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

We’re starting the Area Navigation Systems chapter, and I want to walk you through the first two pages of the FMC’s operational sequence: the POS INIT page and the RTE page. Let’s begin with the POS INIT page. This is the position initialization page. Its job is to allow initialization of heading and position for the IRS — the Inertial Reference System. So before the FMC can navigate, it needs to know where the aircraft is and which way it’s pointing, and that data comes from the IRS. This page is where you feed it that starting information. Look at the display layout. You’ll see dashed lines, like the one at 5R. Those dashed lines indicate where optional data may be inserted to assist the FMC operation. Optional means you don’t have to put something there, but if you do, it helps the computer. Then you have boxed areas, like at 4R. Those indicate where data essential to the operation of the FMC must be inserted. Essential — that’s mandatory. The FMC won’t proceed properly without it. Now let’s trace the fields. At 1R, the FMC displays the last position recorded before shutdown. That’s a memory of where you were when the aircraft was powered down last time. At 2L, you insert the departure airport. At 3L, you insert the gate. The FMC then extracts the airfield reference and gate positions from its database and inserts them at 2R and 3R respectively. So you type the airport identifier and the gate, and the database supplies the actual coordinates. Here’s the key step. At 4R, the FMC is asking for the aircraft position to initialize the IRS. 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 display — and then insert it by selecting LSK 4R. LSK means Line Select Key, the button beside that field. But there’s a faster way. The database has already inserted the position into 3R, the gate position. So you can copy it: select 3R to draw the data down to the scratchpad, then select 4R to insert it into the position field. That saves you typing coordinates. Now, to speed up alignment, particularly if the aircraft has been moved, you can input the magnetic heading from the standby compass at 5R. Why does that matter? The IRS aligns by sensing rotation and acceleration, and if the aircraft has been towed or repositioned, giving it a known heading helps it align faster and more accurately. Once you’ve completed that, the alignment of the IRS will now proceed. And the prompt at 6R now directs the pilot to the route page — the RTE page. So that’s the POS INIT page. Let’s move to the 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. So there are two scenarios: either you pull up a pre-stored company route and check or modify it, or you build a route from scratch because it isn’t in the database. On the RTE page you’ll see fields for ORIGIN — that’s the departure airport, here KMWH. You’ll see DEST — the destination, here KBFI. You’ll see COROUTE — the company route identifier. You’ll see FLT NO — the flight number. You’ll see a VIA field showing the route, here LACRE3.VAMPS — that’s a departure procedure and a waypoint. And you’ll see a TO RUNWAY field, here 13R, and an ACTIVATE prompt. The ACTIVATE function is what you select when you’re satisfied with the route and want the FMC to make it the active flight plan. Let me show you the actual display pages so you can see the layout. That’s the POS INIT page we just walked through. And here’s the RTE page. So the sequence is: initialize position and heading on POS INIT, let the IRS align, then move to RTE to build or verify your route, then activate it. That’s the foundation of how the FMC gets ready to navigate.