EEC Alternate Mode
The EEC can operate in either of two alternate 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 alternate mode is
entered by either retarding the thrust lever 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
illuminate and each EEC reverts to the alternate 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 alternate 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
thrust lever to idle while in the soft alternate mode, the ALTN switch remains illuminated
and the ON indication remains visible. When ALTN is selected manually, the ON indication
is blanked.
Structural Limit Protection
The EEC provides N1 and N2 redline overspeed protection in both normal and alternate
modes. The EGT limit must be observed by the crew because the EEC does not provide
EGT redline exceedance protection.
