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BIT failure isolation cont'd
TM-9-1270-212-14-P Fire Control Subsystem Helmet Directed XM128 P/N 2277716-00 NSN 1270-00-122-9449 Manual
HSS Boresighting Theory. AH-1S
TM 9-1270-212-14&P
the TSU borescope device; the 90-degree left target and
Transistors Q4 and Q5 turn on, energizing K3 and K4. The
the 90-degree right target are located in the same way.
relays connect 28 volts to light the helicopter PLT (pilot
linkage failure) and GNR (gunner linkage failure)
c. For boresighting, degrees of arc are measured in mils.
indicators. If the linkages are placed in their BIT brackets
A mil is the 6400th part of a circle; thus, 1 degree equals
and system operation is normal, reactuating the BIT switch
6400 divided by 360, or 17.77 mils. The targets must be
will clear the failure and display a GO.
placed 1000 units of measure from the helicopter, so that a
deviation of one unit of measure at 1000 units of measure
(5) Added fault isolation is discussed in this para-
distance equals 1 mi1 of angular displacement. The targets
graph. The BIT test, performed in the helicopter, insures
described in this manual are dimensioned for a 1-inch unit
normal system operation and performs limited automatic
of measure. Therefore, the targets must be located 1000
isolation. However, additional testing is required to isolate,
for example, a defective buffer amplifier, since the BIT
inches, or 83 feet 4 inches, forward, left and right of the
test only indicates that one of the eight is defective. The
TSU boresight device.
added testing is accomplished with fire-control subsystem
test set AN/GSM-249,  which individually tests all the
d. To allow accurate azimuth bearing measurements, the
printed circuit cards and the buffer amplifiers within the
three targets must be aligned to the plane of the TSU
EIA as well as the pilot and gunner linkages and the hel-
boresight device on the front of the helicopter within 1/2
met sight. The test set is available at intermediate mainte-
degree, or 8.9 mils. Since the degree may therefore not be
nance and enables repair within the EIA. The use of the test
at true vertical, a plumb bob cannot be used to make the
set is detailed in the maintenance sections of this manual.
targets vertical; rather, a gunner's quadrant is placed on the
TSU boresight device, with the bubble perpendicular to the
centerline of the helicopter. The bubble is leveled and the
1-23. HSS Boresighting Theory. AH-lS(Mod),
gunner's quadrant is transferred to the center target, where
the target is adjusted until the bubble is level when it
a. In the helicopter, the turret weapons are boresighted
parallels the horizontal line of the TSU optics target. The
to the telescopic sight unit (TSU). Since the HSS drives the
gunner's quadrant is then placed on the TSU boresight
TSU, boresighting of the HSS line of sight (LOS) to the
device with the bubble parallel to the centerline of the
TSU LOS assures optimum sighting accuracy. To obtain the
helicopter. The bubble is again leveled and the gunner's
required accuracy, the HSS must be mechanically aligned to
quadrant is transferred to the right target and then to the
the helicopter and then its electrical outputs must be
left target, and each target is adjusted until the bubble is
adjusted to bring the HSS LOS into alignment with the
level when it parallels the horizontal line of the TSU optics
b. The first step in boresighting is to set up three
targets: a center target placed at 0 azimuth with reference
e. The next boresighting step is to mechanically align
to the helicopter, one 90 degrees left, and one 90 degrees
the linkage rails in azimuth. The LOD upper plate (the rail
right. The TSU has mounting accommodations for a tool
clamp base) is rotated until the 0-degree position is
called the TSU boresight device. This device has three
obtained and locked (the 0 line on the rail clamp base is
tapered holes that receive a standard Army borescope and is
aligned with the notch on the swivel pad under this rail
used to accurately locate the three targets in relation to the
clamp base and secured in this position by inserting the
helicopter centerline and roll axis. The borescope is inserted
alignment pin through the hole in the rail clamp base and
mto the forward-looking hole in the TSU borescope device
into the mating 0-degree hole in the swivel pad); the LOD
and the center target (foldout FO-11, sheet 3) is moved
azimuth locking knob is then tightened. .This action
until the intersection of the crosshairs in the borescope
parallels the borescope adapter to the indentations for the
reticle is coincident with the intersection of the crosshairs
gunner linkage rails. Then the LOD is attached to the
forward end of the gunner rails, with the bubble level
of the TSU borescope device target. The boresight device
forward, and the T-bars are tightened. The borescope
target is located vertically the same distance above the
TSU optics target (foldout FO-11, sheet 3) as the TSU
adapter is now parallel to the gunner rails. The borescope is
inserted in the adapter and LOD elevation is adjusted until
boresight device is above the center of the TSU lens. This
only the gunner HSS target is visible. The vertical crosshairs
location insures an accurate placement of all three targets
of the target and the borescope must coincide. To adjust
(foldout FO-11, sheet 3). The borescope is placed in the
the linkage in azimuth and provide coincidence, the nut on
left tapered hole and then in the right tapered hole in
the bottom of each of the mounting-bracket studs must be

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