At position 2L, you’ve got the target Mach number and the target CAS — Page 282, Lesson 276

Let’s start with the descent page on the CDU, because that’s where all the numbers you’re about to see live. I’m looking at Figure 16.14, the active economy path descent page — that’s the ACT ECON PATH DES page. At position 2L, you’ve got the target Mach number and the target CAS. CAS is calibrated airspeed — the speed shown on the airspeed indicator, corrected for instrument and position error. So the FMC is telling you the speed it wants you to fly on this descent, expressed both as a Mach number and as a CAS. At 1L is the end of descent altitude — that’s the altitude you’re aiming to reach at the end of the descent. At 1R is the next descent position and altitude, and the suffix A means “at or above.” So that’s a constraint — you must be at or above that altitude at that position. Position 3L contains the speed transition. This is the speed at which the aircraft transitions from Mach to CAS control, or vice versa, during the descent. The value shown is 10 knots less than the speed stored in the database. And next to it is the transition altitude. If none is defined, it defaults to 240/10000 — meaning 240 knots CAS at 10,000 feet. You cannot type into this field — no input is permitted — but you can remove the data if you want. At 3R you see the next waypoint and its altitude. At 4L is the expected deviation from that required height — in other words, how far off the required altitude the FMC predicts you’ll be when you reach that waypoint. Now, 4R is FPA — that’s the actual flight path angle, based on your current ground speed and rate of descent. Then 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. Finally, access to the associated descent pages is gained at 5R, 6L, and 6R — those are the line-select keys that take you to other descent-related pages. Now let’s move to the principle of operation for a twin IRS, twin FMC system. IRS is the inertial reference system — it gives you position by sensing acceleration. In a twin IRS 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 catch: if there’s a discrepancy between them, the system cannot determine, in isolation, which one is in error. So the FMC needs an external reference — something like a VOR, DME, or GPS — to figure out the correct position. It does this using Kalman filtering. That’s a mathematical technique that blends the external reference with the IRS positions to determine the system position. At the start of a flight, the IRS position predominates — it’s very accurate then. But as the flight progresses, the IRS positions degrade — they drift — so the weighting for the external reference increases. How much it increases depends on the selection of external reference and the range from that reference. Now, 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. Both systems independently determine position, and then the positional information is co-related — compared — to check for gross errors, before being passed to the EFIS, the electronic flight instrument system. This means the position presented on the EFIS may differ from that on each CDU — because the EFIS is showing a co-related position, not either individual one. In independent operation, each FMC works in isolation, with no communication between them. So to tie it together: the descent page gives you the target speeds, the constraints, the flight path angle, and the vertical speed. And the twin IRS/FMC principle explains how the system knows where it is — by blending inertial position with external references, and by co-relating the two FMCs in dual mode to catch gross errors.