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737-300/400/500MAINTENANCE MANUAL
G. In test position 2 switch S3 provides a ground for the B face annunciations and the FMC annunciator. The FMC annunciator reset function can be tested while holding the test switch in position 1 or 2.
8. Performance Computations (Fig. 10)__________________________________
A. General
(1) The FMCS provides optimal thrust, speed, and flight path profile prediction. The optimization is based upon sensor measurements of actual flight path conditions; and the operations definition of the flight cost factors that the pilot wishes to optimize.
B. Economy Cost Factor
(1) Fuel burn is not the only consideration in evaluating trip economy. Time has a tangible value. Speed schedules for optimal operation of the airplane in the climb, cruise, and descent phases of a flight are determined largely as a function of a variable known as cost index. Cost index is a number which relates the direct hourly flying costs to the cost of fuel. It is generally the case that fuel-economical speed schedules are slower schedules. A consequence to this is that decreased fuel burn results in increased trip time. The FMCS cost index is an enterable variable that can be manipulated by the operator. This permits the economy speed schedules for climb, cruise, and descent to be biased for a particular set of operating conditions. Costs can change in the day-to-day operating environment. As costs change the operator can change the value of cost index accordingly, to optimize the performance of the airplane.
C. Wind and Temperature Profile
(1)
The operator can enter winds for various points along the route. These winds, entered at the FMCS CDU, include winds for initial cruise altitude, cruise route leg waypoints, and intermediate altitudes (for descent forcasts). The winds are predicted wind values, which are used for flight planning estimates. The system always uses current FMCS computed wind for its present system wind. For vertical or lateral down-track route predictions, the FMC linearly interpolates between present system wind and the next entered wind. The interpolation is to avoid step changes in wind conditions and thus step transients in performance, speed, and thrust targets.
(2)
Temperature predictions use the present temperature, and a manually entered value for Top-Of-Climb (TOC) temperature. The FMC linearly interpolates between the temperature values, similar to the wind prediction.
D. Speed and Thrust Targets and Thrust Limits
(1) In terms of fuel consumed per nautical ground mile traveled, the variable mach speed schedule offers an improvement over the conventional constant mach schedule. Variable mach is dependent upon cost index, airplane gross weight, and pressure altitude.
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A 737-300/400/500MAINTENANCE MANUAL
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737-300/400/500MAINTENANCE MANUAL
(2) For any value of cost index chosen by the operator, the FMCS continuously computes and updates economy cruise speed in flight, as a function of a parameter known as cost factor. The FMCS evaluates this function continuously in the cruise flight phase to find the speed at which the minimum cost factor occurs. This is the most economical cruising speed.
NOTE: Cost index relates the direct hourly flying costs to
____ the cost of fuel. Cost factor is approximately equal to cost index divided by true airspeed.
(3)
The economy climb and descent speed schedules have been derived from aerodynamic data, to provide minimum trip cost based on cost index and airplane gross weight.
(4)
Other speed schedules are the normal maximum rate and maximum angle climb, and long range cruise schedules. Manually entered speeds can be used, if current flight conditions allow.
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