

This makes sense considering hotter air at the same pressure will be thinner (less dense).

The chart depicts decreasing Vy with higher density altitude.

• Yes, it certainly shows decreased performance with higher temperature at fixed pressure altitude, and that makes sense. My question is about why is also shows decreased performance with lower temperature (higher pressure altitude) at fixed density altitude. Nov 12, 2023 at 22:50
• @Chris always interesting to think whether "higher" refers to pressure or altitude. I believe it refers to what the altimeter reads. So "higher pressure altitude" with the same density altitude would actually mean one is lower than the altimeter is reading, because they are in colder than Standard air. Nov 13, 2023 at 0:49
• @Chris so I don't think the graph is reflecting the same density altitude for all data points. It may be: for a given pressure and density at point A, here is your performance when your altimeter is at 2000, 4000, 6000 etc. Essentially, it is giving best rate of climb Vy under those conditions. Nov 13, 2023 at 0:55
• Density altitude is the x-axis. The points that are clustered together are at different pressure altitudes and approximately the same density altitude. Nov 13, 2023 at 3:43
• @Chris try holding pressure altitude constant for each point/KIAS, and read the points for variable density altitude (corresponding to your climb rate). Aviators (even from the 1970s) read pressure altitude, and calculate density altitude based on temperature deviation from Standard. Nov 13, 2023 at 8:38