Definition
The vertical distances between an aircraft and the terrain or obstacles below it, established by published minimum altitudes on instrument procedures to ensure safe separation from the ground and obstructions.
Plain English
The safe gap between the aircraft and the ground or anything sticking up from it, built into the altitudes pilots are told to fly.
Context Anchor
Seen in instrument flying when using cold-temperature correction tables to make sure published altitudes still keep the aircraft safely above terrain and obstacles.
Derivation
Terrain comes from the Latin terra, meaning earth or land. Clearance comes from clear, meaning free from obstruction. Together, the words point to the clear space needed between the aircraft and the land below, not to permission from air traffic control.
Why Pilots Care
Without applying the required corrections, actual terrain clearances become less than indicated, raising the risk of controlled flight into terrain.
Grounding Statement
On a very cold day, an aircraft may be physically lower over the ground than its altimeter reading makes it appear.
Intuition Check
Do not read clearances here as ATC permission. In this context, terrain clearances means safe vertical space above the ground or obstacles.
Example Sentence 1
On a cold day in mountainous terrain, the crew applied the ICAO cold temperature correction to preserve the required terrain clearances on the approach.
Example Sentence 2
At minus twenty degrees Celsius the aircraft's true terrain clearances were lower than the indicated altitude suggested.