Plongée

Also Plane of Fire. In both field and permanent fortifications the declivity of the superior slope from the crest of the interior slope to the crest of the exterior slope was called the plongée. Like other slopes in fortifications the plongée of the superior slope was expressed as a ratio of the fall of the slope (height) to the distance over which the fall occurred (base). A superior slope with a plongée of 1/4 fell one foot in height for every 4 feet of horizontal thickness of the parapet to the exterior slope. While the limits of the plongée were considered to vary between 1/4 and 1/6, the exact plongée of any particular superior slope was determined by drawing a line from the crest of the interior slope to the point outside the work where fire from the parapet would be required to strike the ground. A plongée greater than 1/4 would reduce the thickness of the parapet protecting troops lining the interior crest to an unacceptable degree while a plongée less than 1/6 would send the defenders fire too high above the heads of attacking troops.

January, 2003