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Figure 1-1. Helmet-direct fire control subsystem (HSS)
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TM-9-1270-212-14-P Fire Control Subsystem Helmet Directed XM128 P/N 2277716-00 NSN 1270-00-122-9449 Manual
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Figure 1-2. HSS helicopter installation
TM 9-1270-212-14&P
Section II. DESCRIPTION AND DATA
assemblies are mounted at a 0-degree azimuth angle. For
1-9. Subsystem Description.
HSS XM128, the pilot linkage has a down pitch angle of 4
degrees and the gunner linkage assembly has a down pitch
The HSS is a helmet-directed sighting subsystem that
angle of 8 degrees. In the HSS XMl36 installation (fig. 1-2,
interfaces with the gun turret and the telescopic sight unit
view B), the EIA is mounted horizontally, the gunner
(TSU) of the TOW missile subsystem in the AH-1S(Mod)
helicopters(fig. 1-2). The HSS
linkage assembly is mounted at a 0-degree azimuth angle,
and AH-1S
and the pilot linkage assembly is mounted at a 5.5-degree
operates from power supplied by the helicopter and con-
left-azimuth angle. For HSS XM136, the pilot linkage has a
sists of the following assemblies:
pitch angle of 0 degrees and the gunner linkage has a down
pitch angle of 11.5 degrees.
Helmet sight (2)
c. The four potentiometers (used for boresighting
Pilot linkage
adjustments) are on the right side of the EIA used in HSS
XMl28 and in a smaJl housing on the end of the EIA used
Gunner linkage
in HSS XMl36. The bonding jumper on the EIA used in the
HSS XM136 is approximately twice as long as that on the
Electronic interface
EIA used in HSS XMl28. The EIA used in HSS XM136 has
an azimuth bias circuit which compensates for the
Extension cable.
5.5-degree offset of the HSS XM136 pilot linkage.
a. The HSS enables the helicopter pilot and
1-11. Helmet Sight Assembly.
copilot/gunner (hereinafter referred to as gunner) to rapidly
acquire visible targets and to direct either the gun turret or
The helmet sight assembly (fig. 1-3) is a lightweight
the TSU to those targets. The helmet-mounted optical sight
aluminum visor housing with a magnetic receptacle
extends over the operator's right eye, and an illuminated
containing a reed switch attached near the top rear and a
reticle pattern is projected into the optical sight.
sight assembly attached to the front. The aluminum visor
Electromechanical linkages sense the helmet position and
generate sight-line signals, which are processed by the
housing is precision attached to the SPH-4 helmet when the
existing visor cover is removed. For initial installation, an
electronic interface assembly (EIA) and used to control the
installation kit containing special fasteners and tools and a
angular position of the turret or the TSU.
small visor knob is furnished with each helmet sight
assembly.
b. Either operator can command the gun turret or TSU
by means of operator-selectable cockpit switches. When
a. The magnetic receptacle holds the steel fastener on
necessary, the gun turret and TSU can be commanded
the front of the arm assembly (fig. 1-4) in place and a latch
simultaneously by the pilot and gunner, respectively.
insures against inadvertent separation. In an emergency, the
1-10. Differences Between Models.
latch allows separation of the steel fastener and magnet at
an axial force of approximately 20 pounds. The magnetic
a. There are two models of the HSS, HSS XM128 (used
receptacle contains a reed switch that senses the presence of
in the AH-1S(Mod) helicopter) and HSS XM136 (used in
the steel fastener. The reed switch closes when the steel
the AH- 1S
helicopters). All of the differences,
fastener is attached and opens when separation occurs.
mechanical and electrical, between the two models result
from differences in HSS installation in the helicopters. The
b. The sight assembly consists of a sight housing and an
only major item that differs between the two models is the
eyepiece. The sight housing contains a lamp assembly
EIA. The helmet sight assemblies, pilot and gunner linkage
(containing three 5-volt lamps), a reticle, and a lens system.
assemblies, and extension cable are identical and inter-
Th sight housing projects the reticle pattern, focused at
changeable between models.
infinity, into a beam splitter within the eyepiece. The beam
splitter superimposes the reticle pattern on the normal
eyepiece image. A detent in the sight housing holds the
b. In the HSS XM128 installation (fig. 1-2, view A), the
eyepiece down in the sighting position over the right eye.
EIA is mounted vertically, and the pilot and gunner linkage

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