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
It tells you how high above the start of the runway you'll be when you reach the lowest altitude the approach lets you descend to before deciding whether to land.
Context Anchor
Seen in instrument approach landing minimums, where it helps connect the published minimum to the runway you intend to land on.
Derivation
A threshold is an entrance. In aviation, the runway threshold is the beginning of the part of the runway available for landing. That makes HAT the height above that runway entrance point.
Why Pilots Care
Determines the exact point at which a pilot must decide to land or go around, directly affecting safety on low-visibility approaches.
Grounding Statement
Picture an invisible level line above the runway threshold; HAT is the height of that line above the runway entrance.
Intuition Check
Threshold does not mean a tolerance or limit here; it means the beginning of the runway available for landing. HAT is measured above that runway point, not above sea level.
Example Sentence 1
The ILS approach showed a DA of 1,123 feet with a HAT of 200, so the pilot knew she'd be 200 feet above the runway threshold when she had to decide whether to land or go around.
Example Sentence 2
At 100 feet HAT the runway environment came into view and the pilot continued to land.