Area Navigation Systems (RNAV) — Page 282, Lesson 275

We're now moving into the cruise phase of the flight, and I want to walk you through the cruise page on the Flight Management System, which is Figure 16.13. In the cruise, all three modes—LNAV, VNAV, and the timing mode—will normally be active. That's the key starting point: lateral navigation, vertical navigation, and time control are all working together to hold your path. Let's look at the layout of this cruise page, which is labelled CRZ. At 1L, the top left, you have the current cruising altitude. In this example it's FL210, which is flight level 210, or 21,000 feet. At 2L, directly below it, is the required cruising speed—and in this case it's the economy cruise speed, the speed that gives you the best fuel efficiency for the conditions. Now, at 3L, the system has computed the engine parameter needed to maintain that speed at 2L. That's shown as EPR/N1—engine pressure ratio or N1 fan speed. This is the thrust setting the autothrottle will command to hold your chosen cruise speed. At 4L, you have the predicted destination fuel, which is the fuel the system calculates you'll have on arrival based on the current plan. Moving across the top center, at 1C, you have the optimum and maximum cruising level for the aircraft's current weight and the ambient conditions. The optimum level is the most fuel-efficient altitude, and the maximum is the highest you can go at that weight. At 1R, on the right side, is the next step altitude—the next higher level you could climb to. At 2R, you have the time and distance to make that step climb, so you know when you'd reach that point. At 3R is the estimated wind velocity, and at 4R is the predicted savings or penalty in making that step climb—whether it's worth going up. The other cruise pages are accessed through 5R, 6L, and 6R. So the cruise page gives you a complete picture: where you are, how fast you're going, what thrust you need, what fuel you'll have left, and whether a step climb is beneficial. Now we move into the descent phase. As in the climb, the LNAV, VNAV, and timing modes are all operative. This is Figure 16.14, the active economy path descent page, labelled ACT ECON PATH DES. At 1L you have the target Mach, and at 2L the target speed. In this example, the Mach is .720 and the speed is 280 knots. At 3L is the flight path angle, shown as FPA, with vertical speed and speed restriction—FPA V/B V/S. At 4L you have the speed restriction, which is 240 knots at 10,000 feet, a typical constraint. At 1R you have the top of descent altitude, shown as 6000A, and at 2R the estimated time and distance to that point—2013 Zulu and 19 nautical miles. At 3R is the end of descent altitude, E/D ALT, with values 3.8, 6.2, and 2360. At 4R you have the vertical deviation, VERT DEV, which is 25 nautical miles—that's how far off the vertical profile you are. At 5R you have the forecast, and at 6R the RTA, or required time of arrival. So in descent, the system is managing your path down—target speed, flight path angle, speed restrictions, and the top of descent point—all to get you to the destination efficiently and on time.