|CHAPTER 3. Navigation
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.
Alternate Navigation Planning
Using the aeronautical chart to locate the top-of-descent point has a second advantage. Since regulations require you to have an alternate means of navigation onboard if the computer does not comply with TSO 146B, the aeronautical chart allows you to check minimum altitudes for VOR reception 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.
Calculating Descents with the FMS
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.
It is a good idea to cross-check the results of your manual descent calculations with the results produced by the computer. Many RNAV units do not display a waypoint for the planned top-of-descent point. However, there may be an “approaching VNAV profile” message that anticipates the descent point and cues the pilot to begin descending. Caution is advised that some systems calculate the vertical flight path dependent on the current airspeed/groundspeed values. Lowering the nose and gaining airspeed in the descent may confuse you into perceiving a false vertical goal or vertical rate, resulting in failure to meet the crossing restriction with some systems. Determine if the system recomputes the airspeed/groundspeed, or if you must enter the descent airspeed during the VNAV programming.
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