HSS Boresighting Theory. AH-1S

TM 9-1270-212-14&P

loosened so the serrated cuts in the rail support can be

borescope is removed. An operator sighting through the

moved in relation to the serrations in the square washers.

TSU and setting the ACQ TRK STOW switch to ACQ

The choice of which support to adjust should be made so as

should see the TSU crosshairs centered on the TSU optics

to keep the slots in the rail supports centered. The

target. If the TSU crosshairs are not aligned, the electrical

serrations provide adjustments in 1-mil increments. The

output signals of the HSS must be altered slightly to

mechanical azimuth should be made as accurate as possible

provide coincidence.

to minimize crosscoupling in subsequent electrical azimuth

adjustments. Use care when tightening the nuts to insure

h. Refer to figure 4-7. Step A shows a typical plot of

the elevation discrepancies recorded at the two 90-degree

that the serrations are matched. Check the alignment with

positions. Note that a straight line between the 90-degree

the borescope before removing the LOD. The pilot rails are

adjusted in like manner, using the borescope to sight on the

positions does not pass through the center point. This

pilot HSS target, except that the LOD rail clamp base is

example represents a composite of roll error (the ends are

secured in the 5.5-degree position for alignment of the pilot

not parallel to the reference lines established by the targets)

linkage in XM136.

and the pitch error (the three points are not in the same

plane). The goal of the adjustments is to make corrections

f. The next boresighting step is to align the rails to the

until the sighting points plot as shown in step D of figure

roll axis of the helicopter. The bubble level on the LOD is

4-7. The roll adjustment is made first by rotating the roll

used to accomplish this alignment. The bubble level has a

adjustment screws at each end of the linkage rails in

5-minute bubble; that is, each mark on the bubble

l/4-turn (90-degree) increments. It is important to turn the

represents 5 minutes of a degree; 5/60ths equal 1/12th

screws at each end in equal amounts to maintain the rail

degree, which equals 1.48 mils. The LOD is used to transfer

ends parallel to each other while flattening the roll plane to

the roll position of the TSU boresight device to the linkage

bring the sightings level, as shown in step B of figure 4-7.

rails. The LOD is placed on the TSU boresight device, with

Note that the three sightings are not yet in the same plane.

the bubble facing forward, and the thumbscrews are

Adjustment of resolver B1 in the linkage affects only the

adjusted to carefully center the bubble. The LOD is

center sighting and is called the pitch adjustment.

carefully attached to the forward and then the aft end of

Therefore, the next adjustment is to raise the center

the gunner and pilot rails, and the rails are adjusted at each

sighting point by adjusting the worm sector attached to the

end to center the bubble. (The LOD rail clamp base is

body of resolver B1. The worm sector provides a l-in-l7 gear

secured to the 5.5degree position for the pilot linkage in

reduction and a resolver body travel of 20 degrees

HSS XM136 and in the 0-degree position for the other

minimum. When the pitch error has been corrected, the

linkages.) The mechanical adjustments are now complete.

typical pattern should look like step C in figure 4-7. The

next step is to correct the elevation of all three by adjusting

NOTE

the EL potentiometer on the EIA until the three sightings

are in the same plane, as shown in step D in figure 4-7. The

Before power is applied, each of the four

final step is to correct azimuth (if necessary) by adjusting

boresight potentiometers on the EIA must be

the AZ potentiometer on the EIA. The methods for the

set to the approximate midpoint of its limits.

pilot linkage are the same except that the LOD rail clamp

g. T h e remaining boresighting adjustments are

base is secured in the 5.5-degree position for the pilot's rails

in HSS XM136.

accomplished with the HSS operating. The LOD is carefully

attached to the forward end of the gunner rails with the

1-24. HSS Boresighting Theory. AH-1S

level forward, and the gunner linkage is attached to the

magnet on the LOD. The LOD elevation and azimuth locks

Refer to TM 9-1090-206-30, Aviation Intermediate

are slightly loosened and the borescope is inserted in the

Maintenance Manual for Armament Subsystem, Helicopter:

LOD adapter. The LOD is moved as required to center the

20-mm Automatic Gun, XM97E1, for composite bore-

borescope crosshairs on the gunner HSS target of the center

sighting procedures.

target. When the borescope is centered on the target, the

LOD elevation and azimuth locks are tightened, and the

1-25