737 MAX Flight Crew Operations Manual
Engines, APU -
Engine System Description
7.20.4
MN-FLT-OH-201
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 DPC 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.
The ALTERNATE MODE EEC schedule provides equal or greater thrust than the
normal mode for the same lever position. Thrust protection is not provided in the
ALTERNATE mode and maximum rated thrust is reached at a thrust lever position
less than full forward.
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.
If the EECs are in alternate mode, advancing the thrust levers full forward
provides some overboost and should be considered only during emergency
situations when all other available actions have been taken and terrain contact is
imminent.
Thrust Control Malfunction Accommodation (TCMA)
Thrust Control Malfunction Accommodation is an EEC function that provides
protection against idle thrust asymmetry conditions while on the ground (RTO or
landing roll). The TCMA function does not compare thrust between the engines,
each engine is monitored independently.
March 1, 2021
