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sends out commands to the DFCS and the autothrottle computer
to control the vertical flight path of the airplane.
Speed targets do not go to the DFCS during takeoff or approach/
go-around operation. Only target N1 (limit value or reduced
value if engine derate has been selected) is used.
Speed and Thrust Limits
The performance sub-function also calculates minimum and
maximum speed and thrust limits for the climb, cruise and
descent phases of the flight. These make sure that the airplane
operates within its flight envelope and the engines are
protected against possible overboost.
Gross Weight
The FMCS calculates airplane gross weight or the flight crew
manually enters the data. Total fuel weight from the fuel
quantity processor unit goes to the FMC. The flight crew can
enter either gross weight or zero fuel weight through the
MCDUs.
If the flight crew enter gross weight, the FMC subtracts total fuel
weight from gross weight to calculate zero fuel weight. If the
flight crew enter zero fuel weight, the FMC adds total fuel weight
and zero fuel weight to calculate gross weight.
Performance Data
The FMC uses this data for the performance calculations:
* Air data
* Cruise altitude
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* Cost index
* Fuel weight
* Engine bleed air sensors
* Model/engine data base.
Air Data
The FMC uses this air data from the ADIRUs:
* Air temperature is used to calculate thrust limits.
* Altitude is used to calculate speed and thrust targets and
limits.
* Airspeed is used to calculate DFCS/Autothrottle commands.
Cruise Altitude
Cruise altitude is entered by the flight crew through the MCDUs.
It is used to establish the altitude for the top of climb (T/C) point.
Cost Index
The cost index is a number which is a ratio of the operator time
costs (such as labor) to the fuel cost.
A low value for cost index means that fuel costs are more
important than time costs. A high cost index means that time
costs are more important than fuel costs. For flights where fuel
costs are high, the performance economy speed schedules are
slower. For flights where time costs are high, the economy
speed schedules are faster.
The flight crew enters the cost index in the MCDU and changes
the data as necessary. The range is from 000 (most economical)
to 200 (time critical). This lets the operator adjust the economy
speed schedules (climb, cruise and descent) for the conditions
and routes they operate.
Fuel Weight
Fuel weight data is sent by the fuel quantity processor unit to the
FMC. It is used by the FMC in the gross weight calculations.
If the total fuel weight data from the fuel quantity processor unit
is invalid, the flight crew can manually input the fuel weight to
the FMC on the MCDUs. Every 30 minutes, the FMC prompts the
flight crew to enter a new fuel weight with the message VERIFY
GW AND FUEL. After the airplane is past the T/D (top of
descent) and a Vref value is entered, the FMC will not show this
message.
Engine Bleed Air Sensors
The FMC receives analog discretes from the air conditioning
system and the thermal antice (TAI) systems for both the
engines and wings. The FMC uses this data to correct
calculated thrust values.
Model/Engine Data Base (MEDB)
The MEDB contains this data:
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* An aerodynamic model of the airplane. This consists of
recommended speed schedules, optimum operating
altitudes, buffet limit envelope and the certified operating
limits for the airplane. Engine out operating conditions
including speed and drag data are also contained in this data
base.
* Fuel flow and N1 thrust models for the engine thrust rating
selected on the airplane. These are used to calculate fuel
flow, thrust limits, engine limits and to compensate for
engine air bleed loads.
* QRH takeoff speed data. This consists of V1, VR, V2 and
takeoff stabilizer trim positions. Gross weight, T/E flap
position and center of gravity are used to calculate these
values.
Flight Envelope Protection
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