adjustment. For the 60-mm computer, he only determines data for two mortars.
Performance Measures GO NO GO
NOTE: Do not score sequentially.
1. Parallel sheaf: —— ——
a. Correctly recorded the sheaf corrections on the computer’s record.
b. Converted the meter corrections to mils using the mil-relation formula without error.
c. Converted the meter corrections to mils using the deflection conversion table without error.
d. Determined new deflection without error.
e. Refired mortar.
f. Used the proper fire commands.
2. Converged sheaf: —— ——
a. Converted the meter distance between mortars to mils using the mil-relation formula.
b. Converted the meter distance between the mortars to mils using the deflection conversion table.
c. Determined the correct deflection for the No. 1 mortar. d. Determined the correct deflection for the No. 3 mortar. e. Determined the correct deflection for the No. 4 mortar.
3. Open sheaf: —— ——
a. Converted the meter distance between mortars to mils using the mil-relation formula.
b. Determined the mil correction to move the mortar impact from 40 meters to 60 meters apart.
c. Determined the correct deflection for the No. 1 mortar. d. Determined the correct deflection for the No. 3 mortar. e. Determined the correct deflection for the No. 4 mortar.
4. Special sheaf: —— ——
a. Plotted target at final adjusted data. b. Indexed target attitude.
c. Plotted impact points for No. 1, 3, and 4 mortars. d. Indexed mortar position attitude.
e. Plotted the No. 1, 3, and 4 mortars.
f. Determined deflection and range for No. 1 mortar. g. Determined deflection and range for No. 3 mortar. h. Determined deflection and range for No. 4 mortar.
NOTE: Deflection for performance measures 4f, 4g, and 4h above have a 10-mil tolerance for deflection and 25 meters for range.
Evaluation Guidance: Score the Soldier GO if all performance measures are passed. Score the Soldier
NO-GO if any performance measure is failed. If the Soldier scores NO-GO, show the Soldier what was done wrong and how to do it correctly.
References
Required Related
STP 31-18B34-SM-TG
Compute Data for a Traversing and/or Searching Mission Using a Plotting Board 071-078-0007
Conditions: As a computer; given a plotting board; data to set up the board as a modified observed
chart; DA Form 2399, Computer’s Record; No. 2 pencil; paper; firing table; and call for fire for a wide or deep target.
Standards: 1. Determined deflection to within 1 mil with a 10-mil tolerance.
2. Determined range to within 25 meters with a 25-meter tolerance. 3. Determined turns to nearest half turn.
4. Determined charge to 1/8 charge.
Performance Steps
NOTE: The information provided in this task applies to all mortars. For 60-mm, use only the information for the No. 1 and 2 mortars. For a six-gun platoon, use the procedure used for the No. 3 and 4 mortars to determine the data for the No. 5 and 6 mortars.
1. Application.
a. Traversing or searching fire is used by mortars when the target is wider or deeper than can be engaged by a parallel sheaf. Wide or deep targets are engaged by using a distributed fire for effect (FFE). This means that the mortar will be manipulated for elevation or deflection between rounds until the number of rounds given in the fire command has been fired. The 4.2-inch mortar will vary range using charges.
b. To effectively engage a target using traversing fire (Figure 1), the attitude of the target cannot be more than 100 mils different than the attitude of the mortar section.
Figure 1. Target Coverage
c. To effectively engage a target using searching fire (Figure 2), the attitude of the target cannot be more than 100 mils different than the azimuth of the gun-target line.
STP 31-18B34-SM-TG
Performance Steps
Figure 2. Target Coverage
2. Traversing fire.
a. Upon receiving the call for fire, the section sergeant determines that, from the size and description of the target, traversing fire must be used to cover it. He then completes the fire direction center (FDC) order on DA Form 2399 (Figure 3).
STP 31-18B34-SM-TG
Performance Steps
b. With the completed FDC order, the computer must now compute the data to fire the mission (Figure 4).
Figure 4. Computed Firing Data
(1) From the FDC order, the computer knows that No. 4 mortar will be used to adjust on the left flank of the target.
NOTES:
1. The following data were used to set up the plotting board: Grid intersection — 1590
Direction of fire — 6085 mils Mounting azimuth — 6100 mils Mortar position — 16808945
Mortar position attitude — 1100 mils Mortar altitude — 400 meters Target altitude — 300 meters Referred deflection — 2800 mils
2. Before this mission can be computed, the four mortars must be plotted individually at the mortar position. During the mission, the computer ensures that the correct plots are used to determine the data required; in other words, during the adjustment, the impact points are aligned with the No. 4 mortar plot.
(2) Using the information in the call for fire, FDC order, and observer corrections, the computer computes the data to adjust the No. 4 mortar onto the left flank of the target and records it on DA Form 2399 (Figure 4).
c. After the adjustment is complete, the computer must—
(1) Plot the 400-meter length of the target on the plotting board using the attitude of the target.
STP 31-18B34-SM-TG
Performance Steps
(2) Divide the target into segments.
(3) Determine the mil width of one segment.
(4) Determine the number of turns it will take to cover one segment. (5) Determine the number of turns between rounds.
d. The computer determines each of these items as follows:
(1) To plot the target on the plotting board, the computer rotates the azimuth disk until the target attitude (taken from the call for fire) is indexed. The computer erases all the plots except the last. Being sure that the attitude is indexed, the computer makes a plot 100 meters above that adjustment plot, then another plot 100 meters above the plot followed by one last plot 100 meters above that plot. These plots represent the start points for each mortar. The area between the plots, and for 100 meters beyond the last plot, is the area each mortar must cover with fire (Figure 5).
(2) The target is now divided into four segments. By determining the remaining data for one segment, the data will apply to all four mortars.
(3) Since each segment of the target is 100 meters wide, if the computer determines the mil width of one segment, the other three will be the same. At this time, the computer has the deflection that it took to hit the adjusting point on the target by the No. 4 mortar (adjusting mortar) (Figure 4). By aligning the adjusting mortar plot with the No. 3 plot on the target, the computer can now determine the deflection from the adjusting mortar to the start point of No. 3 mortar on the target.
Figure 5. Plotting Firing Data on M16 Plotting Board
NOTE: The deflection determined from the adjusting mortar to the No. 3 plot on the target is only used to determine the mil width of one segment of the target. Do not use this deflection for firing data.
(4) By subtracting these two deflections, the computer can determine the mil width of the segment.
STP 31-18B34-SM-TG
Performance Steps
(No. 4 plot deflection — 2997 mils) + (No. 3 plot deflection — -2940 mils) = (Mil width of segment — 57 mils)
(5) Each turn of the traversing handwheel is 10 mils. Dividing the mil width of the segment (57 mils) by 10 gives the computer the total number of turns to cover the segment: 57/10 = 5.7 rounded off to 6 total turns
NOTES:
1. 120-mm mortar: For the 120-mm mortar, each turn of the traversing handwheel is only 5 mils. 2. 60-mm mortar: When the bipod is in the upper saddle of the barrel, one turn of the traversing
handwheel equals 10 mils; when in the lower saddle, one turn equals 15 mils. The upper saddle is used when firing less than 1100 mils elevation. The lower saddle is used when firing more than 1100 mils elevation.
(6) To compute the number of turns to take between each round, the computer must know how many rounds will be fired for each segment. This information is in the FDC order (four rounds). To determine the turns between rounds, divide the total turns by the interval between rounds (there will always be one less interval than the number of rounds: 4 rounds = 3 intervals).
6/2 = 2 = 2 turns between rounds NOTES:
1. Turns are rounded to the nearest half turn.
2. The number of rounds to fire is based on the rule: 4 rounds per 100 meters of target width, or 1 round per 30 meters.
(7) At this point, the computer has only to determine the deflection for No. 1, 2, and 3 mortars, complete the subsequent command, and issue it to the mortar section (Figure 6). To do this, the computer aligns the No. 3, 2, and 1 mortars with their start points on the target and reads the deflection for each mortar. During this procedure, the computer should also determine the range for each mortar to its start point. If there is a range difference of 25 meters or more, the elevation for that mortar must reflect the range difference.
STP 31-18B34-SM-TG
Performance Steps
NOTE: Upon completion of the adjustment phase of the mission, the section is given the command, PREPARE TO TRAVERSE RIGHT (LEFT). The gunners traverse the mortar all the way in the opposite direction of that given, back off two turns (120-mm backs off four turns), and await instructions.
(8) As shown in Figure 7, each mortar is firing a different deflection, but the same charge and elevation. When the last round is fired, the mortars are left as laid and the squad again awaits instructions. As shown in Figure 7, the forward observer (FO) came back with, REPEAT. Rather than fire the mission as it was fired the first time by traversing right, the computer gives the command to traverse left.
STP 31-18B34-SM-TG
Performance Steps
3.Searching/zone fire.
a. In firing a searching zone mission, the adjustment phase of the mission is the same as a regular adjust mission, using the base mortar (No. 2) as the adjusting mortar, except that the mortar is adjusted on either the near end or far end of the target—normally, the far end. NOTE: The following data were used to set up the plotting board:
Grid intersection — 1590 Direction of fire — 6085 mils Mounting azimuth — 6100 mils Mortar position — 16808945 Mortar altitude — 400 meters Target altitude — 370 meters Referred deflection — 2800 mils
b. Upon completion of the adjustment phase of the mission, the computer now computes the data to cover the target with fire. The computer must—
(1) Determine the mil length (depth) of the target.
(2) Determine the number of turns it will take to cover the target. (3) Determine the number of turns between rounds.
c. The computer determines each of those items as follows:
NOTE: The following procedures apply only to the 60-mm and 120-mm mortars.
(1) To determine the mil length of the target, the computer must use the firing table. He subtracts the elevation to hit the near end of the target from the elevation for the far end of the target (adjusting point, 1002).
Range to adjusting point — 2,250, Elevation — 1002 Range to near end — 2,025, Elevation — 1101 [1101 - 1002 = (Length of target) 99 mils]
(2) Each turn of the elevation crank is 10 mils. Dividing the mil length of the target (99 mils) by 10 gives the computer the total turns needed to cover the target:
99.0/10 = 9.9 rounded up to 10 (total turns)
(3) To compute the number of turns to take between each round, the computer must know how many rounds will be fired by each mortar. This information is in the FDC order (8 rounds). To determine the turns between rounds, divide the total turns by the intervals between rounds (there is always one less interval than the number of rounds: 8 rounds = 7 intervals).
10/7 = 1.4 rounded to 1 1/2 (turns between rounds) NOTES:
1. Turns are rounded to the nearest half turn.
2. The number of rounds to fire is based on the rule: 4 rounds per 100 meters of target depth, or 1 round per 30 meters.
d. At this point, the computer has all the information needed to complete the subsequent command and issue it to the mortars (Figure 8).
STP 31-18B34-SM-TG
Performance Steps
Figure 8. Completed Repeat Data
e. To compute fire-for-effect data for a zone mission (4.2-inch mortar).
(1) Upon completion of the adjustment phase of the mission, the computer must now determine the charge data to be placed on each round to be fired in the fire for effect. (2) The number of rounds fired may vary based on the size of the target given in the call for
fire (platoon in open, 100 x 100, or company assembly area, 200 x 200). However, the charge difference will remain constant for each 50-meter spacing between rounds:
( a) 3/8 charge apart when firing without extension. ( b) 4/8 charge apart when firing with extension. ( c) 2/8 charge apart when firing the M329A2 round.
EXAMPLE:
Target description: PLATOON IN OPEN, 100 x 100. For this target, the FO adjusted on the center of target with the No. 2 mortar firing a charge of 21 5/8.
STP 31-18B34-SM-TG
Performance Steps
Three rounds will be fired. To have one round fall 50 meters beyond the
adjustment point, the computer adds 3/8 charge to the base charge of 21 5/8—21 5/8 + 3/8 = 22. For the round to fall short of the base round, the computer subtracts 3/8 charge, 21 5/8 - 3/8 = 21 2/8. When fired with this data, the rounds will impact about 50 meters apart (Figure 9).
Target description: COMPANY ASSEMBLY AREA, 200 x 200. For this target, the FO adjusted on the center of the target, with No. 2 mortar firing a charge of 28 3/8. Five rounds will be fired. The computer must now compute data to place rounds 50 meters and 100 meters beyond and 50 meters and 100 meters short of the base round: 100 meters over 28 3/8 + 1 = 29 3/8 ; 50 meters over 28 3/8 + 4/8 = 28 7/8; Base round charge 28 3/8; 50 meters short 28 3/8 - 4/8 = 27 7/8; 100 meters short 28 3/8 - 1 = 27 3/8; By firing five rounds at these charges, the target area (200 x 200) will be fully covered (Figure 10).
Figure 9. Completed Fire-for-Effect Data
NOTE: Charges of 25 4/8 or above are fired with extension; those below 25 4/8 are fired without extension.
(3) When firing the fire for effect, the crews will mix the rounds to cause a scattered fire for effect on the target.
STP 31-18B34-SM-TG
Evaluation Preparation: SETUP: At the test site, provide a plotting board set up as a modified observed
chart with mortar position plotted, and final plots for two targets. Target No. 1 is for a traversing mission; target No. 2 is for a search mission. Provide a completed computer’s record for the adjustment phase of each mission, plus all other materials given in the task conditions statement.
Brief Soldier: Tell the Soldier that, using the materials and information given, he will compute the final firing data for a traversing mission and then the final firing data for a search mission.
Performance Measures GO NO GO
NOTE: Not to be sequence scored.
1. Traversing mission: —— ——
a. Told the mortars, “Prepare to traverse right (left).”
b. Plotted the location of No. 2, 3, and 4 mortars, or No. 1, 2, and 3 mortars, using the mortar position attitude.
c. Plotted the meter length of the target using the target attitude.
d. Divided the target length into four equal segments and plot those points on the plotting board.
e. Determined the mil width of one segment of the target (within 5 mils). f. Determined the number of turns to cover one segment of the target. g. Determined the number of turns between rounds (nearest half turn). h. Determined the deflection to each mortar start point.
i. Issued the correct fire command to the mortars to fire the traverse part of the mission.
2. Searching/zone mission: —— ——
a. Searching mission (60-mm, 81-mm, 120-mm).
(1) Computed the mil length of the target using the elevation to the near end and far end of the target.
(2) Determined the number of turns to cover the target. (3) Determined the number of turns between rounds.
(4) Issued the correct fire command to the mortars to fire the search part of the mission.
b. Zone mission (4.2-inch mortar).
(1) Determined charge per 50 meters.
(2) Determined number of rounds to use (based on target description). (3) Determined charge for each round.
Evaluation Guidance: Score the Soldier GO if all performance measures are passed. Score the Soldier
NO-GO if any performance measure is failed. If the Soldier scores NO-GO, show the Soldier what was done wrong and how to do it correctly.
References
Required Related
DA FORM 2399
STP 31-18B34-SM-TG
Compute Data for an Illumination Mission Using a Plotting Board 071-078-0008
Conditions: As a computer, given a plotting board set up as a modified observed chart with mortar
position and registration point (RP) plotted; DA Form 2399, Computer’s Record; firing table; No. 2 pencil; paper; call for fire for an illumination mission; and two forward observer (FO) corrections.
Standards: Compute data to place illumination over the target area to include deflections to the nearest
mil with a 10-mil tolerance; ranges to the nearest 25 meters with a 25-meter tolerance; and fuze settings to within one-tenth of a second. For the 4.2-inch mortar, charge to 1/8 charge with a 1/8-charge tolerance.
Performance Steps
NOTE: The information contained in this task has limited data that can be used with the 60-mm mortar. Due to the fixed time of function for the 60-mm illumination, time settings are not used.
1. Purpose of illumination. Battlefield illumination provides friendly forces with light to assist them in ground operations at night. The light provided by the mortar illumination round also allows the FO to detect targets of opportunity or enemy activity in the area. The indiscriminate use of illumination can give away friendly operations as well as reveal enemy operations. Therefore, the fire direction center (FDC), as well as the FO, must keep abreast of friendly activity within their area of responsibility. 2. Types of illumination.
a. The amount of illumination required for a particular mission depends on the observer-target (OT) distance, conditions of visibility, and the size and shape of the area to be illuminated. b. The FO may request any of the following patterns of illumination rounds. Therefore, the
computers must be able to compute the data to provide the pattern requested.
(1) One-gun illumination. Most illumination missions are fired by one mortar. In firing one-gun illumination, the remaining mortars are available to fire high explosive (HE), should a target be detected.
(2) Two-gun illumination. This pattern is used when more light is needed than one round can provide. In providing this pattern, one round each is fired from two different guns
(normally, the flank mortars). The rounds are set to burst at the same time.
NOTE: Mortars may normally fire this pattern by firing two rounds as quickly as possible from one mortar. (3) Two-gun illumination, range spread. This pattern is used when the area to be illuminated
has greater depth than width. When this pattern is called for, the computer computes the data for one mortar to fire rounds 250 meters beyond the target (location given by the FO) on the gun-to-target (G-T) line, and data for one mortar to fire rounds 250 meters short of this point on the G-T line. This procedure causes rounds to burst 500 meters apart. NOTE: For the 4.2-inch and the 120-mm mortar, use 500 meters in place of 250 meters.
(4) Two-gun illumination, lateral spread. This pattern is used when the area to be illuminated has greater width than depth. When this pattern is called for, the computer computes the data for one mortar to fire rounds 250 meters to the left of the target (location given by the FO), and data for one mortar to fire rounds 250 meters to the right of the target. This procedure causes rounds to burst 500 meters apart.
3. Computing illumination data.
a. The computer receives the following call for fire: A3K17
ADJUST FIRE POLAR DIRECTION 6080 DISTANCE 400 VEHICLE NOISE SUSPECTED CONVOY ILLUMINATION
The computer begins computing the data to fire an illumination round over the target area to aid