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feet above takeoff altitude and one of these conditions is true:
* Airspeed more than 300 knots
* Altitude more than 4500 feet above takeoff altitude
* TRA decreases more than 3 percent.
The EEC uses fan trim balance to adjust the engine fan speed
for engine variations. After engine assembly, engine tests are
done to make sure it meets engine performance requirements.
One of these tests is to measure engine thrust and fan speed.
To make sure all engines have the same takeoff and early climb
thrust for the same indicated N1 speed, the fan trim is used to
adjust indicated and commanded N1 speeds. This adjustment is
used to decrease the commanded N1 speed and increase the
indicated N1 speed. When the EEC changes between N1 fan
trim and no N1 fan trim, the transition is slow to make sure there
is not a sudden change in thrust. This adjustment is effective
when these conditions occur:
* Altitude is below 15,000 ft (4572 m)
* Mach number is less than 0.40
* N1 speed is between 75 and 99.54 percent.
There are eight N1 fan trim levels. These fan trim levels are N1
trim 0 through 7 with 0 as no trim and 7 the maximum trim of
2.36 percent. The N1 trim level for the engine is stored in the
engine identification plug. The N1 trim level shows on the EEC
CDU IDENT/CONFIG page of EEC BITE. See the TRAINING
INFORMATION POINT pages in this section for more
information on EEC BITE.
EEC Mode Description
The EEC gets ambient total pressure (PT) from the ADIRUs or
calculates it from ambient total air temperature (TAT) and
ambient static pressure (P0). The EEC gets P0 from the ADIRUs
or from the P0 transducers in the EEC. The EEC gets TAT from
the ADIRUs or from the T12 sensor on the engine.
These are the three EEC operation modes:
* Normal mode
* Soft alternate mode
* Hard alternate mode.
The EEC is in normal mode when all these conditions occur:
* PT is valid
* EEC SWITCH on the P5 aft overhead panel is in the ON
position.
PT is valid when these conditions occur:
* PT signal from both ADIRUs are in limits
ENGINE FUEL AND CONTROL - ENGINE CONTROL - THRUST CONTROL - FUNCTIONAL DESCRIPTION
EFFECTIVITY
GUN ALL 73-21-00
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D633A101-GUN Oct 10/2002
BOEING PROPRIETARY - Copyright#- Unpublished Work - See title page for details.
CFM56 ENGINES (CFM56-7) 737-600/700/800/900 AIRCRAFT MAINTENANCE MANUAL
73-21-00-021
* PT signals agree
* Pitot probe heat for at least one PT probe is on
* Pitot probe heat off, airplane is on the ground and TRA is
less than 53 degrees.
In the normal mode, the EEC calculates Mach number with the
two PT values from the ADIRUs and P0. The Mach number is
one of the parameters used to calculate the N1 reference
speeds. If outside air pressure, temperature, and Mach number
change, the N1 reference speeds also change. This makes sure
engine thrust is satisfactory for airplane performance.
If PT is not valid or the EEC switch is put in the OFF position, the
EEC goes to one of the alternate modes. The EEC energizes the
ALTN light on the P5 aft overhead panel when one of these
occur:
* EEC is in soft alternate mode for 15 seconds
* EEC is in hard alternate mode
* EEC switch is selected to OFF (this puts the EEC in hard
alternate mode).
The EEC goes to the soft alternate mode when PT is not valid. If
PT becomes valid within 15 seconds, the EEC goes back to the
normal mode and the ALTN light does not come on. After the
EEC is in the soft alternate mode for 15 seconds, the ALTN light
comes on. The EEC goes back to the normal mode and the
ALTN light goes off if these conditions occur:
* PT becomes valid
* EEC is in soft alternate mode
* Engine thrust change when the EEC mode changes back to
normal mode is small or thrust levers are near to idle (TRA
less than 51.6 degrees).
In the soft alternate mode, the EEC uses this data to estimate
Mach number:
* Total air temperature (TAT)
* Standard day temperature (from P0)
* Last valid difference between standard day temperature and
static temperature (TO).
The soft alternate mode makes sure the engine thrust does not
have large changes when PT data is not valid. Engine thrust can
be less than normal or engine exceedances can occur if outside
air conditions change while the EEC is in soft alternate mode.
This occurs because the EEC estimates Mach number using
TAT, standard day temperature, and the last valid value of delta
air temperature from standard day. Delta temperature from
standard day is usually calculated from standard day
temperature and static air temperature. While in normal
　
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