Instrument Procedures - Departures — Page 165, Lesson 239

We're moving into the final stretch of the departures chapter now, and I want to walk you through the RNAV and RNP based departure procedures, plus the turns you'll encounter, and how the FMS fits into flying conventional departures. First, let's get the terminology straight. RNAV stands for Area Navigation, and RNP stands for Required Navigation Performance. The key point here is that the general principles that apply to RNAV approach procedures also apply to RNAV departures. So if you've studied approaches, you already know the foundation. These RNAV departures can be based on a few different navigation sources: VOR/DME, which is VHF Omni-directional Range combined with Distance Measuring Equipment; DME/DME, which uses two DME stations; basic GNSS, which is Global Navigation Satellite System; and finally, RNP criteria. Now, here's a practical reality: most aircraft equipped with an FMS, a Flight Management System, are capable of following RNAV procedures. But—and this is a big but—the procedures themselves may contain constraints on which system you're allowed to use. So you can't just assume your FMS will handle everything. Let me be very specific about the RNP requirement, because this is a critical pre-flight item. To use an RNP based procedure, your aircraft's RNAV system must be approved for the published RNP value. That's a certification requirement. And you must confirm before flight that the related VOR/DME stations are in fact working. I want to emphasize that—you physically verify the ground stations are operational before you go. Before flight, you also must verify that the aircraft will be able to meet the RNP requirements for the segments you're going to fly, and you must continue to monitor the system accuracy during the flight. So it's a two-part check: a pre-flight verification of capability, and an in-flight monitoring of accuracy. Now let's talk about turns. There are four kinds of turn that may be specified for an RNAV procedure. I want you to remember all four. First, a turn at a fly-by waypoint. Second, a turn at a fly-over waypoint. Third, a turn at an altitude or height. And fourth, a fixed radius turn, which is generally used for RNP based procedures. So the distinction between fly-by and fly-over is important—a fly-by waypoint is where you start turning before you reach it, and a fly-over is where you fly over it before turning. And the fixed radius turn is the one that's typically associated with the higher-precision RNP world. Finally, let's look at using the FMS to follow conventional departure procedures. This is where the FMS helps you fly a non-RNAV, conventional departure. Where FMS is available, you may use it when flying the conventional departure procedures that are defined, provided two conditions are met. First, the procedure must be monitored using the basic display normally associated with that procedure. So you're not just staring at the FMS screen—you're cross-checking the raw data on the conventional instruments. Second, the tolerances for flight using raw data on the basic display must be complied with. In other words, you still have to fly within the same tolerances as if you were flying raw data, even though the FMS is doing the work. So to tie it all together: RNAV departures give you flexibility, but they come with strict system approval and pre-flight verification requirements. The four turn types define how you navigate the procedure. And when you use the FMS on a conventional departure, it's a tool, not a replacement for monitoring the raw data and staying within its tolerances.