6-50 Chp6
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FM 6-50, MCWP 3-1.6.23 CHAPTER 6 MINIMUM QUADRANT ELEVATION 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. a. Piece-to- crest range (PCR) is the horizontal distance between the piece and the crest, expressed to the nearest 100 meters. Procedures for measurement are discussed in 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 tired. 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 paragraph 6-4. minimum quadrant elevation for the charge computed. 6-1
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FM 6-50, MCWP 3-1.6.23
6-3. MEASURING ANGLEOF SITE TO CREST
During advance party operations, site to crest is measured
with the M2 compass or M2A2 aiming circle (refer to chapter4 for these procedures). If the M2 compass is used, add20 mils to allow for the accuracy of the M2 compass. Duringposition improvement, the chief of section verifies the angleof site to the crest and reports it to the platoon leader. Toverify the angle of site to crest, the chief of section, sightsalong the bottom edge of the bore, has the tube traversedacross the probable field of fire and has the tube elevateduntil the line of sight clears the crest at the highest point.He then centers all bubbles on the elevation mount, andreads the angle of site to the crest from the elevation counter.The angle of site is read and expressed to the nearest wholemil. 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
RANGEa. There are five methods that can be used to measurepiece-to-crest range:
(1) Taping, the most accurate method, is normally tootime consuming.
(2) Subtense is fast and accurate.
(3) Map measurement is fast and accurate if theobstacle 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 whenother methods are not feasible.
b. Regardless of the method used, PCR must be verified by the gunnery sergeant or platoon leader before computingminimum quadrant elevation. He can do this by measuring by one of the five methods listed above and comparing theresults.
6-5. COMPUTATION FOR FUZES OTHERTHAN ARMED VARIABLE TIMEa. The gunnery sergeant or platoon leader performs thecomputations indicated in this section if the sum of angles1 and 2, Figure 6-1, exceeds 300 mils or if the rapid firetables are not available. All angles are determined andexpressed to the next higher whole mil. He performs thecomputation for all howitzers. The example below showshis computations.
b. One howitzer section may report a sight to crest that isunusually high. If the platoon leader determines that it isdue to a single narrow obstruction (such as a tree), thatpiece may be called out of action when firing deflections
that would engage the obstruction. This would enable theplatoon to use the next lower minimum quadrant. Otheralternatives are to remove the obstruction or move theweapon.
EXAMPLEGun 1 has a range to crest of 1,100 meters; the angle of site to crest reported is +16 mils. Gun 1 is an M109A2155-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 thechief of section . . . . . . . . . . . . . . . . . . . . . . . .+l6 milsAngle 2: Determines the vertical clearance in mils.He uses the GST to divide the vertical clearance inmeters (5 meters) by the PCR in thousands (1.1).This value, read under the M gage point, is 4.6, andis expressed to the next higher whole mil.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
+5 mils
Note: The value for angle 2 can also be extracted fromthe rapid fire tables (Appendix K).
Angle 3: Determines comp site by multiplying thecomp site factor corresponding to the piece-to-crestrange (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. Thecomp 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 forcomp site has been determined, it must beexpressed to the next higher whole mil.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
+1 milAngle 4: Determines elevation for the PCR (TFT,Table F, column 2). If this value is not a wholenumber, it is expressed to the next higher whole mil(74.1 expressed to 75) . . . . . . . . . . . . . . . . . . . . . . . . . ..+75 milsAngle 5: Determines the value of 2 forks (TFT,Table F, column 6) at PCR (2 x +2
mils) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .+4 milsTotal: Add angles 1-5 to determine the platoonleader'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 eachhowitzer in his firing unit. The highest value is the XOmin QE for his tiring unit with this charge.
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FM 6-50, MCWP 3-1.6.23
6-6. COMPUTING FOR ARMED VTFUZES (LOW-ANGLE FIRE)
The method of computing the XO’s minimum QE for firinga projectile fuzed with an M513, M514, M728, or M732fuze depends on the method in which the fuze is used.a. The proximity (VT) fuze is designed to arm 3 secondsprior to the time set on the fuze; however, some VT fuzeshave armed as early as 5.5 seconds prior to the time set onthe 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 settingring is set to the whole second, the time determined isexpressed up to the next higher whole second. A VT fuzeis designed so that it will not arm earlier than 2 secondstime 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 thepiece-to-crest range. The QE determined for PD fuzed roundsis safe for VT fuzes if the time set is greater than the minsafe time determined (paragraph a above). If the platoonleader finds it necessary to fire a VT fuze with a time lessthan the min safe time, the vertical clearance for the minimumQE must be increased to ensure the fuze will not functionas it passes over the crest.
c. If the projectile is to be tired with the VT fuze set ata time less than the minimum safe time, allowance must bemade for vertical clearance of friendly elements. Verticalcrest clearances for M513, M514, M728, and M732 VTfuzes 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, thevertical clearance shown in Table 6-1 should be increased by 50 percent. If the projectile is to be tired 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 lessthan the minimum safe time is fired, is based on PCR and
a greater vertical clearance as indicated in Table 6-1 insteadof the 5 meters as stated in paragraph 6-2c.
f. The following is an example of computations to determineminimum QE for armed VT fuzes.
EXAMPLEThe 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 +16mils. The PCR is 1,700 meters. The platoon leadercomputes the minimum QE as follows:
ANGLE 1: The angle of site to crest reported by thechief of section . . . . . . . . . . . . . . . . .+l6milsANGLE 2: The vertical clearance in mils. Divide thevertical clearance in meters (100 meters) by thePCR of 1,700 meters in thousands (1.7). By readingunder the M gage point of the GST, derive a value of +59 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . +59 milsANGLE 3: Comp site for PCR of 1,700 meters (FT155-AM-2, Table G, charge 4 GB, column 12, range2,000 meters gives a comp site factor of 0.010.) 16+59 = 75. 75 x 0.010 = +0.750 expressed to thenext higher whole mil . . . . . . . . . . . . . . . . . . . .+1 milsANGLE 4: Elevation at PCR (TFT, Table F, column2).(90.0).. . . . . . . . . . . . . . . . . . . . . . . . . . . .+9O milsANGLE 5: The value of 2 forks at PCR (TFT, TableF, column 6). (2 x+2 mils) . . . . . . . . . . . . . . . +4 milsMinimum 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, MINIMUMSAFE TIME 12.0 SECONDS (M514), FUZE ARMED VT.The platoon leader will compute the min QE for each gunin his firing unit. The highest value is the XO min QE forhis firing unit with this charge.
g. If the fuze setting to be fired is equal to or greaterthan the minimum safe time, the minimum QE determinedfor fuzes quick and time applies. If the fuze setting to befreed is less than the minimum safe time, the minimumquadrant elevation determined for armed VT applies.
h. Table 6-2 is a recapitulation of the steps for computingminimum quadrant elevation. It should be used as a reference by the platoon leader and in training individuals to computeminimum quadrant elevation.
i. The XO min QE is compared to the min QE to theminimum range line as computed by the FDC. The greaterof these two values is placed on the safety T.
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FM 6-50, MCWP 3-1.6.23
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