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G
ECU o
input
v e r n e r i n
PC L Collective input input g
c. Hydromechanical Unit- The hydromechanical unit is mounted to, and driven by the AGB. The HMU also receives filtered fuel through the AGB core passages. The HMU contains a high pressure pump that delivers fuel to the combustion chamber.  The vane pump is self priming and is driven directly by the AGB (9,947 rpm); it pressurizes the fuel to about 300 psi at IDLE, or to 400-832 psi at FLY.  The HMU Main fuel metering control responds primarily to three main inputs: Collective pitch through the LDS, Power Control Lever through the PAS, and ECU inputs for governing. The HMU also responds to Compressor Inlet Temperature (T2), Compressor Discharge Pressure (P3) and Ng speed. In addition to fuel flow metering the HMU positions variable geometry (VG) linkage through a hydraulic piston extending from the left side of the HMU.

Hydromechanical Unit 9 Functions:
1.
 Fuel Pumping (high pressure Ng driven vane pump).  

2.
 Fuel flow Metering (main metering valve controlled by the pilot, HMU, and ECU).  

3.
Collective pitch compensation through LDS (incr/decr fuel flow via collective movement). 

4.
Acceleration/Deceleration fuel flow limiting (limits metering valve movement rate).  

5.
Ng limiting and Ng shutdown (max. Ng at 103%, shutdown at 110%).  

6.
Variable Geometry positioning (IGV, variable vanes, and AISBV).  

7.
Torque motor to trim Ng governor (ECU/HMU interface, pilot override through ECU lockout).

8.
 PAS override allowing manual control with ECU inoperative (ECU lockout).

9.
Open Vapor Vent for fuel system priming (PCL in lockout position opens a HMU case vent, allows pilot to purge fuel system using prime boost, or submerged boost pump).

Ng Limiting- Ng limiting can be a function of Ng governing.  The HMU mechanically computes and sets the maximum Ng speed IAW the T2 sensor input, or a physical limitation in which the Ng speed is "Max Fuel Flow limited", the resultant Ng speed with metering valve full open.  The type of limiting that is in effect depends on the T2 (engine inlet temperature). Under warm climatic conditions higher Ng speeds are allowed with the maximum Ng limited to 103 % Ng (the fuel flow limited).  In warm conditions, TGT limiting will most likely limit the Ng prior to Ng limiting.  In extreme cold conditions, less Ng speed is allowed. The Ng limiting (Ng governing) may be entered prior to the TGT limiting.  The Ng speed - T2 charts can be found in TM 1-2840-248-23 (Engine, Aircraft Turboshaft Models T700).
ECU inputs: TGT LIMITING, Np GOVERNING, and LOAD SHARE. ECUfeedback : fromLVDT inHMUto stabilize torque motor amplifier,failure of LVDT causes oscillation. malfunction.
Power Control Lever
Sets/Limits maximumpower
　
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