Plain English Meaning

FMSs are the flight computers that plan and manage where an aircraft flies and how it gets there. The 's' just means more than one — either two computers in the same aircraft, or the systems used across many aircraft.

Full Definition

The plural form of Flight Management System (FMS) — the onboard computer systems that integrate navigation, performance, and guidance data to manage an aircraft's flight path. Each FMS combines inputs from sensors such as GPS, inertial reference units, and radio navigation aids with a stored database of airports, airways, procedures, and waypoints to compute lateral and vertical flight paths, , and . The plural is used when referring to multiple units (often two installed for in a single aircraft) or to FMSs across different aircraft.