Definition
A significant deviation from an aircraft's assigned route, typically defined in oceanic and remote airspace as a lateral track deviation of 10 nautical miles or more from the cleared flight path. Gross Navigation Errors are reportable events that are investigated by the relevant air traffic authority because they compromise the lateral separation that keeps aircraft safely apart in airspace without radar coverage.
Plain English
A large off-track error -- the aircraft has wandered far enough from its cleared route to be a safety concern, and the event has to be reported and looked into.
Context Anchor
Seen in oceanic, remote-area, and long-range navigation procedures, where aircraft may be far from radar coverage and must follow assigned routes very accurately.
Derivation
Gross' here comes from the Latin 'grossus,' meaning large or thick. In this context it means 'large in size' -- not 'disgusting' as in everyday speech. So a Gross Navigation Error is simply a large navigation error, big enough to matter.
Why Pilots Care
Such errors can cause loss of separation from other traffic, safety violations, or formal incident reporting requirements.
Grounding Statement
Think of it as being far enough off the assigned route that air traffic control can no longer treat the aircraft as being where it was expected to be.
Intuition Check
Gross does not mean gross weight here. It means a large or serious navigation error.
Example Sentence 1
After the crew loaded the wrong waypoint into the flight management system, the aircraft drifted 12 miles off track, triggering a Gross Navigation Error report.
Example Sentence 2
ATC instructed an immediate turn after confirming the aircraft had made a gross navigation error.