These are the speeds the FMS wants you to fly during the descent — Page 282, Lesson 276

Let's start with the descent page on the FMS, because that's what Figure 16.14 shows us. When you select the active economy path descent page — that's the ACT ECON PATH DES page — you're looking at the FMS's plan for getting the aircraft down efficiently. At position 2L on that page, you'll see the target Mach number and the target CAS, which is your calibrated airspeed. These are the speeds the FMS wants you to fly during the descent. At 1L, you have the end of descent altitude — that's the altitude you should reach when the descent is complete. At 1R, you have the next descent position and its altitude, and the suffix A after it means "at or above" — so that altitude is a minimum, you can be at or above it. Now, position 3L contains the speed transition. This is the speed at which the aircraft transitions from one speed regime to another during the descent. The value shown here is 10 knots less than what's stored in the database, and next to it is the transition altitude. If no transition is defined, it defaults to 240/10000 — meaning 240 knots at 10,000 feet. You can't type input into this field, but you can remove the data if you need to. At 3R, you have the next waypoint and its altitude, and at 4L you see the expected deviation from that required height — in other words, how far off the FMS predicts you'll be from that altitude when you reach the waypoint. At 4R, we have FPA, which is the actual flight path angle — that's the angle of your descent path based on your current ground speed and your rate of descent. Next to it, V/B is the vertical bearing — that's the flight path angle required to achieve the required height at the next position. And V/S is the actual rate of descent, in feet per minute. So FPA tells you what you're actually doing, V/B tells you what you need to do, and V/S is the vertical speed. Finally, at 5R, 6L, and 6R, you have access to associated descent pages — those are the other descent-related pages you can jump to from here. Now let's move to the principle of operation for a twin IRS, twin FMC system. In this setup, the left FMC normally receives information from the left IRS, and the right FMC from the right IRS. The systems compare the IRS positions, but here's the key limitation: if there's a discrepancy between them, they can't determine in isolation which system is in error. So the FMC needs input from an external reference — something outside the IRS — to figure out the correct position. This is done using Kalman filtering. The external reference is compared with the IRS positions to determine the system position. At the start of a flight, the IRS position will predominate — it's the primary source. But as the flight progresses, the IRS positions degrade — they drift over time — and the weighting for the external reference increases. That weighting is commensurate with the selection of external reference and the range from that reference. So the farther you get from your reference, the more the system relies on it differently. There are four possible modes of operation for a twin FMS system. In the dual mode, one FMC acts as the master and the other as the slave. The systems independently determine position, and the positional information is co-related — that means it's cross-checked — to check for gross errors, before being passed to the EFIS, which is your electronic flight instrument system. One important consequence: the position presented on the EFIS may differ from that on each CDU, because the EFIS shows the co-related position, not each individual FMC's position. Then there's independent operation, where each FMC works in isolation with no communication between them. That's the other end of the spectrum — no co-relation, no cross-checking. So to tie it together: the descent page gives you the speed and altitude targets, the transition, and the flight path information you need to manage the descent. And the twin FMC principle tells you how the system determines its position — using IRS inputs, an external reference, and Kalman filtering to blend them, with different modes of operation depending on how the two FMCs interact.