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control head. This is a five-position, center springloaded
to neutral switch used to control the heading
hold mode. Pressing the switch forward to the ENGA 2-38. VERTICAL FIN.
position engages the heading hold mode and pressing The vertical fin (51, fig. 2-1) incorporates mounting
it rearward to the DISENG position disengages the hardware which allows the fin to be rotated, creating a
heading hold. Pressing the switch left (L) or right (R) lower vertical profile for easier storage and
will trim the helicopter left or right. transportability.
2-37. HORIZONTAL STABILIZER.
The horizontal stabilizer (50, fig. 2-1) is located near
the center of the tailboom. The horizontal stabilizer aids
in trimming the helicopter in level flight and increases
SECTION VI. HYDRAULIC SYSTEM
2-39. HYDRAULIC SYSTEM. If indicator is extended, reset by pressing in. If indicator
remains in during runup, helicopter is flyable. If
The hydraulic system reduces crew member effort and indicator extends again, shut down helicopter and make
fatigue by minimizing cyclic, collective, and antitorque appropriate entry in DA Form 2408-13-1 and 2408-13-
control input and feedback forces. 1E. Do not fly helicopter until appropriate maintenance
action has been accomplished.
2-40. RESERVOIR AND SIGHT GLASS.
The hydraulic reservoir is located on the cabin roof and 2-42. SOLENOID AND SYSTEM RELIEF
is mounted on a bracket, which is forward and left of VALVES.
the transmission. A window is provided on the cowling
for inspection of the sight glass. A sight glass is a. Solenoid Valve. Solenoid valve is designed to
provided to determine when the reservoir needs provide pressure to the system when it is de-energized.
servicing. Normal fluid level is indicated when hydraulic The solenoid valve is de-energized when the HYD SYS
fluid completely fills sight glass. switch is in the HYD SYS position or in the event of
loss of electrical power to the valve. Placing the HYD
SYS switch to the OFF position will energize the valve
2-41. HYDRAULIC FILTERS. and pump pressure will be blocked with the system
pressure connected back to the reservoir.
Hydraulic filters are installed for both pressure and
return sides of the hydraulic system. When a filter b. System Relief Valve. The hydraulic system
becomes clogged it will give a warning by raising a red contains a relief valve located between the pressure
indicator button. The red indicator pops out when set and return portions of the hydraulic system. The unit
differential pressure across the filter is exceeded. Once will protect the system from overpressurization in the
actuated, the indicator will remain extended until it is event of a hydraulic pump malfunction.
reset manually. When the indicator is in the reset
position, it will be hidden from view. An access door is
provided on right side of the cowling for inspection and 2-43. HYDRAULIC SYSTEM SWITCH.
servicing.
The hydraulic system switch is located on the SCAS
NOTE control panel (1, fig. 2-17). The switch is labeled HYD
SYS and OFF. This switch controls the activation and
While an extended red indicator indicates an deactivation of the hydraulic system. The circuit is
impending filter stoppage, its operation is powered by the DC essential bus and is protected by
also affected by low temperature, pressure the HYD SYS circuit breaker on aft overhead console
surges, and excessive vibration. panel (fig. 2-8).
2-37
TM 1-1520-248-10
2-44. PRESSURE CAUTION MESSAGE. switch will de-energize, resulting in a LOW HYD
PRESS caution message being displayed on the MFD.
The hydraulic system contains a pressure switch
located on the pressure manifold. When hydraulic
pressure drops below safe operating limits the pressure
SECTION VII. POWER TRAIN SYSTEM
2-45. POWER TRAIN SYSTEM. 2 - 4 8 . T R A N S M I S S I O N A N D M A S T
The power train system consists of the freewheeling ASSEMBLY.
assembly, input driveshaft, transmission and mast The transmission and mast assembly transfers the
assembly, oil cooler fan assembly, tail rotor driveshaft engine torque to the main rotor system with a two-stage
assemblies, and tail rotor gearbox. Also included are gear reduction. The assembly is mounted to the roof of
the related components such as temperature and the helicopter, forward of the engine, by the pylon
pressure indicators, torque indicator, electromagnetic assembly. There are two restraint spring assemblies to
chip detectors, oil pump, and oil filter. restrain the pitching motion of the transmission
assembly. The transmission is equipped with several
sensors and indicators (para. 2-50) to ensure proper 2-46. FREEWHEELING ASSEMBLY. operation and give immediate indication of any
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TECHNICAL MANUAL OPERATOR’S MANUAL FOR ARMY OH-58D HELICOPT(20)
