R R  VSV Control - Schematic Figure 007
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R R  VBV Control - Schematic Figure 008
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 (4) Transient Bleed Valve (TBV) Control The TBV function is to improve the stall margin of the HP Compressor during engine starting and acceleration through the Start Bleed System. The TBV position is controlled by the ECU, working through the HMU, as a function of the following parameters: N2K25 (corrected core speed), N2 (physical core speed), N2 switched accel, N2 min. The ECU logic calculates the bleed air to unload the HPC 9th stage to give the optimum stability for transient mode operation. The 9th stage bleed air is sent to the Low Pressure Turbine Nozzle first stage providing an efficient start stall margin. A feedback valve position is used by the ECU to indicate the TBV position.

 (5) High Pressure Turbine Clearance Control (HPTCC) The HPTCC valve modulates air from 4th and 9th stages to turbine shroud. The ECU controls the valve position. A feedback signal is used by the ECU to indicate the valve position to the desired value.

 (6) Low Pressure Turbine Clearance Control (LPTCC) The LPTCC valve modulates the fan air flow to the LPT casing. The ECU controls the valve position. A feedback signal is used by the ECU to indicate the valve position to the desired value.

 (7) Oil/Fuel Temperature Control The IDG oil is cooled by engine fuel through an oil/fuel heat exchanger (IDG oil cooler). For some aircraft operation, extra heat rejected in fuel is carried out of the engine fuel system through the fuel return valve (FRV) in order not to exceed defined temperature limits (either engine fuel/oil temperature or IDG oil temperature) (Ref. 73-10-00). ECU performs this temperature control using the engine oil temperature. ECU has two actions depending upon the temperature values and the aircraft flight conditions:

 - command the FRV in order to permit a fuel return to the aircraft tank

 - increase the engine speed (which leads to decrease the temperature of the cooling fuel flow). The "open FRV" command can be overriden to shut off by the aircraft system from a discrete signal sent through EIU. The fuel return valve controls 2 levels of flows back to tank controlled by the ECU. The ECU logic is based on engine oil temperature which is correlated with IDG oil temperature. Each level of flow is corresponding to a level of IDG oil temperature (Ref. 73-10-00).

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 A "close" command from the HMU (master lever OFF) interrupts fuel return flow to the aircraft (Ref. 73-10-00). The FRV shuts off (no fuel return):

 - when IDG oil temperature does not exceed defined temperatures

 - when FADEC receives an aircraft discrete inhibition signal from the fuel system (through the EIU).

 - when the engine is shutdown (N2 below 50 percent)

 - when fuel flow to the burner is above 5520 pph FADEC automatically increases the engine idle speed when the engine oil temperature is above limit allowing to decrease the cooling fuel flow temperature. Excessive engine oil temperature is warned to the cockpit by an ARINC word.

R **ON A/C 001-049, 051-099, 101-105, 151-199, 201-211,
 (8) Burner Staging Valve Control
 (Ref. 73-10-00)

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 C. Engine Limits Protection
 (1) General The ECU prevents inadvertent overboosting of the expected rating (N1 LIMIT or N1 TARGET) during power setting. ECU provides engine overspeed protection for N1 (FAN ROTOR SPEED) and N2 (CORE ROTOR SPEED) in order to prevent engine exceeding certified limits. (Ref. Fig. 009) The ECU monitors EGT and sends an appropriate message to the cockpit display in case of abnormally high EGT. The ECU also provides Max PS3 protection through the fuel control limitations (Ref. Para. 2.B.). Mechanical protections are also provided on WF and fuel pressure by mechanical means.

 (2) Overspeed Protection
 The engine overspeed protection is provided by :

 - Two electrical N2 governors (one per channel)

 - Two electrical N1 governors (one per channel)

 - One mechanical N2 governor which is located in HMU. The electrical governors operate with the engine control laws or the fuel metering valve to limit rotors speed to red line value.

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