Flight Management System — Page 279, Lesson 327

We're starting a new topic now: the Flight Management System, or FMS. I want to walk you through the initial pages you'd see on a Boeing aircraft, and then we'll get into the climb profile logic. First, the summary of the initial pages. These are the pages you'll see on the Boeing series currently in use, but remember, companies modify these for their own use, so the information may vary. Let's go through them in order. The IDENT page. This is the identification page. It shows the aeroplane model and configuration, the operational programme identifier, the drag and fuel flow factors, and the navigation data base identifier and its cycle. So this page tells the system exactly what aircraft it's flying and which software and navigation database are loaded. Next, the POS INIT page — that's the position initialisation page. It shows the IRS position reference, the IRS heading reference, and the GMT, UTC, or time zone display. IRS stands for Inertial Reference System, which gives the aircraft its position and heading. This page lets you verify and initialise those references. Then the RTE page — the route page. It shows the origin airport, the destination airport, the flight number, and route activation. This is where you build and activate your flight plan. Finally, the PERF INIT page — the performance initialisation page. This one has a lot on it: gross weight, fuel quantity, zero fuel weight, fuel reserves, cost index, cruise altitude, and then two Boeing 747-specific items — spare fifth engine carriage, and altitude step size. So this page sets up the performance parameters the FMC will use to compute the flight profile. Now let's get into the operational procedures for climb vertical navigation, or VNAV. This is where the cost index comes into play. Entering a cost index of "ZERO" provides economy speeds representing a maximum range cruise. So a zero cost index means the FMC will fly at the speed that gives you the best range — maximum range cruise. Here's the VNAV profile the FMC commands. It's a climb with climb thrust, and it must remain within all airspeed and altitude constraints that are part of the SID entered into the active route. SID is the Standard Instrument Departure — the published departure procedure. So the FMC climbs at climb thrust, respecting the speed and altitude restrictions in that SID. Then, after those constraints, it climbs at economy speed to the entered cruise altitude. Now here's an important failure condition. If, when flying the climb speed profile, it would cause a violation of an altitude constraint, the UNABLE NEXT ALT message is displayed. That's the exact message. The pilot must then select a different speed on the FMS-CDU — that's the Control Display Unit — that provides a steeper climb angle. So the fix is to slow down to climb steeper. Once you reach cruise altitude, the FMC commands cruise at economy speed until the top of descent point — the TOD. And here's a key limitation: a number of cost index modifications are allowed until passing the top of descent point. For example, Long Range Cruise, or LRC, and "selected speed" cruise may also be entered. So you can change the cost index and select different cruise modes, but only up to the TOD. After that point, you've passed the window for those modifications. Let me tie that together. The cost index of zero gives you maximum range cruise. The FMC climbs with climb thrust, obeying the SID constraints, then economy speed to cruise altitude. If a constraint would be violated, you get UNABLE NEXT ALT and you must pick a slower speed for a steeper climb. At cruise, it's economy speed until TOD, and you can modify the cost index — like switching to LRC or selected speed — but only before you pass TOD. That figure shows the FMCs decoupled for fully independent mode operation — which is not usual in normal operations, but it's worth knowing that the system can be configured that way.