Definition
A radio navigation error caused when a transmitted signal reflects off the surface of the Earth — particularly hilly or mountainous terrain — before reaching the aircraft's receiver. The reflected signal arrives slightly later or from a different direction than the direct signal, distorting the bearing or position information the receiver displays.
Plain English
An inaccuracy in a navigation signal that happens because the signal bounces off the ground on its way to the aircraft, instead of arriving cleanly in a straight line.
Context Anchor
Seen in instrument flying discussions of space-wave radio signals, especially when flying near rough or mountainous ground.
Derivation
Terrain comes from a Latin word meaning earth or land. Error comes from a Latin word meaning to wander or go astray. Together, the term points to a navigation reading that goes astray because of the land between the transmitter and the aircraft.
Why Pilots Care
Can produce incorrect bearing information that leads to navigation mistakes unless the pilot cross-checks with other sources.
Grounding Statement
Picture a radio signal reaching the airplane after bouncing off a hillside instead of traveling cleanly in a straight path.
Intuition Check
Terrain error does not mean the pilot made an error about the terrain. It means the terrain caused an error in the received radio navigation signal.
Example Sentence 1
Flying through a mountainous area, the instructor pointed out that small fluctuations in the bearing indicator were likely due to terrain error.
Example Sentence 2
Flying higher reduced the terrain error and gave a steadier VOR bearing.