# Military

## CHAPTER 6

#### 6-1. RESPONSIBILITIES

The platoon leader is responsible for determining the lowest quadrant elevation that can be safely fired from his platoon position that will ensure projectiles clear all visible crests.

 Note: All references to platoon leader will apply to the executive officer as well. Also, the gunnery sergeant is responsible for these same duties prior to the platoon's occupation of position.

#### 6-2. ELEMENTS OF COMPUTATION

Use of the rapid fire tables (Appendix K) is the fastest method of computing minimum quadrant elevation (min QE). Manual computations are more accurate than the rapid fire tables and must be used if the sum of the vertical angle and the angle needed for a 5-meter clearance is greater than 300 mils. The tables were not constructed for the value of the sum of Angles 1 and 2 to exceed 300 mils. Figure 6-1 shows the elements of minimum quadrant elevation.

b. Angle 1, Figure 6-1, is the angle of site to crest for the weapon. See paragraph 6-3 for measuring procedures. The largest site to crest will not necessarily yield the largest minimum quadrant.

c. Angle 2, Figure 6-1, is the vertical angle required to clear the top of the crest. For quick, time, and unarmed proximity (VT) fuzes, a vertical clearance of 5 meters is used. For armed VT fuzes, see paragraph 6-6.

d. Angle 3, Figure 6-1, is the complementary angle of site (comp site). It is the comp site factor (from the TFT, Table G) for the appropriate charge at the piece-to-crest range multiplied by the sum of Angles 1 and 2. If the PCR is not a listed value, use the next higher listed PCR. Site (si) is the sum of Angles 1, 2, and 3.

e. Angle 4, Figure 6-1, is the elevation for the appropriate charge corresponding to the PCR. A large PCR may cause the value of Angle 4 to override the effects of site to crest (Angle 1). Therefore, minimum quadrant must be computed for each weapon and each charge to be fired.

f. Angle 5, Figure 6-1, is a safety factor equivalent to the value of 2 forks (from the TFT, Table F) for the appropriate charge at the PCR.

g. The greatest sum of Angles 1-5, Figure 6-1, is the minimum quadrant elevation for the charge computed.

#### 6-3. MEASURING ANGLE OF SITE TO CREST

During advance party operations, site to crest is measured with the M2 compass or M2A2 aiming circle (refer to chapter 4 for these procedures). If the M2 compass is used, add 20 mils to allow for the accuracy of the M2 compass. During position improvement, the chief of section verifies the angle of site to the crest and reports it to the platoon leader. To verify the angle of site to crest, the chief of section, sights along the bottom edge of the bore, has the tube traversed across the probable field of fire and has the tube elevated until the line of sight clears the crest at the highest point. He then centers all bubbles on the elevation mount, and reads the angle of site to the crest from the elevation counter. The angle of site is read and expressed to the nearest whole mil. This angle of site and PCR are reported as part of the section chief's report (paragraph 2-20).

#### 6-4. MEASURING PIECE-TO-CREST RANGE

a. There are five methods that can be used to measure piece-to-crest range:

(1) Taping, the most accurate method, is normally too time consuming.

(2) Subtense is fast and accurate.

(3) Map measurement is fast and accurate if the obstacle can be located (for example, a lone tree will not appear on a map).

(4) Pacing is time-consuming and dependent on the distance and accessibility to the crest.

(5) Estimation is least accurate, but it is used when other methods are not feasible.

b. Regardless of the method used, PCR must be verified by the gunnery sergeant or platoon leader before computing minimum quadrant elevation. He can do this by measuring by one of the five methods listed above and comparing the results.

#### 6-5. COMPUTATION FOR FUZES OTHER THAN ARMED VARIABLE TIME

a. The gunnery sergeant or platoon leader performs the computations indicated in this section if the sum of angles 1 and 2, Figure 6-1, exceeds 300 mils or if the rapid fire tables are not available. All angles are determined and expressed to the next higher whole mil. He performs the computation for all howitzers. The example below shows his computations.

b. One howitzer section may report a sight to crest that is unusually high. If the platoon leader determines that it is due to a single narrow obstruction (such as a tree), that piece may be called out of action when firing deflections that would engage the obstruction. This would enable the platoon to use the next lower minimum quadrant. Other alternatives are to remove the obstruction or move the weapon.

 EXAMPLE Gun 1 has a range to crest of 1,100 meters; the angle of site to crest reported is +16 mils. Gun 1 is an M109A2 155-mm howitzer, and charge 3 green bag will be fired. The platoon leader does the following: Note: All computations are derived from AM-2 TFTs, graphical firing tables (GFTs), and graphical site tables (GSTs). Angle 1: Records the angle of site reported by the chief of section . . . . . . . . . . . . +l6 mils Angle 2: Determines the vertical clearance in mils. He uses the GST to divide the vertical clearance in meters (5 meters) by the PCR in thousands (1.1). This value, read under the M gage point, is 4.6, and is expressed to the next higher whole mil. . . . . . . . . . . . +5 mils Angle 3: Determines comp site by multiplying the comp site factor corresponding to the piece-to-crest range (or the next higher listed range in the TFT, Table G, if that range is not listed) by the sum of angles 1 and 2. Angle 1 + Angle 2 = +21. The comp site factor corresponding to 1,500 meters (range 1100 is not listed in Table G) is +0.010. Therefore, +21 x 0.010 = +0.210. Once a value for comp site has been determined, it must be expressed to the next higher whole mil. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .+1 mil Angle 4: Determines elevation for the PCR (TFT, Table F, column 2). If this value is not a whole number, it is expressed to the next higher whole mil (74.1 expressed to 75) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . +75 mils Angle 5: Determines the value of 2 forks (TFT, Table F, column 6) at PCR (2 x +2 mils) . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .+4 mils Total: Add angles 1-5 to determine the platoon leader's minimum quadrant elevation. 16 + 5 + 1 + 75 + 4 = 101 mils QE, charge 3 GB Therefore, the min QE for Gun #1, Chg 3GB, is 101 mils. The platoon leader will compute the min QE for each howitzer in his firing unit. The highest value is the XO min QE for his firing unit with this charge.

#### 6-6. COMPUTING FOR ARMED VT FUZES (LOW-ANGLE FIRE)

The method of computing the XO's minimum QE for firing a projectile fuzed with an M513, M514, M728, or M732 fuze depends on the method in which the fuze is used.

a. The proximity (VT) fuze is designed to arm 3 seconds prior to the time set on the fuze; however, some VT fuzes have armed as early as 5.5 seconds prior to the time set on the fuze. Because of the probability of premature arming, a safety factor of 5.5 seconds must be added to the time of flight corresponding to the PCR. Since time on the setting ring is set to the whole second, the time determined is expressed up to the next higher whole second. A VT fuze is designed so that it will not arm earlier than 2 seconds time of flight, which makes it a bore-safe fuze.

b. In combat situations, the platoon leader determines the minimum safe time and minimum quadrant elevation at the piece-to-crest range. The QE determined for PD fuzed rounds is safe for VT fuzes if the time set is greater than the min safe time determined (paragraph a above). If the platoon leader finds it necessary to fire a VT fuze with a time less than the min safe time, the vertical clearance for the minimum QE must be increased to ensure the fuze will not function as it passes over the crest.

c. If the projectile is to be fired with the VT fuze set at a time less than the minimum safe time, allowance must be made for vertical clearance of friendly elements. Vertical crest clearances for M513, M514, M728, and M732 VT fuzes fired over ordinary terrain are shown in Table 6-1.

d. If the projectile is to be fired over marshy or wet terrain, the average height of burst will increase. Therefore, the vertical clearance shown in Table 6-1 should be increased by 50 percent. If the projectile is to be fired over water, snow, or ice, the vertical clearance shown in Table 6-1 should be increased by 100 percent.

e. The minimum QE for fuze VT, when a fuze setting less than the minimum safe time is fired, is based on PCR and a greater vertical clearance as indicated in Table 6-1 instead of the 5 meters as stated in paragraph 6-2c.

f. The following is an example of computations to determine minimum QE for armed VT fuzes.

 EXAMPLE The howitzer is a 155-mm M109A3, charge 4 green bag, and armed variable time fuzes (M514) are to be fired. The angle of site reported by the chief of section is +16 mils. The PCR is 1,700 meters. The platoon leader computes the minimum QE as follows: ANGLE 1: The angle of site to crest reported by the chief of section . . . . . . . . . +l6 mils ANGLE 2: The vertical clearance in mils. Divide the vertical clearance in meters (100 meters) by the PCR of 1,700 meters in thousands (1.7). By reading under the M gage point of the GST, derive a value of +59 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . +59 mils ANGLE 3: Comp site for PCR of 1,700 meters (FT 155-AM-2, Table G, charge 4 GB, column 12, range 2,000 meters gives a comp site factor of 0.010.) 16 +59 = 75. 75 x 0.010 = +0.750 expressed to the next higher whole mil . . . . . . . . . . . . . . . . . . . .+1 mils ANGLE 4: Elevation at PCR (TFT, Table F, column 2).(90.0).. . . . . . . . . . . . . +90 mils ANGLE 5: The value of 2 forks at PCR (TFT, Table F, column 6). (2 x +2 mils) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . +4 mils Minimum safe time (time of flight [TOF] at the PCR = 5.6 + 5.5 = 11.1 or 12.0 seconds) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.0 seconds The MINIMUM QUADRANT ELEVATION FOR THIS GUN IS 170 MILS, CHARGE 4 GREEN BAG, MINIMUM SAFE TIME 12.0 SECONDS (M514), FUZE ARMED VT. The platoon leader will compute the min QE for each gun in his firing unit. The highest value is the XO min QE for his firing unit with this charge.

g. If the fuze setting to be fired is equal to or greater than the minimum safe time, the minimum QE determined for fuzes quick and time applies. If the fuze setting to be freed is less than the minimum safe time, the minimum quadrant elevation determined for armed VT applies.

h. Table 6-2 is a recapitulation of the steps for computing minimum quadrant elevation. It should be used as a reference by the platoon leader and in training individuals to compute minimum quadrant elevation.

i. The XO min QE is compared to the min QE to the minimum range line as computed by the FDC. The greater of these two values is placed on the safety T.