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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 distance, air speed, and current altitude are known. This ...
The top of climb may be calculated manually with considerable effort. [ 1 ] Alternatively, when manual planning and monitoring a VFR flight, TOC is an elegant and efficient way for a pilot to eliminate all the vaguery and variability of departing any airport (the turns assigned, changes of runway the pilot cannot control).
The first thing the VNAV calculates for the descent is the top of descent point (TOD). This is the point where an efficient and comfortable descent begins. Normally this will involve an idle descent, but for some aircraft an idle descent is too steep and uncomfortable.
Keep climbing whenever the aircraft is light enough to climb to the next highest available flight level, until it is time for descent. If it is possible to stay at the next higher cruise flight level for 20 minutes or more before "top of descent " ( TOD ) then it is cost-effective to make the climb.
In aviation, the rule of three or "3:1 rule of descent" is a rule of thumb that 3 nautical miles (5.6 km) of travel should be allowed for every 1,000 feet (300 m) of descent. [1] [2] For example, a descent from flight level 350 would require approximately 35x3=105 nautical miles.
It is also helpful to calculate the top of descent, or the point at which the pilot would plan to commence the descent for landing. The flight time will depend on both the desired cruising speed of the aircraft, and the wind – a tailwind will shorten flight times, a headwind will increase them.
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An onboard navigation system displays a constant rate descent path to minimums. The VNAV path is computed using aircraft performance, approach constraints, weather data, and aircraft weight. The approach path is computed from the top of descent point to the end of descent waypoint, which is typically the runway or missed approach point. [1]