FDC General Knowledge for Fire Control Specialist (13J) Practice Test

Chart Deflection and Chart Angle-T are expressed in mils. This unit is chosen because mils divide a circle into 6,400 equal parts, giving a finely gridded angular measure that translates directly into linear moves on a plotting chart as distance increases. With the small-angle idea, the lateral offset on the chart is roughly the range to the target times the angle in radians, and one mil is about 0.00098 radians. That means the deflection scales predictably with distance, so quick mental math is possible: at 1,000 meters, one mil ≈ 1 meter of offset on the chart; at 2,000 meters, ≈ 2 meters, and so on. Degrees or radians would be less convenient for fast field calculations, and grads aren’t the standard used in this context, so mils are the practical and consistent choice for chart deflection and Chart Angle-T.