sentences:  Top of descent
  
  In aviation, the top of descent, also referred to as the TOD or T/D, is the computed transition from the cruise phase of a flight to the descent phase, or the point at which the planned descent to final approach altitude is initiated.The top of descent is usually calculated by an on-board flight management system, and is designed to provide the most economical descent to approach altitude, or to meet some other objective (fastest descent, greatest range, etc.