Area Navigation Systems (RNAV) — Page 276, Lesson 268

Let's start with the IDENT page, because it's the first thing you'll see on the CDU — the Control Display Unit — after the FMS, the Flight Management System, powers up. When you apply power, the FMS runs an internal test sequence. Once that test passes, it presents the IDENT page. This is your identity page, and it's the gateway to initializing the whole navigation system. Now, look at the layout. The screen is divided into lines, numbered 1 through 6, with a left column and a right column. So you have 1L, 1R, 2L, 2R, and so on, down to 6L and 6R. These are the line selection keys — the LSKs — that you press to select data on a particular line. At 1L and 1R, you'll see the aircraft model and the engine thrust, both pulled from the performance database. So for a 737-400, you'd see the model at 1L and the thrust rating at 1R. At 2L, you have the identification of the permanent navigation database. This tells you which nav database is loaded. Then at 2R and 3R, you have the currency periods of that navigation data — the dates during which the data is valid. This is critical, because if your nav data is out of date, your navigation can be based on stale information. At 4L, you have the identification of the operating programme — the software version running the FMS. And at 5R, you have the date of the supplementary data. Now, here's the key point: the only information you can actually change on this display is the current nav data at 2R. If that data is out of date, a prompt will appear in the scratchpad — that's the input line at the bottom of the screen. To change it, you select LSK 3R to downselect the next period of data to the scratchpad. That pulls the next valid data period into the scratchpad. Then you press 2R to insert that data into the active data line. So the sequence is: 3R to bring it down, then 2R to put it in. Now, notice the prompts. At 6R, you have the prompt for the next page in the initialization sequence — that's how you move forward. At 6L, you have the prompt for the page index — that takes you back to a menu of pages. And one more important behavior: wherever any input data is used on other CDU pages, that data will automatically propagate to those pages. So if you enter something here, it flows through to the rest of the system automatically. You don't have to re-enter it elsewhere. Before we go further, let me tie this back to the bigger picture. If the aircraft is flying a standard company route, you insert the route designator. Otherwise, you input the route manually. Data is initially typed into the scratchpad at the bottom of the screen, then inserted in the appropriate position using the line selection keys. Once a valid position has been input, it's passed to the IRS — the Inertial Reference System. So the IDENT page is your starting point. It confirms what's loaded, lets you update the nav data if needed, and then you move on through the initialization sequence.