Definition
In RNAV operations, system error is the total difference between the aircraft's actual position and the position the navigation system says the aircraft should be in. It is the combined result of three components: path definition error (how accurately the desired path is described in the navigation database), flight technical error (how well the pilot or autopilot keeps the aircraft on that path), and navigation system error (how accurately the equipment determines the aircraft's position).
Plain English
It is the total amount by which an RNAV aircraft can be off the intended track when you add up every source of error in the navigation chain -- the database, the pilot or autopilot's tracking, and the equipment itself.
Context Anchor
Seen in RNAV departure discussions when the procedure or equipment must keep the aircraft within a required accuracy limit.
Derivation
Error comes from the Latin errare, meaning “to wander.” That fits the aviation meaning: the aircraft or its indicated position has wandered away from the intended path. System points to the whole navigation-and-flying setup, not just one instrument.
Why Pilots Care
Determines whether the aircraft can safely fly an RNAV procedure without exceeding allowed position tolerances.
Intuition Check
Do not read system error as meaning the equipment has failed. Here it means the measured or expected difference between the intended RNAV path and the aircraft’s actual position or tracking.
Example Sentence 1
RNAV 1 departure procedures require total system error to remain within 1 NM of the centerline for 95 percent of the flight time.
Example Sentence 2
If system error grows too large, the aircraft may no longer qualify for the published departure procedure.