Definition
A height value, in feet, representing the difference between the published Minimum Descent Altitude (MDA) for a non-precision approach and the highest runway elevation within the touchdown zone (the first 3,000 feet of the landing runway, sometimes referred to as the touchdown GS or glide slope reference area). It expresses how far above that runway elevation the aircraft will be when it reaches the MDA.
Plain English
How high you will be above the highest part of the touchdown zone of the runway when you level off at the lowest altitude allowed on the approach.
Context Anchor
Seen in instrument approach information, especially when comparing an approach’s minimum altitude to the runway elevation.
Derivation
MDA stands for minimum descent altitude: the lowest altitude a pilot may descend to on certain instrument approaches unless the runway environment is in sight and landing can be made safely. “Touchdown” refers to the part of the runway where the airplane is expected to first land.
Why Pilots Care
Provides the minimum safe altitude for obstacle clearance until the runway environment is visible.
Grounding Statement
Picture the runway first, then picture a flat altitude level above it; this term is the height between those two levels.
Intuition Check
Do not read this as a distance along the runway. It is a vertical height above the runway’s touchdown area.
Example Sentence 1
During the approach briefing, the pilot noted that the MDA above the highest runway elevation in the touchdown GS was 420 feet, giving a clear picture of how high the aircraft would be over the runway at minimums.
Example Sentence 2
Clearance to land was not received until the aircraft reached the MDA above the highest runway elevation in the touchdown GS.