CAUTION: DO NOT MOVE THE EEC MODE SWITCH FROM NORM TO ALTN WHEN THE ENGINE IS OPERATING AT HIGH POWER. THE ENGINE MAY OVERBOOST IF YOU DO.
Normal Mode (EPR)
The normal position of the switch is on (in). This shows a mechanical indication of NORM. This position puts the EEC into its normal (EPR) mode. EPR is the thrust rating parameter for the engine.
In the EPR mode, the EEC uses this data to set the proper thrust:
*
Thrust lever angle (TLA)
*
Inlet temperature
*
Altitude
*
Mach number
*
Bleed status.
You can set thrust with the thrust levers at specified positions. There are specific thrust lever positions for takeoff, maximum continuous, and maximum climb thrust ratings. There is a linear EPR relationship between these thrust lever rating positions and idle.
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ENGINE FUEL CONTROL -OPERATION
Alternate Mode (N1)
The non-normal position of the switch is out. This hides NORM and shows a lighted ALTN. When the switch is out, the EEC is in alternate (N1) mode. The EPR indication goes out of view when the EEC is in the alternate mode.
In the alternate mode, the EEC controls the engine with N1 speed. It uses this data to adjust the engine N1:
*
TLA
*
Inlet temperature
*
Altitude.
In alternate mode with the thrust levers at a specific position, the thrust changes as the flight conditions change. It is necessary to adjust the thrust levers as the airplane flies in order to maintain the correct thrust.
Some faults make the EEC change to the alternate mode automatically. When this happens, the EEC turns on the ALTN light in the EEC mode switch.
The EEC uses the alternate mode during reverse thrust operation. This is because EPR is not accurate when the engine is in reverse thrust.
Thrust Lever
The thrust levers move TLA resolvers. The resolver sends angle position to the EEC. This signal lets the EEC set the proper thrust.
See the engine controls chapter for more information about the thrust levers and resolvers (CHAPTER 76).
Fuel Control Switch
The fuel control switches have two positions: RUN and CUTOFF. In RUN, the switch sends signals to the EEC to enable fuel flow. The EEC makes the decision when to start fuel flow. In CUTOFF, the switch controls the FMU to shut off metered fuel flow to the injectors. It also closes the engine fuel spar valve.
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