In
aviation
Aviation includes the activities surrounding mechanical flight and the aircraft industry. ''Aircraft'' includes fixed-wing and rotary-wing types, morphable wings, wing-less lifting bodies, as well as lighter-than-air craft such as hot a ...
, 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
In aeronautics, the final approach (also called the final leg and final approach leg) is the last leg in an aircraft's approach to landing, when the aircraft is lined up with the runway and descending for landing.Crane, Dale: ''Dictionary of ...
altitude is initiated. The top of descent is usually calculated by an on-board
flight management system
A flight management system (FMS) is a fundamental component of a modern airliner's avionics. An FMS is a specialized computer system that automates a wide variety of in-flight tasks, reducing the workload on the flight crew to the point that mode ...
, 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 distance,
air speed
In aviation, airspeed is the speed of an aircraft relative to the air. Among the common conventions for qualifying airspeed are:
* Indicated airspeed ("IAS"), what is read on an airspeed gauge connected to a Pitot-static system;
* Calibrated a ...
, and current altitude are known. This can be done by finding the difference between current altitude and desired altitude, dividing the result by the desired rate of descent, and then multiplying that figure by the quotient of the ground speed (not airspeed) and 60. ((C-T)/RoD)*(KGS/60)=TOD. The result dictates how far from the destination descent must begin.
See also
*
Flight management system
A flight management system (FMS) is a fundamental component of a modern airliner's avionics. An FMS is a specialized computer system that automates a wide variety of in-flight tasks, reducing the workload on the flight crew to the point that mode ...
*
Autopilot
An autopilot is a system used to control the path of an aircraft, marine craft or spacecraft without requiring constant manual control by a human operator. Autopilots do not replace human operators. Instead, the autopilot assists the operator' ...
*
Top of climb
References
{{Flight phases
Flight phases