The Flight Management System, FMS, is comprised of 2 FMGEC—Flight Management Guidance Envelope Computers. Inside each of the FMGEC computers lay the operating systems that manage our A330.
FMGEC: Flight Management. Flight Guidance. Flight Envelope. Computers.
FM: Flight Management—The on-board computerized Dispatcher responsible for flight planning, performance, navigation, and communicating data via the PFDs, NDs, and MCDUs.
FG: Flight Guidance—The on-board, computerized pilot responsible for commanding the autopilot, flight directors, and authothrust.
FE: Flight Envelope—The on-board, computerized Flight Engineer, responsible for computing the flight envelope, maneuvering speeds, reactive windshear detection, and gross weight and CG computations and warnings.
Under normal operations the two FMGECs work together. Data entered into either MCDU (Multipurpose Control Display Unit) is shared with the other computer. They work together, and just like other glass planes there is a master. The selection of the A/P or A/T (first on) will determine which FMGEC becomes that master. If one FMGEC has a problem, the other can handle the operation, but the pilot must switch control on the Switching Panel. No automatic switching of a FMGEC computer.
In the example below, the number 2 FMGC failed and the pilot selects both on 1, and now the First Officers MCDU interacts with the number one FMGEC.
(Airbus) MCDU is a CDU (Boeing)
MCDU: Pronounced the McDoo, is your Multipurpose Control Display Unit, called a CDU (control display unit) on the Boeing. The long-range goal is to get to the destination, and this computer airplane can do that itself, with a little help. By programming the MCDU, the pilot can set up the plane for success to takeoff, climb, manage speed, level off, descend, and fly an arrival to an auto-landing at destination. Flying the plane in this automated manner is all about utilizing Managed Guidance—the operation is fully automated and the computer is directing the plane.
(Airbus) FCU is a MCP (Boeing)
FCU: The Flight Control Unit is nothing more than Boeing’s MCP (mode control panel). This is where the pilots intervene to deviate from their original programmed plan. ATC wants you to slow down, turn to a heading, or gives you an unexpected level off, this how you'll manage the flight. Or, the pilot just wants to fly their plane. When we intervene, we use Selected Guidance—The pilots is commanding the plane.
Normal operations is to fly the plane with a combination of both. Many pilots rely on the managed guidance. Smart pilots takes control and use selected guidance, to keep proficient at their flying skills.