Flight Management System — Page 279, Lesson 328

Let’s start with the economics of the Flight Management System, because that sets the stage for everything else. The FMS has a concept called the Cost Index, and it’s the heart of how the system decides what speed to fly. The Cost Index is determined by dividing the aeroplane operating cost, expressed in dollars or pounds per hour, by the fuel cost, expressed in dollars or pounds per pound or litre of fuel. So you’re taking the cost of running the aircraft per unit time and dividing it by the cost of the fuel itself. That ratio becomes a single number the FMC uses to balance two competing costs. Now, there’s a note here about Time Related Cost. It’s a function of speed selected. The higher the speed in climb, cruise, or descent, the higher the “trip fuel cost” — you burn more fuel going fast — but the lower the “trip time cost,” because you get there sooner. So there’s a trade-off. The Economy Cruise Mode will yield the lowest operating cost based on that Cost Index. If you set the Cost Index to zero, you get what’s called “minimum trip fuel operation.” That means the FMC will fly cruise at “maximum range” cruise speed and use a low speed descent, because you’re prioritizing fuel savings over time savings. Now let’s move to the operational side, specifically Cruise Lateral Navigation, or LNAV. The FMC generates lateral steering commands, and normally those commands are great circle tracks between the waypoints that make up the active route. A great circle is the shortest path between two points on a sphere, and that’s what the FMC follows between waypoints. However, when a procedure stored in the FMS data base is entered into the active route, the FMC can supply commands to fly a constant heading, a constant track, or follow a DME arc, as required to comply with that procedure. So it’s not always a straight great circle — it adapts to the procedure. How does the FMC know where it is? It determines present position by using inputs from the IRS or INS, DME, VOR, and other navigation systems fitted. IRS stands for Inertial Reference System, INS for Inertial Navigation System — those are the inertial sources. DME is Distance Measuring Equipment, and VOR is VHF Omnidirectional Range. The FMC takes all those inputs, calculates its present position, and uses that to generate lateral steering commands along the active leg to the active waypoint. To function at all, the FMC requires position information from at least one IRS or INS. While the aircraft is on the ground, the FMC calculates present position based only on information received from the IRS or INSs — no DME or VOR on the ground, just the inertial systems. Here’s an important accuracy point. The FMC present position is normally the combination of all IRS or INS positions. Since inertial systems accumulate position errors as a function of time — they drift — the position information being used by the FMC is slowly accumulating errors. You can detect these position errors by observing the various positions of the individual IRS or INSs on the CDU, the Control Display Unit. If an extended ground delay occurs and a significant map error is noticed, the IRS or INS should be realigned and present position re-entered. So if you’re sitting on the ground a long time and the map looks wrong, realign the inertial systems and re-enter your position. Now let’s talk about Descent. When a programmed “arrival” is entered, the FMC calculates a descent path based on the procedure’s airspeed and altitude constraints and the End of Descent, abbreviated E/D. The E/D is a waypoint with an altitude and airspeed constraint that coincides with a final approach fix or runway threshold. So the descent path is built to meet those constraints at that specific point. For VFR and non-precision approaches, the FMC computed path is built to a point that is 50 feet over the approach end of the runway. VFR is Visual Flight Rules, and a non-precision approach is one without vertical guidance. It’s the flight crew’s responsibility to not descend below “DH” — that’s Decision Height — until adequate visual contact has been achieved. During a missed approach, LNAV guidance is available to the missed approach point and altitude. Finally, let’s look at Accuracy. Radial error rates of less than 0.05 NM per hour are not uncommon. That means the position error grows less than five hundredths of a nautical mile per hour. Introduction of Ground Positioning by Satellite, GPS, as a navigation input will improve overall performance. But it must be stressed that 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 crew watching it. So the key thread here: the FMC blends inertial, DME, and VOR data to know where it is, it uses the Cost Index to decide how fast to fly for economy, and it builds descent paths to meet procedure constraints — but the crew’s monitoring is what ultimately guarantees accuracy.