Definition
DA has two distinct aviation meanings. (1) Density Altitude — pressure altitude corrected for non-standard temperature. It is the altitude at which the standard atmosphere would have the same air density as the actual conditions. Higher temperatures, higher field elevations, and higher humidity raise density altitude, reducing engine power, propeller efficiency, and wing lift. (2) Decision Altitude — on a precision instrument approach (such as an ILS), the altitude at which the pilot must have the required visual references in sight to continue the approach to landing. If those references are not visible at DA, the pilot must execute a missed approach. DA is referenced to mean sea level (MSL) and read on the altimeter.
Plain English
DA stands for one of two things. Density Altitude is how thin or thick the air really is, expressed as the altitude where the standard atmosphere would feel the same — hot, high, or humid days make the airplane perform as if it were much higher up. Decision Altitude is the height on an instrument approach where you must be able to see the runway environment; if you can't, you go around.
Context Anchor
Seen in aircraft performance planning when DA means density altitude, and on instrument approach charts or approach briefings when DA means decision altitude.
Derivation
Both meanings share the abbreviation DA but come from different uses of the word 'altitude.' Density refers to how packed together the air molecules are; Decision refers to the moment a pilot must decide to land or go missed. The shared acronym is a common source of confusion, which is why aviation texts almost always spell out which DA is meant.
Why Pilots Care
Higher density altitude reduces engine power output, propeller thrust, and wing lift, resulting in longer takeoff rolls and shallower climb gradients.
Intuition Check
Do not assume DA has one fixed meaning. In performance planning it usually means density altitude; in instrument approach minimums it usually means decision altitude.
Example Sentence 1
On a 95°F afternoon at a 5,000-foot airport, the density altitude was over 8,000 feet, so the pilot recalculated takeoff distance before departing.
Example Sentence 2
The pilot checked density altitude before takeoff to confirm the available runway length was adequate for the reduced performance.