FADEC Operation

[B73567AMT]

Using FADEC control system used on B737-Next Gens as the model, here is my question:

Once the engine has been started and running, start levers in IDLE, will a total failure in Channels A & B result in an engine shutdown or will the engine continue to run and the speed that the failure occured?

A total failure meaning the inability to power the channels using aircraft power or the EEC Alternators.

This is what I think...

The HPSOV (Fuel Valve) in the HMU is controlled by the Start levers or Fire Handle. The solenoid which closes the HPSOV is only powered when the Start levers are in CUTOFF or the fire handle is pulled.

The FMV (Fuel Metering Valve in the HMU) is driven by the FMV EHSV. The EHSV (Electro-Hydraulic servo valve) is electrically driven by a torque motor commanded by electrical signals from the EEC.

I think that if all electrical power to the EEC is lost, but the startlevers are left in IDLE and the fire handle NORM, then the EEC cannot electrically command the FMV closed. It cannot command any of the torque motors in the HMU, so the VSVs, VBVs and turbine colling valves, etc will remain where they are. Therefore, the engine will continue to run at that setting.
It may not be able to be controlled, but it will run.

Anybody have anything else to add.

[avioniker]

ENGINE FUEL AND CONTROL - ENGINE CONTROL - HMU - FUNCTIONAL DESCRIPTION ALL General The electronic engine control (EEC) sends control signals to servo system in the hydromechanical unit (HMU). The electro-hydraulic servo valves (EHSVs) 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 (HPTACC) valve Low pressure turbine active clearance control (LPTACC) valve Variable bleed valves (VBV) Variable stator vanes (VSV). AAL ALL P, 3AR-3AY, 3BA-3BH, 3BJ-3BN, 3BP, 3BR-3BY, 3CA-3CH, 3CJ-3CN, 3CP, 3CR-3CY, 3DA-3DH, 3DJ-3ZZ PRE SB CFM56-7B 73-44 The EEC also sends a signal to the burner staging valve (BSV) solenoid in the HMU. This valve supplies servo fuel to open and close the BSV. 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 independent of the EEC. When closed this valve stops fuel flow to the combustor. The mechanical overspeed governor makes sure the N2 rotor does not turn too fast.

HMMMMMMMM, sounds right to me.....:confused:

[DoctorA300]

AMT,
The FMV goes to minimum flow stop. Dependant on OAT N2 would be around 50%.
Brgds
Doc

[B73567AMT]

DoctorA300...
How did you come across that info??