EEC modes of operation
The EEC can operate
in three modes, NORMAL, SOFT ALTERNATE and HARD ALTERNATE.
The
NORMAL MODE operation is indicated by the white ON light in the EEC button on
the ENGINE CONTROL panel. The EEC calculates and controls the required thrust
N1 by means of the total ambient pressure (PT) input from the ADIRU, or
calculates this with the total air temperature (TAT) and the ambient static
pressure (P0). The EEC calculates the Mach number reference to the N1 speed
requirement for the selected thrust setting. If the Mach number, temperature
and/or pressure changes, the reference (thrust) N1 changes where the EEC
adjusts this to the required N1 by controlling the fuel supply to the nozzles
through the HMU. (Hydro Mechanical Unit)
If
these numbers are not valid, the EEC changes to the SOFT ALTERNATE mode when
detected for more than 15 seconds. If this happens, the amber ALT light
illuminates on the EEC button below the ON indication accompanied by a MASTER
CAUTION indication. The EEC now calculates the required thrust N1 by estimating
the Mach number looking at the total air temperature (TAT), standard day
temperature (from P0) and the last valid difference between the standard day
temperature and static temperature (TO). As the EEC estimates the Mach number,
lower thrust N1 can be the result but also a possible engine limit exceedance
may occur when the OAT changes during the SOFT ALTERNATE mode operation.
SOFT
ALTERNATE automatically changes to the HARD ALTERNATE mode when the respective
thrust lever is retarded below the 19° TLA position from IDLE, and the SOFT
ALTERNATE mode is active for more than 15 seconds. This will be the case when
reaching TOD and the lever(s) retard towards IDLE, but the indication does not
change as the ON and ALT light remain illuminated.
The
QRH will direct you to select both EEC’s in the “manual” HARD ALTERNATE mode if
one EEC refers to SOFT ALT to prevent thrust lever stagger. This manual
selection is done by pressing the EEC button, extinguishing the white ON
indication leaving the amber ALT indication illuminated. Prevent this selection
in high thrust settings (retard to mid range) as large thrust changes can occur
in that range. When a too high thrust change is commanded, the EEC will not
enter the HARD ALTERNATE mode.
To calculate thrust N1 the EEC uses now static pressure (P0) to assume the Mach number together with an assumed OAT for the highest thrust N1 setting requirement. This makes sure that the required aircraft performance is met, but can cause thrust exceedances resulting in excessive EGT’s. A good reference is the FMC computed green marker on the N1 indicator to prevent overboosting the engine.
Both EEC’s require input from two DEU’s to operate in the NORMAL mode.
If only one DEU is energized both EEC’s receive 1 DEU input and change to the
SOFT ALTERNATE mode indicated by two amber ALT lights on the ENGINE CONTROL
panel, accompanied by a MASTER CAUTION. Of course you’ll also see an amber
DSPLY SOURCE indication on both PFD’s indicating that there is 1 DEU providing
info to all 6 DU panels.To calculate thrust N1 the EEC uses now static pressure (P0) to assume the Mach number together with an assumed OAT for the highest thrust N1 setting requirement. This makes sure that the required aircraft performance is met, but can cause thrust exceedances resulting in excessive EGT’s. A good reference is the FMC computed green marker on the N1 indicator to prevent overboosting the engine.
