Definition
The height of the Decision Altitude (DA) or Minimum Descent Altitude (MDA) above the highest runway elevation in the touchdown zone (the first 3,000 feet of the landing runway). HAT is published on instrument approach charts for straight-in approaches and is expressed in feet.
Plain English
How far above the runway you actually are when you reach the lowest altitude the approach lets you fly down to. It tells you, in plain feet, how close to the runway your minimums put you.
Context Anchor
Seen on instrument approach charts, usually in parentheses next to a straight-in landing minimum.
Derivation
Straight description: 'height above touchdown' literally means how high you are above the part of the runway where the wheels are expected to touch down. The reference point is the runway, not sea level.
Why Pilots Care
It sets the exact height at which the pilot must decide to land or go around when flying an instrument approach in low visibility.
Grounding Statement
On an approach chart, HAT is the “how high above the touchdown area” number for that runway.
Intuition Check
Do not read HAT as your aircraft’s current height above the runway. It is a published chart value showing how high the approach minimum is above the runway touchdown area.
Example Sentence 1
With a HAT of 200 feet and a reported ceiling of 100 feet, the pilot expected to go missed at decision altitude.
Example Sentence 2
At 250 feet HAT the pilot acquired the runway lights and continued to landing.