Definition
An integrated airborne navigation system that combines inputs from multiple sensors -- such as inertial reference units, GPS, VOR/DME, and air data computers -- and presents a single, blended navigation solution to the pilot and to other onboard systems. It serves as the central point from which position, track, and guidance information is distributed to the flight management system, autopilot, and cockpit displays.
Plain English
A system on the aircraft that pulls together position information from several different sources, decides on the best overall answer, and feeds that single answer to the rest of the cockpit.
Context Anchor
Seen in FMS and instrument cockpit discussions when selecting which navigation source is feeding the course display or autopilot.
Derivation
Master' here means the controlling or central source -- the system that other systems take their reading from. Combined with 'navigation system,' it describes the central navigation source feeding the rest of the aircraft.
Why Pilots Care
Selecting the correct master navigation system prevents the autopilot from following erroneous position data that could lead to course deviations or airspace violations.
Intuition Check
Do not read “master” as meaning “best” or “most accurate.” Here it means the navigation source currently selected to be in control.
Example Sentence 1
After takeoff, the crew confirmed the master navigation system was using GPS as its primary position source.
Example Sentence 2
When the FMS detected a GPS fault, the crew switched to the inertial reference unit as the new master navigation system.