In
aviation
Aviation includes the activities surrounding mechanical flight and the aircraft industry. ''Aircraft'' include fixed-wing and rotary-wing types, morphable wings, wing-less lifting bodies, as well as lighter-than-air aircraft such as h ...
, the top of descent, also referred to as the TOD or T/D, is the computed transition from the
cruise phase
Cruise is the phase of aircraft flight from when the aircraft levels off after a climb (aeronautics), climb until it begins to descent (aeronautics), descend for landing. Cruising usually comprises the majority of a flight, and may include small ...
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 A ...
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 mod ...
, 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 it is flying through (which itself is usually moving relative to the ground due to wind). In contrast, the ground speed is the speed of an aircraft with respect to the surf ...
, 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
In aeronautics, the rate of climb (RoC) is an aircraft's vertical speed, that is the positive or negative rate of altitude change with respect to time. In most ICAO member countries, even in otherwise metric countries, this is usually expressed ...
, and then multiplying that figure by the quotient of the
ground speed
Ground speed is the horizontal component of the velocity of an aircraft relative to the Earth’s surface, also referred to as "speed over the ground". It is vital for accurate navigation that the pilot has an estimate of the ground speed that wil ...
(not airspeed) and 60. ((C-T)/RoD)*(KGS/60)=TOD. The result dictates how far from the destination descent must begin.
See also
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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 mod ...
*
Autopilot
An autopilot is a system used to control the path of a vehicle without requiring constant manual control by a human operator. Autopilots do not replace human operators. Instead, the autopilot assists the operator's control of the vehicle, allow ...
*
Top of climb
References
{{Flight phases
Flight phases