Area Navigation Systems (RNAV) — Page 276, Lesson 267

Let's start with the Control and Display Unit — the CDU. This is the pilot's primary way of talking to the Flight Management Computer, which we call the FMC. Think of the CDU as the keyboard and screen, and the FMC as the brain behind it. The CDU is used before flight to initialize the performance and navigation requirements for the flight — that means you set up both how the aircraft will perform and where it will go before you ever start moving. Look at the layout. You have an alphanumeric keypad — that's the letters and numbers — plus specific function keys. Alongside the display are the line select keys, which we abbreviate as LSK. These are the buttons right next to the screen. You use them to insert or select data into the FMC, and to move through the various function pages. So if you want to put a value into a field on the screen, you press the LSK that sits beside that field. Now the display format. At the top is the title of the selected page. If the function you've chosen has more than one page, you'll also see the page number — for example, "1 of 3." In the centre of the display are up to 10 data fields — five on the left and five on the right — and those are the ones you access with the LSKs. At the bottom of the screen are two or more page select fields, and below them is the scratchpad. The scratchpad is where you type in or modify data before you insert it into the appropriate data field. So you key in a value, it appears in the scratchpad, and then you use the LSK to move it up into the field you want. Let me show you the CDU layout so you can see how these parts fit together. Now, the pre-flight initialization. When you're setting up the FMC in navigation mode, you have a checklist of things to verify and input. First, check the correct database is installed — that's the IDENT page. Then input the aircraft position — that's the POS INIT page. You also input the departure and destination aerodromes, the intended SID and STAR procedures — those are the Standard Instrument Departure and Standard Terminal Arrival Routes — and the planned route, which again is done on the POS INIT page. So the key idea here is that the CDU is your interface, and before flight you use it to tell the FMC where you are, where you're going, and how you plan to get there. The database check ensures the navigation data is current and valid, and the position initialization gives the system a known starting point.