Definition
A condition in which an aircraft's vertical distance above the ground or obstacles has decreased to a level that no longer provides an adequate safety margin, triggering an alert from a terrain awareness system such as a GPWS or TAWS.
Plain English
The aircraft is getting too close to the ground or to something on the ground, and the warning system is telling the pilot to climb or change course before it becomes dangerous.
Context Anchor
Seen in terrain alerting system discussions, especially when an onboard system warns that the aircraft’s height and path are putting it too near terrain.
Derivation
Terrain comes from the Latin word terra, meaning earth or land. Clearance comes from the idea of being clear of something, meaning there is enough open space. Together, terrain clearance means having enough space between the aircraft and the ground.
Why Pilots Care
Controlled flight into terrain (CFIT) is one of the leading causes of fatal accidents. A terrain clearance alert is often the last warning a crew receives before impact, so recognizing and responding to it immediately is critical.
Analogy
It is like driving toward a low bridge. If there is not enough space between the top of the vehicle and the bridge, the clearance is unsafe.
Grounding Statement
If the aircraft is descending toward rising ground and there is not enough altitude to pass safely over it, the terrain clearance is unsafe.
Intuition Check
Clearance here does not mean permission from air traffic control. It means physical space between the aircraft and the terrain; unsafe means that space is not enough.
Example Sentence 1
The TAWS alerted the crew to unsafe terrain clearance during the descent, and the pilot immediately initiated a climb.
Example Sentence 2
The pilot immediately climbed to restore safe altitude after the system detected unsafe terrain clearance.