CHAPTER 3. Navigation

Common Error: Not Considering Winds During Descent Planning

A common error in planning a descent is failing to consider winds and their effect on groundspeed. As illustrated in Figure 3-34, if you fail to take into account a 20-knot tailwind, your groundspeed will be faster than you planned, and you will reach the target waypoint before reaching the assigned altitude.

Essential Skills

  • Determine the descent airspeed to be used with attention to turbulence, aircraft descent profile, and powerplant cooling restrictions.
  • Program, observe, and monitor the top of descent, descent rate, and level-off altitude.
  • Plan and fly a descent to a crossing restriction.
  • Recognize and correct deviations from a planned descent path, and determine which factor changed.

Intercept And Track Course

Intercepting and Tracking a Different Course to the Active Waypoint

Figure 3-37 illustrates a common situation. Air traffic control instructs you to fly to a waypoint via an inbound course different from the desired track calculated by the FMS. In the example in Figure 3-37, you are en route to SUNOL intersection. The FMS has calculated a desired track of 060 degrees, but ATC has instructed you to fly a heading of 080 degrees to intercept a 009-degree course to SUNOL.

The FMS is set to take the aircraft to SUNOL intersection, but via an inbound course different from the one ATC has cleared you to follow. Therefore, you need to be a means of programming the FMS to follow your choice of course instead of the desired track that it has identified.

The Nonsequencing Mode

Every IFR-capable FMS/RNAV unit offers an alternative mode of operation, the nonsequencing mode, which allows you to perform this particular task. Like the OBS knob which allows you to select VOR radials, the nonsequencing mode allows you to select courses to or from an active waypoint. The nonsequencing mode differs from the sequencing mode in two important ways:

  • Nonsequencing mode allows you to select a different inbound course to the active waypoint. For this reason, some manufacturers refer to the nonsequencing mode as OBS (hold or suspend) mode, which suggests similarity to the OBS knob found on traditional VOR indicators. As the OBS knob allows you to select inbound VOR radials, the nonsequencing mode allows you to select inbound courses to an active waypoint.
  • Nonsequencing mode stops the waypoint sequencing feature of the FMS/RNAV unit. If engaged in nonsequencing mode, the FMS/RNAV program does not automatically sequence to the next waypoint in the flight plan when the aircraft arrives at the active waypoint.

Every FMS/RNAV offers a way to switch to the nonsequencing mode. There is typically a button marked OBS (or Hold), and an OBS or course selection knob to select an inbound course to the active waypoint. Figure 3-38 illustrates the procedure for one particular FMS.

Once you switch to nonsequencing mode and select the inbound course of 009°, the navigation indicator reflects aircraft position with respect to the 009° course. The navigation indicator in Figure 3-38 shows that you are west of course. The assigned heading of 080° provides an acceptable intercept angle. As you fly the 080° heading, the needle centers as you reach the 009° course. Once the 009° course is reached and the needle has centered, you can turn to track the 009° course inbound to SUNOL.

It is important to remember that the nonsequencing mode suspends the FMS/RNAV’s waypoint sequencing function. If you reach SUNOL and the unit is still set in the nonsequencing mode, it will not sequence on to the next waypoint. Generally, once established on a direct course to waypoint or navaid, switching back to sequencing (releasing the Hold or Suspend function) mode allows the FMS/RNAV to continue to the programmed point and thence onward according to the programmed routing. Setting the computer back to the sequencing mode is usually accomplished by pressing the OBS (Hold or Suspend) button again.

 ©AvStop Online Magazine                                                                                                                                                      Contact Us              Return To Books

AvStop Aviation News and Resource Online Magazine

Grab this Headline Animator