Flight Management System — Page 279, Lesson 330

We're starting the Flight Management System chapter, and I want to walk you through the Control and Display Unit — the CDU. This is the crew's primary interface with the Flight Management Computer System, so let's get the layout and key functions firmly in your head. First, the CDU itself is a lighted switch panel. The keys on it perform various functions, and they break down into three major groups. Memorize these three: Alphanumeric, Function and Mode, and Line Select Keys, often abbreviated LSK. Let's take them in order. The Alphanumeric Keys are the letter and number keys. Their function is simple: pressing any alphanumeric key results in that character appearing in the scratch pad. The scratch pad is the entry line at the bottom of the CDU screen where you build up an entry before you commit it to a field. Next, the Function and Mode Keys. These are used for three broad jobs: initializing the system, accessing flight planning functions and status, and modifying the flight plan. They also let you select climb, cruise, or descent information for preplanning or modification. Now, each of these keys has a specific label and a specific job, and I want to go through them one by one because they're the heart of the unit. First, CLB, which stands for climb. Pressing CLB displays the current or alternate climb mode for assessment and selection. So you can see what the system is currently doing in the climb, or you can select an alternative. From this page, the cruise altitude is enterable, and so is a speed/altitude restriction — meaning you can constrain the climb with a specific speed or a specific altitude limit. Next, CRZ, which stands for cruise. This displays the current or alternate cruise mode for assessment and selection. And here's where the performance data lives: you get information about optimum altitude, step-climb savings, and turbulence penetration N1 targets. Let me unpack those. Optimum altitude is the altitude at which the aircraft achieves best fuel economy for the current conditions. Step-climb savings tells you how much fuel you'd save by climbing to a higher, more efficient altitude. And turbulence penetration N1 targets — N1 is the low-pressure compressor speed, essentially the engine fan speed — gives you the engine power setting to use when flying through turbulence to protect the engine. Then DES, which stands for descent. Like the others, it displays the current or alternate descent mode for assessment and selection. The target speed is enterable, and again a speed/altitude restriction can be set. But here's the new material: the descent page also provides information on Flight Path Angle, abbreviated FPA, Vertical Speed, abbreviated V/S, and Vertical Bearing, abbreviated V/B. All of this information is provided for crew reference. So FPA is the angle of the descent path relative to the horizontal, V/S is the rate of climb or descent in feet per minute, and V/B is the vertical bearing — the vertical angle to a point. These give you the tools to manage the descent precisely. Next, INIT REF, which stands for initialization/reference. This key allows access to the data pages required for start-up of the FMCS and the IRS. FMCS is the Flight Management Computer System, and IRS is the Inertial Reference System — the gyro-based system that provides attitude and position data. So this is where you initialize the systems before flight. Additionally, the operator may select various reference data and maintenance pages from here. Then we have N1 LIMIT. This key permits manual command of the active N1 limit, and selection of any Reduced Climb N1 Limit that may apply. N1, as I mentioned, is the low-pressure turbine/compressor speed — the fan rpm. So this key allows the crew to select an engine rpm limit. The reduced climb limit is a derated power setting used to save engine wear. The excerpt notes this allows the crew to select an engine, "LP Turbine", rpm — LP meaning low pressure. Now, an important operational note: the N1 Limit key may be shown as a menu key on the master, or left-hand, CDU, and it can be used to find data within the system. So on the left CDU, that key position functions as a menu access point. Next, DEP ARR, which stands for departures/arrivals. This is used for selection of the procedures and runways at the origin and destination airports. So you pick your departure procedure and runway at the origin, and your arrival procedure and runway at the destination. Then RTE, which stands for route. This key permits flight plan data entries and is a primary means for lateral flight plan alteration. Lateral means horizontal — so changing the horizontal path of the flight, the waypoints you fly over. Next, LEGS, which stands for route legs. This displays and accepts entries of detailed data concerning each leg of the flight plan, for both the lateral and vertical paths. So a leg is one segment of the route between two waypoints, and this page lets you see and modify the details of each one, both horizontally and vertically. Finally, HOLD. This key permits planning or initiation of holding at a designated waypoint. Holding is the racetrack pattern you fly while waiting for clearance, and this key lets you set that up at a specific waypoint. So to tie it together: the CDU is your window into the FMS. Alphanumeric keys feed the scratch pad, the Line Select Keys — which we'll cover next — let you pick fields on the screen, and the Function and Mode keys give you access to every major function: climb, cruise, descent, initialization, engine limits, departures and arrivals, route, legs, and holding. Each one has a specific page and a specific set of data it manages.