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73-21-00-014
Purpose
The hydromechanical unit (HMU) supplies fuel for engine servo
system operation and combustion. The HMU receives fuel
metering signals and engine servo command signals from the
EEC and the airplane.
Location
The HMU is at the 8:00 position. The HMU attaches to the aft
side of the fuel pump assembly. The fuel pump assembly
attaches to the aft side of the accessory gearbox (AGB). You
open the left fan cowl to get access to the HMU.
Physical Description
The HMU has electrical and fuel connections. These connect the
HMU to the EEC, to airplane control systems, and to engine
servo systems. These connections are necessary for correct
engine operation.
These are the HMU electrical connections:
* EEC channel A electrical connector
* EEC channel B electrical connector
* High pressure shutoff valve (HPSOV) solenoid
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* Burner staging valve (BSV) solenoid.
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These are the HMU hydraulic (fuel) connections:
* Metered fuel line (on the top of the HMU, not shown)
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* Burner staging valve (BSV) fuel line
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* Variable bleed valve (VBV) OPEN fuel pressure line
* VBV CLOSE fuel pressure line
* Variable stator vane (VSV) ROD (open) fuel pressure line
* VSV HEAD (close) fuel pressure line
* Pressure case regulated (PCR) fuel pressure line
* Low pressure turbine active clearance control (LPTACC)
valve fuel line
* High pressure turbine active clearance control (HPTACC)
valve fuel line
* Transient bleed valve (TBV) fuel line
* Overboard drain line.
Training Information Point
The raised letters on the HMU identify the different servo fuel
connections.
ENGINE FUEL AND CONTROL - ENGINE CONTROL - HYDROMECHANICAL UNIT
EFFECTIVITY
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BOEING PROPRIETARY - Copyright#- Unpublished Work - See title page for details.
CFM56 ENGINES (CFM56-7) 737-600/700/800/900 AIRCRAFT MAINTENANCE MANUAL
73-21-00-015
ENGINE FUEL AND CONTROL - ENGINE CONTROL - HYDROMECHANICAL UNIT
ENGINE FUEL AND CONTROL - ENGINE CONTROL - HYDROMECHANICAL UNIT
EFFECTIVITY
GUN ALL PRE SB CFM56-7B 73-44 73-21-00
Page 43
D633A101-GUN Jun 10/2003
BOEING PROPRIETARY - Copyright#- Unpublished Work - See title page for details.
CFM56 ENGINES (CFM56-7) 737-600/700/800/900 AIRCRAFT MAINTENANCE MANUAL
73-21-00-015
ENGINE FUEL AND CONTROL - ENGINE CONTROL - HYDROMECHANICAL UNIT
ENGINE FUEL AND CONTROL - ENGINE CONTROL - HYDROMECHANICAL UNIT
EFFECTIVITY
GUN ALL POST SB CFM56-7B 73-44 73-21-00
Page 44
D633A101-GUN Jun 10/2003
BOEING PROPRIETARY - Copyright#- Unpublished Work - See title page for details.
CFM56 ENGINES (CFM56-7) 737-600/700/800/900 AIRCRAFT MAINTENANCE MANUAL
73-21-00-015
General
The electronic engine control (EEC) sends control signals to
servo system in the hydromechanical unit (HMU). The electrohydraulic
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).
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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.
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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.
Fuel Metering Valve (FMV)
The EEC controls the FMV with servo fuel pressure from the
FMV EHSV. The FMV resolver sends the position of the FMV
back to the EEC. Fuel flow through the open FMV causes the
high pressure shutoff valve (HPSOV) to open.
The EEC can fully close the FMV on the ground during engine
start for these conditions:
* Exhaust gas temperature is more than the limits for engine
start
* Engine goes to idle speed during start, but speed then
decreases below 50% N2 speed and EGT goes above the
start limit
* EEC senses fuel flow but no EGT for 15 seconds after the
start lever is put to idle with TAT more than 2C or 20 seconds
after the start lever is put to idle if TAT is less than 2C, (wet
　
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