The altitude which determines an airplane's performance

As a pilot, there are several altitudes we encounter. PHAK Chapter 8 lists five of them, and each one answers a different question.

The five altitudes

• Indicated -- The altitude displayed on the face of the altimeter.
• True -- The height above sea level. It's commonly used on charts to indicate the height of terrain and obstacles.
• Absolute -- The height above the ground, also referred to as AGL (above ground level).
• Pressure -- What the altimeter reads with 29.92 in the Kollsman window. When in flight, it is the indicated altitude corrected for non-standard pressure.
• Density -- Pressure altitude corrected for non-standard temperature.

Why density altitude

Lift, drag, thrust and horsepower all come down to atmospheric density. Heat reduces density. The altimeter doesn't accurately reflect temperature changes, even though it materially impacts performance. As a matter of fact, PHAK Chapter 11 says the airplane "will perform as though it were operating at an altitude equal to the existing density altitude."

Take an airport with an elevation of 1,000 feet on a 95°F afternoon with 29.92 on the altimeter. Density altitude works out to roughly 3,600 feet. You should expect a longer takeoff roll and a slower climb than a takeoff with temperature below freezing.

"But my POH charts use pressure altitude"

They do -- and PHAK even calls pressure altitude "important as a basis for determining aircraft performance." Now look across the top of the chart. You'll find columns for outside air temperature. Pressure altitude plus temperature gets you density altitude, so the chart is doing the calculation for you.