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Descent (Part Two)

in Navigation

Managing Speed


Up to this point the focus has been on the task of losing excess altitude. For example, in the situation shown in Figure 3-27, you are faced with the requirement to reduce altitude from 11,000 feet to 3,000 feet. Most descent scenarios also present the challenge of losing excess speed. In piston aircraft of modest performance, losing excess speed seldom requires much forethought. Slowing from a cruising speed of 120 knots to an approach speed of 100 knots requires little planning and can be accomplished quickly at almost any point during a descent. Flying higher performance aircraft requires a closer look at concepts of excess altitude and excess speed.

Figure 3-27. The descent planning task.

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

Higher performance piston engines usually require descent scheduling to prevent engine shock cooling. Either the engines must be cooled gradually before descent, or power must be constant and considerable in the descent to prevent excessive cooling. In such instances, a much longer deceleration and gradual engine cooling must be planned to prevent powerplant damage. Additionally, the turbulence penetration or VA speeds should be considered with respect to weather conditions to avoid high speeds in turbulent conditions, which could result in overstressing the airframe. Drag devices such as spoilers can be of great advantage for such maneuvers. In the scenario in Figure 3-27, a cruising speed of 270 knots is inappropriate as the aircraft descends below 10,000 feet, and even more so as it enters Class C airspace. Therefore, descent planning must include provisions for losing excess airspeed to meet these speed restrictions.

Some sophisticated FMSs are able to build in a deceleration segment that can allow the aircraft to slow from the cruise speed to the desired end-of-descent speed during the descent. This type of navigation system allows you to maintain the cruise speed up until the top-of-descent point and calculates the deceleration simultaneously with the descent. A deceleration segment is illustrated in Figure 3-32.

Figure 3-32. A deceleration segment planned by a more sophisticated FMS.

Figure 3-32. A deceleration segment planned by a more sophisticated FMS. [click image to enlarge]

Simple FMS units such as GPS RNAV receivers assume that you will slow the aircraft to the planned descent speed before reaching the top-of-descent point. ATC timing may preclude this plan.

Descent Flying Concepts

Probably the most important descent flying concept to understand is that a planned descent is basically a “pathway in the sky,” similar to the glideslope associated with an ILS procedure. If you start down at the planned top-of-descent point, fly a groundspeed of 180 knots, and descend at 1,000 feet per minute (fpm), you will be flying on a fixed path between the top-of-descent point and the bottom-of-descent point. If you maintain the 180-knot and 1,000-foot-perminute descent, you will cross a point 18 NM from ECA at exactly 9,000 feet, a point 12 NM from ECA at 7,000 feet, and a point 6 NM from ECA at exactly 5,000 feet, as shown in Figure 3-33.

Figure 3-33. Planned descent path as a wire in the sky.

Figure 3-33. Planned descent path as a wire in the sky. [click image to enlarge]

If you are at a different altitude at any of these points, you will not cross ECA at the required 3,000 feet unless corrective action is taken. Four things can cause you to drift from a planned descent path:

  1. Not following the planned descent rate
  2. Not following the planned descent speed
  3. Unexpected winds
  4. Navigation system not recalculating for airspeed change of descent

Figure 3-34 shows the effect of each situation on the position of the aircraft with respect to the planned descent path.

Figure 3-34. Drifting off the planned descent path.

Figure 3-34. Drifting off the planned descent path.

Flying the Descent

The key to flying a descent is to know your position relative to the pathway-in-the-sky at all times. If you drift off the path, you need to modify the descent speed and/or descent rate in order to rejoin the descent path. Many FMSs do not give a direct indication of progress during a descent. You must be very familiar with the indirect indications of the VNAV descent. In this case, follow the planned descent rate and speed as closely as possible and be mindful of altitude and position while approaching the crossing restriction fix.

Determining Arrival at the Top-of-Descent Point

All navigation systems provide some type of alert informing the pilot of arrival at the planned top of descent point, and that it is time to begin the descent at the speed and rate entered into the FMS.

If air traffic control is able to accommodate your request, the ideal point to begin the descent is at the planned top-of-descent point. If air traffic control is unable to accommodate such a request, one of two scenarios will ensue: an early descent or a late descent.


Early Descents

Beginning descent before reaching the planned top-of-descent point means you must set aside descent planning and proceed without the benefit of vertical guidance offered by the navigation system. If, during the descent, the navigation computer does not display position with respect to the planned descent path, you must simply do the best possible to arrive at the crossing restriction at the assigned altitude. If the navigation system does display position with respect to the planned descent path, you can usually recapture the planned descent path and resume flying with vertical guidance from the computer. The basic technique is to initiate descent at a reasonable descent rate that is less than the planned descent rate. If you follow this initial descent rate, you will eventually intercept the planned descent path, as shown in Figure 3-35.

Figure 3-35. Early descent scenario.

Figure 3-35. Early descent scenario.

Late Descents

Beginning the descent beyond the planned top-of-descent point means that you will have the same amount of excess altitude, but a shorter distance and time to lose it, as shown in Figure 3-36.

Figure 3-36. Late descent scenario.

Figure 3-36. Late descent scenario. [click image to enlarge]

Since flying beyond the planned top-of-descent point leaves less time to lose excess altitude, your goal is to minimize the “overrun” distance by slowing the aircraft as soon as a late descent scenario is suspected. A lower speed means you will cover less distance in the same amount of time, and thus be left with more time to lose altitude.

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

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

 

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