Area Navigation Systems (RNAV) — Page 276, Lesson 273

We're starting fresh on the Area Navigation Systems chapter, and I want to walk you through the heart of how the Flight Management Computer, the FMC, lets you build a route without needing a physical beacon at every point. This is the essence of RNAV. First, let's talk about how you define a waypoint. The FMC gives you a few ways to create a position that isn't in its database. The first is a place bearing/distance waypoint, or PBD. You define it by giving a range and a bearing from a known navigational aid or reporting point. For example, you might input something like `TRN250.0/76`. That means a bearing of 250 degrees and a distance of 76 nautical miles from the aid TRN. The FMC then creates a new waypoint and designates it `TRN01`, assuming it's the first or only position you've specified with reference to TRN. Now, a critical formatting rule: the decimals are optional, but the bearing must always be a 3 or 5 digit group, and the distance can be 1 to 5 digits. So you could write 250/76 or 250.0/76, but the bearing group always has that structure. The second type is a course interception waypoint. This is a position defined where a bearing from any valid database position intersects with a course, like an airway, or with a bearing from another database-defined position. The input format looks like `GOW167.0/TRN090.5`. Here, the FMC produces a PBD waypoint, which in this case would be designated `GOW01`. Again, the bearings must be either 3 or 5 digits. So you're essentially finding the point where two lines cross—one from GOW and one from TRN—and that intersection becomes your waypoint. Now, let's move to the climb phase, because this is where you see the FMC's vertical navigation, or VNAV, and lateral navigation, LNAV, working together. Normally in the climb, the VNAV, LNAV, and timing functions are all operative. I want you to look at the climb page, the CLB page, on the control and display unit. At line 1L, you have the planned initial cruising altitude, if one exists and the climb is active. At 1R, you have the current climb restriction. The suffix 'A' indicates altitude, so you know that value is in feet. At 2L, you get the economy speed for the climb—that's the speed the FMC calculates for best fuel efficiency. At 3L, you have any speed restriction, which defaults to 250 knots and 10,000 feet. That's the standard speed limit below 10,000 feet. If ATC imposes a different speed or altitude restriction, you can input it to 3L from the scratchpad. At 2R, you have the ETA and the distance to go to the next position. At 3R, you get the height error at the next point. For example, it might show the aircraft will be 310 feet low, meaning the FMC predicts you'll cross that point 310 feet below the required altitude. The climb engine N1 is displayed at 4R—that's the fan speed of the engine, a key power parameter. The prompts at lines 5 and 6, both left and right, direct you to the other climb mode pages. And there's a note about RTA, which is required time of arrival, to be used if ATC specifies an RTA. So, in summary, you have two ways to create waypoints—PBD and course interception—and then a climb page that gives you the full picture of your vertical and lateral plan, with speed restrictions, altitude targets, and predicted errors. That's the core of how the FMC manages your climb.