Flight Management System — Page 279, Lesson 327

Let me walk you through the Flight Management System chapter. We're starting with the summary of the initial pages you'd see on a Boeing series aircraft currently in use. Remember, this information can vary from company to company, because operators modify the system for their own use. First, the IDENT page. This is the identification page. It shows the aeroplane model and configuration, the operational programme identifier, drag and fuel flow factors, and the navigation data base identifier and cycle. So the drag and fuel flow factors are correction factors the company enters to match the actual performance of their specific aeroplane. The navigation data base identifier and cycle tells you which version of the navigation database is loaded and its effective cycle. Next, the POS INIT page — that's position initialisation. It displays the IRS position reference, the IRS heading reference, and the GMT, UTC, and time zone display. IRS stands for Inertial Reference System, which provides your position and heading reference. Then the RTE page — the route page. It shows the origin airport, the destination airport, the flight number, and route activation. Then the PERF INIT page — performance initialisation. This one has a lot: gross weight, fuel quantity, zero fuel weight, fuel reserves, cost index, cruise altitude, and on the B747 specifically, spare fifth engine carriage, plus altitude step size. Now let's get into the operational procedures for climb vertical navigation, which is abbreviated VNAV. This is where the cost index really matters. Entering a cost index of ZERO provides economy speeds representing a maximum range cruise. So a zero cost index means the system will fly at the speed that gives you maximum range. The VNAV profile that the FMC commands is a climb with climb thrust, to remain within all airspeed and altitude constraints that are part of the SID entered into the active route. SID is the Standard Instrument Departure. So the FMC climbs at climb thrust, respecting the speed and altitude restrictions built into your departure procedure. Then, after that, it climbs at economy speed to the entered cruise altitude. 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 message you'd see on the FMS-CDU. The pilot must then select a different speed on the FMS-CDU that provides a steeper climb angle. So you're trading speed for climb gradient to satisfy the constraint. Once you reach cruise altitude, the FMC commands cruise at economy speed until the top of descent point, abbreviated TOD. And a number of cost index modifications are allowed until you pass that top of descent point. For example, Long Range Cruise, abbreviated LRC, and "selected speed" cruise may also be entered. So you have flexibility to change your cost index strategy during the cruise phase, but only up until TOD. Let me make sure you've got the key relationships. The cost index of zero gives you maximum range cruise economy speeds. The VNAV climb profile respects SID constraints, then climbs at economy speed. If a constraint would be violated, you get UNABLE NEXT ALT and must select a steeper climb speed. At cruise, economy speed until TOD, and you can modify the cost index up to TOD with options like LRC or selected speed cruise. That's the core of the Flight Management System summary and the climb VNAV procedure.