Definition
DA/HAT is a paired value shown on instrument approach charts for precision and approach-with-vertical-guidance procedures. DA (Decision Altitude) is the altitude, expressed in feet above mean sea level (MSL), at which the pilot must decide whether to continue the approach to landing or execute a missed approach. HAT (Height Above Touchdown) is the same point expressed in feet above the elevation of the touchdown zone of the runway. Both numbers refer to the same physical point in space; they are simply measured from two different references.
Plain English
Two numbers describing the same decision point on an approach. One tells you the altitude on your altimeter at that point; the other tells you how high you are above the runway you're aiming for.
Context Anchor
Seen in instrument approach minimums, including approach plates and radar approach figures, where landing minimums are listed.
Derivation
Decision Altitude names exactly what it is: the altitude at which a decision is made. Height Above Touchdown is literally the height above the touchdown zone. The pairing exists because altimeters read MSL, but pilots also need to know how high they actually are above the runway.
Why Pilots Care
It marks the exact point where continuing without visual contact becomes unsafe and a missed approach must begin.
Intuition Check
Do not treat DA and HAT as interchangeable. DA is the number you fly on the altimeter; HAT is how high that point is above the touchdown area.
Example Sentence 1
The controller advised the pilot that the DA/HAT for the ILS approach was 1,250 feet MSL / 200 feet above touchdown.
Example Sentence 2
Before starting the final descent the controller confirmed the DA/HAT so the crew knew exactly when to decide.