RNG PULL SCAN PUSH ON BRT PROC CRSR MSG OBS ALT NRST CLR ENT D MENU DTK 050 TK 12.5nm ECAi LEG ALT 2 *CRSR* *CRSR* V n v i n 2 : 1 5 11000 to : 3000 ECA : -00 nm GS : 180 -1000fpm 12 5 nm The VNAV function then calculates a top-of-descent  point and displays the time remaining to reach it. 2 VNAV allows you to enter a descent waypoint and altitude, descent speed, and descent rate. 1 ECA ft 0 0 0 3 ft 0 0 0 0 1 ft 0 0 0 1 1 Deceleration segment The aircraft must slow from 270 knots to 180 knots to obey speed restrictions. knots 0 7 2 knots 0 8 1 Figure 3-32. A deceleration segment planned by a more sophisticated FMS. 3-22 Figure 3-31. Planning a descent with an advanced avionics unit. 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. 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 V$_{A}$ 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. 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. 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. 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. 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  Managing Speed