Definition
The height, in feet above the runway threshold, at which an aircraft following the published glidepath of an instrument approach will cross the beginning of the runway. It is shown in the profile view of an instrument approach chart and is referenced to the runway threshold elevation, not mean sea level.
Plain English
How high above the start of the runway you will be when you fly across it on a normal approach.
Context Anchor
Seen on an instrument approach profile view near the runway end, especially when checking the descent path to the runway.
Derivation
Threshold comes from Old English 'therscold,' meaning the doorstep or entry point of a building. The runway threshold is the entry point of the usable landing surface, so the threshold crossing height is simply how high you are when you cross that entry point.
Why Pilots Care
It determines the correct touchdown point, ensures safe clearance over the threshold, and prevents landing short or floating too far down the runway.
Intuition Check
Threshold crossing height does not mean the airplane must always cross the threshold at exactly that height. It is the published planned height for the stated descent path.
Example Sentence 1
The approach chart showed a glidepath angle of 3.00 degrees and a TCH of 50 feet, giving us a standard descent to the runway.
Example Sentence 2
A lower than published TCH can result in landing short of the intended touchdown zone.