HSS Boresighting Theory. AH-lS(Mod)

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

TSU LOS.

target.

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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