Flight Management System — Page 279, Lesson 328

Let's start with the economics of the Flight Management System, because that's where this section opens. The FMS is constantly balancing two competing costs: time and fuel. The book puts it as "Time Related Cost," which is a function of the speed you select. The relationship is a direct trade-off. If you fly faster in climb, cruise, or descent, your "trip fuel cost" goes up — you burn more fuel per mile. But at the same time, your "trip time cost" goes down, because you're spending fewer minutes in the air. So the FMS is always weighing that exchange. That's where the "Cost Index" comes in. The Economy Cruise Mode uses the Cost Index to find the speed that yields the lowest overall operating cost. The Cost Index itself is a number you compute by dividing the aeroplane's operating cost — expressed in dollars or pounds per hour — by the fuel cost, which is dollars or pounds per pound or litre of fuel. So it's a ratio of time cost to fuel cost. If you set the Cost Index to zero, you get what's called "minimum trip fuel operation." That means the FMS will plan a cruise at "maximum range" cruise speed and a low-speed descent, because you've told it fuel is everything and time is worth nothing. Now let's move to the operational side, starting with Cruise Lateral Navigation, which we call LNAV. The FMC — the Flight Management Computer — normally guides you along great circle tracks between the waypoints that make up your active route. A great circle is the shortest path between two points on a sphere, and that's the default. But here's the important part: when you enter a procedure that's stored in the FMS database into the active route, the FMC can switch modes. It can supply commands to fly a constant heading, or a constant track, or follow a DME arc — whatever the procedure requires. So it's not locked into great circles; it adapts to the published procedure. Now, how does the FMC know where you are? It determines present position using inputs from the IRS or INS — that's the Inertial Reference System or Inertial Navigation System — plus DME, VOR, and other navigation systems fitted to the aircraft. It takes that calculated present position and uses it to generate lateral steering commands along the active leg, steering you toward the active waypoint. But there's a critical requirement: to function at all, the FMC needs position information from at least one IRS or INS. And while the aircraft is on the ground, the FMC calculates present position based only on the IRS or INS inputs — no DME or VOR on the ground. Here's a subtlety you need to understand. The FMC's present position is normally the combination of all the IRS and INS positions. But inertial systems accumulate position errors as a function of time — they drift. So the position the FMC is using is slowly accumulating errors. You can detect these errors by observing the various positions of the individual IRS or INS units on the CDU, the Control Display Unit. And here's the practical procedure: if you have an extended ground delay and you notice a significant map error, you should realign the IRS or INS and re-enter the present position. That's the fix. Let's move to Descent. When you enter a programmed "arrival," the FMC calculates a descent path based on the procedure's airspeed and altitude constraints, and the End of Descent, which we call the E/D. The E/D is a waypoint that has an altitude and airspeed constraint, and it coincides with a final approach fix or the runway threshold. So it's the point where your descent is supposed to end. For VFR and non-precision approaches, the FMC builds its computed path to a point that is 50 feet over the approach end of the runway. That's a specific number — 50 feet. And it's the flight crew's responsibility not to descend below "DH," the Decision Height, until adequate visual contact has been achieved. During a missed approach, LNAV guidance remains available to the missed approach point and altitude. Finally, Accuracy. The book tells us radial error rates of less than 0.05 nautical miles per hour are not uncommon. That's the drift rate of the system. And the introduction of GPS — Ground Positioning by Satellite — as a navigation input will improve overall performance. But the book stresses something important: the skill of the operator and the need for constant and careful monitoring will always be a deciding factor. The system is only as good as the pilot watching it.