Flight Management System — Page 279, Lesson 324

I want to walk you through the Flight Management System now, and we're going to start with the two data bases that live inside the FMC, because everything the system does depends on them. First, the performance data base. Its purpose is to reduce the need for the flight crew to refer to the Cruise Control Manual during flight, and to provide the FMC with the information it needs to calculate pitch and thrust commands. So think of it as the system's memory of how this aircraft performs. All the reference data you would normally need can be displayed on the FMS-CDU — that's the Flight Management System Control Display Unit, the screen and keypad you interact with. The data stored in this data base includes aircraft drag and engine characteristics, and maximum and minimum speeds. Now, here's a nice detail: maintenance can refine the data base for each individual aircraft by entering factors for drag and fuel flow. So the performance data base isn't just generic — it can be tuned to match the specific airframe you're flying. Next, the FMC navigation data base. This one includes most of the information you would normally determine by referring to navigation charts. And that information can be displayed on the FMS-CDU or the AMD — the AMD is the associated display, the map display. This eliminates most of the cockpit chart reading in aircraft without an FMC. The geographic area covered includes all areas where the aircraft is normally flown. The stored information includes the location of navigation aids, airports, runways, and other airline-selected information such as SIDs, STARs, approaches, and company routes. SIDs are Standard Instrument Departures, STARs are Standard Terminal Arrival Routes — the published procedures for leaving and entering the terminal area. Now, here's a really important operational detail about the navigation data base. The FMC contains two sets of navigation data, and each set is valid for 28 days. Each set corresponds to the normal revision cycle for navigation charts. During preflight, the flight crew can select which set is active, and the FMC uses the active set for all navigation calculations. The contents of the navigation data base are updated by maintenance every 28 days. So when the navigation chart revision date arrives, the new data is already in the FMC and ready for selection. That's the beauty of it — you don't have to wait for the effective date; the data is loaded ahead of time, and you just switch to it. Now let's move to the operational procedures, specifically the initial actions. When you apply power to the aircraft, the FMS immediately executes a self-test procedure. Upon its successful completion, the IDENT page of the system is displayed. The IDENT page of the FMC allows the crew to confirm — and that's where the excerpt cuts off, but that's the start of your preflight flow: power on, self-test, IDENT page, and then you confirm what's on that page. Let me show you a figure that illustrates how the FMCs can be configured. That figure shows the FMCs may be decoupled to provide fully Independent Mode operation, though that's not usual in normal operations. It's a configuration you should be aware of, but it's not the standard setup. So to tie it together: the performance data base feeds the pitch and thrust calculations, the navigation data base feeds your routing and position information, and both are managed through the FMS-CDU. The 28-day cycle on the navigation data is your key operational rhythm — you'll be selecting the active set during preflight, and maintenance keeps it current.