Definition
The height of the Decision Height (DH) or Minimum Descent Altitude (MDA) above the highest runway elevation in the touchdown zone (the first 3,000 feet of the runway). HAT is published on instrument approach charts for straight-in approaches and is expressed in feet above that touchdown zone elevation, not above the ground generally or above sea level.
Plain English
How many feet you are above the runway's touchdown zone when you reach the lowest altitude the approach allows you to descend to before you must see the runway.
Context Anchor
Seen on instrument approach charts, especially in the landing minimums area for ILS and other straight-in approaches.
Derivation
“Touchdown” comes from the aircraft touching down, or making contact with the runway. In this term, it does not mean the exact spot where your wheels will touch; it points to the runway’s touchdown zone elevation, the height reference used for approach minimums.
Why Pilots Care
It establishes the minimum height at which the pilot must acquire visual references to continue the landing.
Intuition Check
Do not read “touchdown” as the exact place your airplane will land. Here it means height above the runway’s published touchdown zone elevation reference.
Example Sentence 1
On this ILS, the DH is 1,123 feet MSL with a HAT of 200 feet, so at minimums we'll be 200 feet above the touchdown zone.
Example Sentence 2
At the published height above touchdown the runway environment came into view and the pilot continued to land.