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

We're now in the cruise phase of the flight, and I want to walk you through what the cruise page, the CRZ page, is showing you. In the cruise, all three modes — LNAV, VNAV, and the timing mode — will normally be active at once. That's the first thing to hold onto: lateral navigation, vertical navigation, and time control are all working together. Let's look at the layout of this CRZ page, because it's a dense display. At 1L, the top left, you have the current cruising altitude. In this example it's FL210 — that's flight level 210, meaning 21,000 feet on the standard pressure setting. At 2L, directly below it, is the required cruising speed, and in this case it's the economy cruise speed — the speed the flight management system has computed to give you the best fuel economy for this leg. At 3L is something very important for engine management: the computed EPR or N1 required to maintain that speed at 2L. EPR is engine pressure ratio, and N1 is the low-pressure compressor speed, expressed as a percentage. The system is telling you, "to hold this economy speed, set the engines to this EPR or this N1." At 4L you have the predicted destination fuel — the fuel the system expects to have on board when you arrive. Now let's move across to 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 where you get the best fuel efficiency; the maximum is the highest you can go at this weight and temperature. These are computed values, not fixed numbers — they change as you burn fuel and as conditions change. Over on the right side, at 1R, is the next step altitude. A step climb is a climb to a higher flight level later in the cruise, once you've burned enough fuel to make it efficient. At 2R you have the time and distance to make that step climb — how long and how far until you should start it. At 3R you have the estimated wind velocity, and at 4R the predicted savings or penalty in making the step climb indicated at 1R. So the system is telling you: if you climb to this next level, here's what you gain or lose in fuel. Finally, the other cruise pages are accessed through 5R, 6L, and 6R — those are the line-select keys on the right and left edges of the display. You press the key next to the label to bring up a different cruise page. Now let's move into the descent phase. As in the climb, the LNAV, VNAV, and timing modes are all operative. The page here is the ACT ECON PATH DES page — that's the active economy path descent page. "Active" means this is the one currently in use, and "economy path" means the descent profile is being computed for best fuel economy. At the top, at 1L, you have the target Mach and speed — in this example .720 Mach and 280 knots. That's the speed the system wants you to fly during the descent. At 2L you have the time and distance to the next waypoint — 2004.3 Zulu time, 19 nautical miles. Zulu time is UTC, Greenwich Mean Time. At 3L you have the flight path angle, vertical speed, and speed restriction — FPA, V/S, and SPD REST. The flight path angle is the descent angle, the vertical speed is your rate of descent in feet per minute, and the speed restriction is a limit the system is applying. Below that, at 4L, you have the target altitude — 6000A, where the "A" indicates an altitude constraint, and the time to reach it, 2013 Zulu. On the right side, at 1R you have the economy descent speed — TOMACEY is a waypoint name, and the speed to fly to it. At 2R you have the waypoint and altitude — MACEY at 6000 feet. At 3R you have the vertical deviation, 25 nautical miles — that's how far off the vertical profile you are. At 4R you have the forecast wind, and at 5R the RTA, the required time of arrival — the time constraint the system is working to meet. So in the descent, the system is managing your speed, your path angle, your altitude constraints, and your time of arrival all at once, just as it did in the climb and cruise.