90 100 120 140 160 180 200 220 240 260 280 300 1600 1700 1800 1900 1200 1300 1400 1500 700 800 900 1000 1100 600 2000 500 GROUND SPEED (knots) DESCENT GRADIENT RATE (ft./min) A descent ratio table is provided for use in planning and executing descent procedures under known or approximate ground speed  conditions and rates of descent. The ratio expresses the number of nautical miles needed to descend 1,000 ft.  DESCENT RATIO TABLE 3.0 2.5 2.1 1.9 1.7 1.5 1.4 1.3 1.2 1.1 1.0 0.9 0.9 0.8 0.8 0.7 3.3 2.8 2.4 2.1 1.9 1.7 1.5 1.4 1.3 1.2 1.1 1.0 1.0 0.9 0.9 0.8 3.7 3.1 2.6 2.3 2.0 1.8 1.7 1.5 1.4 1.3 1.2 1.1 1.1 1.0 1.0 0.9 4.6 3.9 3.3 2.9 2.6 2.3 2.1 1.9 1.8 1.7 1.6 1.5 1.4 1.3 1.2 1.2 5.3 4.4 3.8 3.3 3.0 2.7 2.4 2.2 2.1 1.9 1.8 1.7 1.6 1.5 1.4 1.3 6.0 5.0 4.3 3.8 3.3 3.0 2.7 2.5 2.3 2.1 2.0 1.9 1.8 1.7 1.6 1.5 6.7 5.6 4.8 4.2 3.7 3.3 3.0 2.8 2.6 2.4 2.2 2.1 2.0 1.9 1.8 1.7 7.3 6.1 5.3 4.6 4.1 3.7 3.3 3.1 2.8 2.6 2.4 2.3 2.2 2.0 1.9 1.8 8.0 6.7 5.7 5.0 4.4 4.0 3.6 3.3 3.1 2.9 2.7 2.5 2.4 2.2 2.1 2.0 8.7 7.2 6.2 5.4 4.8 4.3 3.9 3.6 3.3 3.1 2.9 2.7 2.5 2.4 2.3 2.2 9.3 7.8 6.7 5.8 5.2 4.7 4.2 3.9 3.6 3.3 3.1 2.9 2.7 2.6 2.5 2.3 10.0 8.3 7.1 6.3 5.6 5.0 4.5 4.2 3.8 3.6 3.3 3.1 2.9 2.8 2.6 2.5 Figure 3-28. Descent ratio table. Figure 3-29. Descent formula. Descent Rate (ft/min) X Groundspeed (NM/hr) 60 (min/hr) = NM required Cruising Altitude (ft) – Descent Altitude (ft) 11,000 ft – 3,000 ft 1,000 ft/min 180 NM/hr 8,000 ft 1,000 ft/min 8 X = 24 NM X = X 3 NM/min = 3 NM 60 (min/hr) Figure 3-30. Top-of-descent point on an en route chart. Calculating Descents with the FMS along the route of flight in case VOR navigation is required  at any time. The airway that leads to the ECA VOR lists a  minimum en route altitude (MEA) of 3,000 feet, which is  the clearance altitude. Building a descent with an FMS follows the familiar process  of entering the basics of the descent into the system, letting  the system do the math, and then reviewing what the system  has produced. Most FMS units offer a descent planning or  vertical navigation (VNAV) page that allows you to enter  the details of your descent.  Figure 3-31  shows the VNAV  page for one manufacturer’s system. Note that there is an  entry for each of the descent planning concepts discussed  above. Computers perform the calculations using the same  formulas and data. 3-21