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, 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.). The top of descent may be calculated manually as long as the distance, air speed, and current altitude are known. This can be done by finding the difference of your current altitude, and your desired altitude, and dividing that by your desired rate of descent. Then multiplying that by the quotient of your ground speed (not airspeed) and 60. ((C-T)/RoD)*(KGS/60)=TOD. The answer to that equation will represent how far from the target you must begin your descent.