CHAPTER 3. Navigation


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 bottomof- descent point.

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.”

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