# Descent (Part One)

Making the transition from cruise flight to the beginning of an instrument approach procedure sometimes requires arriving at a given waypoint at an assigned altitude. When this requirement is prescribed by a published arrival procedure or issued by ATC, it is called a crossing restriction. Even when ATC allows a descent at the pilot’s discretion, you need to choose a waypoint and altitude for positioning convenient to start the approach. In either case, descending from a cruising altitude to a given waypoint and altitude requires both planning and precise flying.

## Elements of Descent Planning Calculations

Figure 3-27 illustrates the basic descent planning task. The task begins with an aircraft flying at an assigned cruising altitude. The aircraft must descend to an assigned altitude and reach that assigned altitude at a designated bottom-ofdescent point. The next step is to choose a descent rate and a descent speed. The ultimate goal is to calculate a top-ofdescent point, which is the point at which, if you begin the descent and maintain the planned descent rate and airspeed, you will reach the assigned altitude at the designated bottom-of- descent point.

Figure 3-27. The descent planning task. [click image to enlarge]

In a basic aircraft, you must rely on manual calculations to perform the descent planning task. In an advanced avionics aircraft, there are two descent planning methods available: (1) manual calculations, and (2) the vertical navigation features of the FMS unit. Skillful pilots use both methods and cross-check them against one another in order to reduce the possibility of error and help keep the pilot “in the loop.”

Manual Descent Calculations

The simplest technique for calculating the distance required to descend uses a descent ratio. The table in Figure 3-28 lists a descent ratio for many combinations of planned descent speeds and descent rates. Calculating a descent is a simple matter of looking up the descent ratio for your target descent rate and groundspeed, and multiplying the descent ratio by the number of thousands of feet in altitude that you must descend. For example, suppose you are asked to descend from 11,000 feet to meet a crossing restriction at 3,000 feet. Since there is a 200-knot speed restriction while approaching the destination airport, you choose a descent speed of 190 knots and a descent rate of 1,000 feet per minute (fpm). Assuming a 10-knot headwind component, groundspeed in the descent is 180 knots. Referring to the table in Figure 3-28, the planned descent speed and rate indicate a ratio of 3.0. This means that you will need 3 NM for every 1,000 feet of descent. You must descend a total of 8,000 feet (11,000 feet – 3,000 feet). A total of 24 NM is needed to descend 8,000 feet (3 NM × 8 = 24 NM), and must, therefore, begin the descent 24 NM away from the end-of-descent point.

Figure 3-28. Descent ratio table. [click image to enlarge]

Another technique for calculating descents is to use the formula shown in Figure 3-29. A descent table can be found in the front of each set of U.S. Terminal Procedures on page D-1. Working through the formula for the ECA VOR crossing restriction example, 8 minutes is needed to descend 8,000 feet at the planned descent rate of 1,000 fpm. At your planned descent speed of 180 knots, you will cover 3 NM per minute. Thus, in 8 minutes, you will cover 24 NM. Once again, you must start the descent 24 NM prior to ECA to meet the crossing restriction.

Figure 3-29. Descent formula.

Coordinating Calculations with Aeronautical Charts

Regardless of which method is used, it is always a good idea to locate the top-of-descent point chosen on the aeronautical chart. Figure 3-30 shows a chart that covers the area surrounding the ECA VOR. A top-of-descent point 24 NM prior to ECA is located 3 NM before PATYY intersection.

Figure 3-30. Top-of-descent point on an en route chart.