aircrews 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 or  altitude requires both planning and precise flying.  ATC may ask the pilot to descend to and maintain a  specific altitude. Generally, this clearance is for en route  traffic separation purposes, and pilots need to respond to  it promptly. Descend at the optimum rate for the aircraft  being flown until 1,000 feet above the assigned altitude,  then descend at a rate between 500 and 1,500 fpm to the  assigned altitude. If at any time, other than when slowing  to 250 KIAS at 10,000 feet MSL, the pilot cannot descend  at a rate of at least 500 fpm, advise ATC.  The second type of clearance allows the pilot to descend  “… at pilot’s discretion.” When ATC issues a clearance to  descend at pilot’s discretion, pilots may begin the descent  whenever they choose and at any rate of their choosing.  Pilots are also authorized to level off, temporarily, at any  intermediate altitude during the descent. However, once  the aircraft leaves an altitude, it may not return to that  altitude.  A descent clearance may also include a segment where the  descent is at the pilots’ discretion—such as “cross the Joliet  VOR at or above 12,000, descend and maintain 5,000.”This  clearance authorizes pilots to descend from their current  altitude whenever they choose, as long as they cross the  Joliet VOR at or above 12,000 feet MSL. After that, they are  expected to descend at a normal rate until they reach the  assigned altitude of 5,000 feet MSL.  Clearances to descend at pilots’ discretion are not just an  option for ATC. Pilots may also request this type of clearance  so that they can operate more efficiently. For example, if  a pilot was en route above an overcast layer, he or she  might ask for a descent at his or her discretion to allow the  aircraft to remain above the clouds for as long as possible.  This might be particularly important if the atmosphere  is conducive to icing and the aircraft’s icing protection  is limited. The pilot’s request permits the aircraft to stay  at its cruising altitude longer to conserve fuel or to avoid  prolonged IFR flight in icing conditions. This type of descent  can also help to minimize the time spent in turbulence by  allowing pilots to level off at an altitude where the air is  smoother.  Inappropriate descent planning and execution during  arrivals has been a contributing factor to many fatal  aircraft accidents. Since the beginning of commercial jet  operations, more than 9,000 people have died worldwide  ATC is not always responsible for safe terrain clearance for  the aircraft under its jurisdiction. Many times ATC issue en  route clearances for pilots to proceed off airway direct to  a point. Pilots who accept this type of clearance also are  accepting the shared responsibility for maintaining safe  terrain clearance. Know the height of the highest terrain  and obstacles in the operating area and your position in  relation to the surrounding high terrain.  The following are excerpts from CFIT accidents related  The controller reported that he was scanning his radarscope  for traffic and noticed that the MD-80 was descending  through 6,400 feet. He immediately instructed a climb  to at least 6,500 feet. The pilot returned to 6,500 feet, but  responded to ATC that he had been cleared to 5,000 feet.  When he had read back 5,000 feet to the controller, he  received no correction from the controller. After almost  simultaneous ground proximity warning system (GPWS)  and controller warnings, the pilot climbed and avoided  the terrain. The recording of the radio transmissions  confirmed that the aircraft was cleared to 7,000 feet and  the pilot mistakenly read back 5,000 feet then attempted  to descend to 5,000 feet. The pilot stated in the report: “I  don’t know how much clearance from the mountains we  had, but it certainly makes clear the importance of good  communications between the controller and pilot.”  The inability of controllers and pilots to properly  communicate has been a factor in many CFIT accidents.  Heavy workloads can lead to hurried communication and  the use of abbreviated or non-standard phraseology. The  importance of good communication during the arrival  phase of flight was made evident in a report by an air traffic  controller and the flight crew of an MD-80.  The basic causes of CFIT accidents involve poor flight crew  situational awareness, or SA. One definition of SA is an  accurate perception by pilots of the factors and conditions  currently affecting the safe operation of the aircraft and the  crew. The causes of CFIT are the flight crews’ lack of vertical  position awareness or their lack of horizontal position  awareness in relation to the ground, water, or an obstacle.  More than two-thirds of all CFIT accidents are the result  of an altitude error or lack of vertical SA. CFIT accidents  most often occur during reduced visibility associated with  instrument meteorological conditions (IMC), darkness, or  a combination of both.  because of controlled flight into terrain (CFIT). CFIT is  described as an event in which a normally functioning  aircraft is inadvertently flown into the ground, water, or an  obstacle. Of all CFIT accidents, 7.2 percent occurred during  the descent phase of flight.  Controlled Flight Into Terrain (CFIT)  3-9 