Electronic Engine Control
Each engine has a full authority digital Electronic Engine Control, or EEC. The EEC is a computer that monitors and analyses engine parameters to maximise performance.
Normally, the EEC sets thrust based on thrust lever position and continually computes maximum thrust limits. The auto throttle computer then uses the max thrust data to calculate thrust lever angles.
Electrical power is supplied to each EEC by a dedicated alternator that is independent of the aircraft’s electrical system. These operate when the engines are at >15% N2. Each EEC has two independent control channels and can automatically transfer between them should the operating channel fail.
Data for the EEC is received from various aircraft systems through the DEUs, and sent in digital and analogue format to other systems. It interfaces with the following engine control systems: ?
- HMU to meter fuel into engines
- Engine Air Control systems to adjust stator vane angles
- Fuel Flow Transmitters
- Ignition and starting systems
- Thrust reverser operation
EEC Modes
The EECs are controlled on the engine panel and can be placed in one of three different modes: ?
- ON → normal mode. Engine ratings set by EEC from atmospheric conditions and bleed demand.
- ALTN → Hard alternate mode. EEC provides rated thrust or higher.
- ON + ALTN → Soft alternate mode.
In normal mode, the EEC uses sensed flight conditions and bleed air demands to calculate N1 thrust rating. It then compares desired thrust to actual N1 and adjusts fuel flow to achieve commanded N1.
Should required signals to operate in normal mode be unavailable, the EEC automatically switches to soft alternate mode. In this mode, the EEC uses the last valid parameters to define engine parameters. Note that this means thrust rating shortfalls or exceedances may occur as flight conditions change.
The soft alternate mode remains in place until hard alternate mode is set by either the thrust lever is retarded to idle or manually selecting it via the EEC switch.
Hard alternate mode is always equal or greater than normal mode thrust for the same lever position.
The ENGINE CONTROL lights illuminate when there are engine control system faults. The lights only operate on the ground - they are inhibited from 80 kts on takeoff to 30 seconds after landing. They indicate that the aircraft cannot be dispatched.
Idle Modes
The EEC automatically selects ground minimum, flight minimum, and approach idle.
As expected, ground min idle is used for ground operations, while flight minimum idle is selected for most phases of flight.
Ground idle is 59% N2.
Approach idle is selected in flight if the flaps are in the landing configuration or if engine anti-ice is on for either engine. This mode will remain active until after touchdown and ground idle is selected.
The function of Approach Idle is to maintain higher N1 and N2 RPM to improve engine acceleration in the event of a go around. In flight, if a fault prevents the EEC from receiving flap or anti-ice status signals, Approach Idle is active below 15,000 MSL.
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