Flight Management System — Page 279, Lesson 330

I want to walk you through the Control and Display Unit — the CDU — which is the crew's primary interface with the Flight Management System. Think of it as the keyboard and screen through which you talk to the FMS. The keys on its lighted switch panel break down into three major groups: alphanumeric keys, function and mode keys, and line select keys, which we abbreviate as LSK. Let me define each group. The alphanumeric keys are the letters and numbers. Pressing any alphanumeric key results in that character appearing in the scratch pad — that's the small entry line at the bottom of the CDU screen where you build up your input before you commit it. The function and mode keys are used for 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. The line select keys, the LSKs, are the keys along the sides of the screen that let you move data from the scratch pad into a specific field, or select a line of data. Now let's go through the function and mode keys one by one, because each has a specific job. First, CLB — that's climb. It displays the current or alternate climb mode for assessment and selection. From here, cruise altitude is enterable, as is a speed or altitude restriction. So if you want to plan a climb with a particular speed limit or altitude constraint, this is where you do it. Next, CRZ — cruise. It displays the current or alternate cruise mode for assessment and selection. Beyond that, it gives you information about optimum altitude, step-climb savings, and turbulence penetration N1 targets. Optimum altitude is the most fuel-efficient cruise altitude for your current conditions. Step-climb savings tells you how much fuel you'd save by climbing to a higher altitude. And turbulence penetration N1 targets — N1 is the low-pressure turbine rotor speed, expressed as a percentage — gives you the engine speed target to use when flying through turbulence. Then DES — descent. It displays the current or alternate descent mode for assessment and selection. Target speed is enterable here, as is a speed or altitude restriction. It also provides Flight Path Angle, abbreviated FPA, Vertical Speed, abbreviated V/S, and Vertical Bearing, abbreviated V/B — all provided for crew reference. Flight Path Angle is the angle of your descent path relative to the horizontal, Vertical Speed is your rate of climb or descent in feet per minute, and Vertical Bearing is the bearing to a point in the vertical plane. Next is INIT/REF — 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. It also lets the operator select various reference data and maintenance pages. Then we have the N1 LIMIT key. This permits manual command of the active N1 limit, and selection of any Reduced Climb N1 Limit that may apply. In plain terms, it allows the crew to select an engine rpm — specifically the low-pressure turbine rpm, which is N1. So you're manually commanding the engine speed limit for the current phase of flight. Now, an important detail: the N1 Limit key may be shown as a menu key on the master — that's the left-hand — CDU, and it can be used to find data within the system. So on the left CDU, that key might serve as a menu rather than a direct N1 limit function. Next, DEP/ARR — departures and arrivals. This key is used for selection of the procedures and runways at the origin and destination airports. So you pick your departure procedure and arrival procedure here. Then RTE — route. This permits flight plan data entries, and it's a primary means for lateral flight plan alteration. Lateral means the horizontal path — your waypoints and airways across the ground. LEGS — 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 for each individual leg, you can see and modify the details of both the horizontal and vertical profile. Finally, HOLD. This permits planning or initiation of holding at a designated waypoint. Holding is that racetrack pattern you fly while waiting for clearance, and this key lets you set it up at a specific waypoint. So to tie it together: the CDU is your control panel for the FMS. The alphanumeric keys feed the scratch pad, the function and mode keys give you access to each phase of flight and system function, and the line select keys let you place data where it needs to go. Each key has a specific purpose — climb, cruise, descent, initialization, engine limits, departures and arrivals, route, legs, and holding.