Range formulas in FMS?

[domyalex]

Ok, this question is just for the sake of curiosity.
While reading about the thread of the 747 that crossed the pond on 3 engines, I wondered about how does the FMS calculate the available flight range at a given time.

Does it just look at the actual fueld burn, ground speed, available fuel in the tanks and then gives an estimate of the distance that can be traveled?
Does it also consider actual and forecasted winds on the route?
What about different FLs changes programmed along the flight plan?
What about the weight of the a/c (as burning fuel is making the aircraft lighter and not by a small margin)? Is it factored using diff equations or any other method? I know the NASA guys must consider very carefully how weight will change during the takeoff of the shuttle and use some rather complex formulas.

I'm almost finishing my System Engeneering degree and have been always kinda interested in how the avionics really work in the inside.

 

[dseagrav]

I dunno, I figured the complexity of the calculations in FMS was directly related to the cost of said FMS.

 

[LAXSaabdude]

Not sure about what was installed in the 747, but in the Universal unit on the EMB 145, it is purely a "snapshot" approach. It takes the current groundspeed, current fuel consumption, and current distance, and calculates the numbers. It doesn't figure in any anticipated weight changes, altitude changes, wind changes, or power reductions for descent.

First time I turned the FMS to the Performance page about a minute after takeoff, it was a little unnerving to see that we would run out of gas looong before we got to our destination. Of course, we were at about 2000', at climb power, and doing only about 200kts! The numbers dont really start do get accurate until you are leveled off at cruise altitude, and speed and power have stabilized.

LAXSaabdude.

 

[typhoonpilot]

The 747-400 would probably have an FMS similar to to the 777, which is a Honeywell Pegasus system. The Pegasus does take into account the forecast winds that are entered as well as current winds. It uses a mixing algorithm to combine winds measured at the aircraft versus the forecast winds downroute. At 200 NM in front of the aircraft the FMS uses 50% of the measured wind vector and 50% forecast wins; at distances over 200 NM, the FMS uses successively less measured wind until the wind used is very near 100% forecast wind at roughly 500 NM.

The FMS does look at step climbs as programmed under normal conditions. It has an engine out function which will also incorporate step climbs into it's predictions. How that would vary on two engine versus four engine aircraft, I don't know.

The FMS calculates weight based on the initial entered weight minus the engine fuel flow rate.


TP

 

[Nalo Boy]

smoke & mirrors