What is the source of magnetic variation information in a Flight Management System (FMS)?
Magnetic variation is calculated by each IRS based on the respective IRS position and the aircraft magnetic heading.
The main directional gyro which is coupled to the magnetic sensor (flux valve) positioned in the wingtip.
The FMS calculates MH and MT from the FMC position.
Magnetic variation information is stored in each IRS memory; it is applied to the true heading calculated by the respective IRS.