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

Right, let's get into the heart of the Flight Management System. We're on the IDENT page now, which is the very first page the Control Display Unit, or CDU, shows you after the system boots up. When you first apply power to the aircraft, the Flight Management System, the FMS, runs an internal test sequence. Once that test passes, it presents this IDENT page. Think of it as the system's identity card and its data status report all in one. Let's walk through the layout. The screen is divided into left and right columns, and each line is numbered. The line selection keys, which we call LSKs, are the buttons right next to the screen on both sides. You use those to interact with the data on each line. At 1L and 1R, we have the aircraft model and the engine thrust rating. This comes straight from the performance database. So, for example, you'd see "737-400" and the engine thrust figure, like "23.5K," which means 23,500 pounds of thrust. Now, the navigation database. At 2L, we have the identification of the permanent navigation database. This is the fixed, baseline set of navigation data. At 2R and 3R, we see the currency periods of that navigation data. This is critical. It tells you the effective dates—the "from" and "to" dates—that this database is valid for. You'll see something like "JAN 01 JAN 28/89," meaning it's good from January 1st to January 28th. At 4L, we have the identification of the operating programme. This is the software version running the FMS. At 5R, we have the date of the supplementary data. That's additional data that's loaded separately. Here's the key operational point. The only information you can actually change on this display is the current navigation data at 2R. If that data is out of date—meaning today's date falls outside the currency period shown—a prompt will appear in the scratchpad at the bottom of the screen, warning you. To update it, you select LSK 3R. That downselects the next period of data, pulling it into the scratchpad. Then you select 2R to insert that new data into the active data line. That's the sequence: 3R to bring it down, 2R to put it in. Now, look at the bottom corners. At 6R, there's the prompt for the next page in the initialization sequence. That's your "POS INIT" prompt, which takes you to position initialization. At 6L, there's the prompt for the page index, which is your way back to the main menu. One last, very important concept: propagation. Where any input data is used on other CDU pages, the data will automatically propagate to those pages. So if you change something here, it automatically updates everywhere else that data is referenced. You don't have to manually re-enter it on each page. So, to summarize the flow: power on, internal test, IDENT page. You verify the aircraft model, the engine thrust, the nav database ID and its validity dates, the operating program, and the supplementary data date. You update the nav data if it's expired, and then you're ready to move on to the next step in the initialization sequence.