Area Navigation Systems (RNAV) — Page 276, Lesson 269

We're starting a new topic now: the FMS control and display unit, and the pages you'll actually work with on the ground before departure. I want to walk you through the POS INIT page first, because that's where you initialize the inertial reference system, the IRS. Look at the POS INIT page layout. It's a screen with line-select keys down both sides, numbered 1L through 6L on the left and 1R through 6R on the right. Those are the LSKs, the line select keys. You press one to move data into or out of the field on that line. Here's the key distinction I want you to remember from the start. On all displays, the dashed lines — like the one at 5R — indicate where optional data may be inserted to assist the FMC operation. Optional. The boxed areas — like at 4R — indicate where data essential to the operation of the FMC must be inserted. Essential. So dashed means optional, boxed means mandatory. That's a professional distinction you'll see on every page. Now let's walk the page. At 1R, the FMC displays the last position recorded before shutdown. That's your starting reference. At 2L you insert the departure airport, and at 3L you insert the gate. When you do that, the FMC extracts the airfield reference and gate positions from its database and inserts them at 2R and 3R respectively. So you type the identifiers on the left, and the database supplies the actual coordinates on the right. At 4R, the FMC is asking for the aircraft position to initialize the IRS. This is the boxed, essential field. You have two ways to fill it. You could input the position manually in the scratchpad — that's the entry line at the bottom of the screen — and then insert it by selecting LSK 4R. But here's the efficient way: the database has already inserted the position into 3R. So you can copy it by selecting 3R to draw the data down to the scratchpad, and then select 4R to insert it into the field. Copy from the gate position, paste into the IRS position field. Now, to speed up alignment — particularly if the aircraft has been moved — you can input the magnetic heading from the standby compass at 5R. That's the dashed, optional field. It helps the IRS align faster because it gives the system a known heading reference. Once you've completed that, the alignment of the IRS will proceed. And the prompt at 6R now directs you to the route page, the RTE page. So let's move to that RTE page. The route pages are used to insert, check and/or modify a company route, or to insert a route not held in the database. That's the purpose: you're either loading a company route or building one from scratch. On the RTE page you'll see fields for the origin, the destination, the runway, the flight number, and the route itself. You'll see the origin and destination identifiers, the runway in use, and the flight number. And the route line shows the departure procedure and the en-route structure — for example, a departure like LACRE3.VAMPS, which is a standard instrument departure, a SID, followed by the first waypoint. The key action on this page is the ACTIVATE prompt. When you've inserted or modified the route, you activate it. That tells the FMC this is the active flight plan, the one it will navigate. Let me show you the physical unit and the power-up sequence, because that ties it together. Here's the control and display unit itself — the CDU. And when power is applied, the FMS executes an internal test sequence. When the test is successfully completed, the unit is ready for you to start initializing. So the flow is: power up, the FMS runs its internal test, then you go to the POS INIT page to initialize the IRS with position and heading, and then the prompt directs you to the RTE page to build or activate your route. That's your ground sequence before departure.