B737NG HMU & EEC Functional Description
Skytrucker,
Firstly the HMU(heart of the system),
The electronic engine control(EEC) sends control signals to servo systems in the Hydromechanical unit (HMU).The electro-hydraulic servo valves (EHSV`s) in the HMU change these signals to hydraulic fuel pressure for these components:
- fuel metering valve(FMV)
- transient bleed valve(TBV)
-high pressure turbine active clearance control valve(HPTACC)
-low pressure turbine active clearane control valve(LPTACC)
-variable bleed valves(VBV`s)
-variable stator vanes(VSV`s)
-burner staging valve(BSV) optional on engines
The high pressure shutoff valve (HPSOV) and mechanical overspeed governor are also in the HMU.The start lever and the fire handle control the HPSOV independant of the EEC.When closed this HPSOV stops fuel flow to the combuster.The mechanical overspeed governor makes sure the N2 rotor does not turn too fast.
Also note that there is no mechanical link between the thrust levers and the EEC,the thrust levers move Resolvers that electronically signal the EEC of the DESIRED Thrust.
Secondly the EEC(the brains of the system),
Each EEC has two computers.Each computer can control the engine.One computer is in active control while the other is in standby.The computers are called Channel`s A & B.The two channel communicate through a cross channel data link (ccdl).
Each EEC has driver circuits which changes digital command signals to analog signals that go to the HMU.
Each EEC has sense circuits whish read signals from various sensors on the engine and airplane.The active channel can read input data from either channel via the CCDL.The active channel chooses the best signal OR averages the signal to calculate the value it uses to control the engine.
If the active channel is not valid,the standby channel becomes the active.If one EEC channel is not valid the EEC stays in dual channel mode.Dual channel mode lets the active channel use the sense circuits from both channel for engine control.If one channel is not valid,a fault is stored in the BITE memory.Many of these EEC faults cause the ENGINE CONTROL and MASTER CAUTION lights in the flight deck to come on.
The EEC is usually in dual channel mode,it will go to single channel mode when the EEC Alternator supplies power to only one channel.The EEC channel with the alternator power becomes the active channel and the other the standby.The standby channel get its power from the airplane transfer bus.The EEC also goes to single channel mode when the EEC channels can`t communicate via the CCDL.When in single channel operation the active EEC channel uses only its sense circuits to control the engine.
When both channels opertae normally,channels A & B alternate between active & standby each time the engine is started.
Thirdly EEC Alternate MODES.
There are two alternate modes, the soft alternate & hard alternate modes also called soft & hard reversionary modes.
The EEC goes to soft alternate mode when PT(total pressure) as sensed by the ADIRU`s is not valid.If PT becomes valid,the EEC goes back to normal mode and the ALTN light(in the EEC switch) does not come on.After the EEC is in the soft Alternate mode for more than 15 seconds the ALTN light comes on.The EEC goes back to normal mode and the ALTN light goes off if these conditions occur:
-PT becomes valid
- EEC is in soft alternate mode
-engine thrust change change when the EEC mode changes back to normal is small OR thrust levers are near to idle.
The EEC goes to HARD ALTERNATE mode when these conditions occur:
-EEC is in soft alternate mode for more than 15 seconds(ALTN light on) & the thrust leveris less than 19 degrees above idle.
-the EEC switch is in the off position(ie SELECTED by Crew).
In the hard alternate mode the EEC uses static pressure to get an assumed Mach number (used for thrust req`d calculations).
The EEC will return to normal mode if one of these occur:
-EEC switch goes from off to on position and PT is VALID
OR
-the engine is shutdown & restarted,PT is valid and the EEC switch is in the ON position.
Hope this helps