Electronic Engine Control (EEC) Switches

ON – In view (white):

  • Indicates normal control mode is selected
  • Engine ratings calculated by EEC from sensed atmospheric conditions and bleed air demand
  • When ON is not in view, the EEC has been manually selected to the alternate mode.

ALTN – In view (amber):

  • Indicated EEC has automatically switched to alternate control mode or it has been selected manually
  • EEC provides rated thrust or higher.

NOTES

  • Both ON and ALTN may be in view if EEC has automatically switched to soft alternate mode.
  • EGT limits must be observed in both normal and altemate control modes.

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    EEC Alternate Mode

    The EEC can operate in either of two altemate modes, soft or hard. If required signals are not available to operate in the normal mode, the EEC automatically changes to the soft alternate mode. When this occurs, the ALTN Switch illuminates and the ON indication remains visible. In the soft alternate mode, the EEC uses the last valid flight conditions to define engine parameters which allows the mode change to occur with no immediate change in engine thrust. Thrust rating shortfalls or exceedances may occur as flight conditions change. The soft alternate mode remains until the hard altemate mode is entered by either retarding the throttle to idle or manually selecting ALTN with the EEC Switch on the aft overhead panel.

NOTE

Loss of either DEU results in a loss of signal to both EECs. The EEC ALTN Lights ifluminate and each EEC reverts to the altemate mode to prevent the engines from operating on a single source of data.

When the hard alternate mode is entered, the EEC reverts to the altemate mode thrust schedule. Hard alternate mode thrust is always equal to or greater than normal mode thrust for the same lever position. If the hard alternate mode is entered by reducing the throttle to idle while in the soft altemate mode, the ALTN Switch remains illuminated and the ON indication emains visible. When ALTN is selected manually, the ON indication is blanked.

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