Top of Descent
The top of descent ( TOD or T / D ; literally: the highest point of the descent; or: the starting point of the descent) is the point in aviation at which the flight phase of the cruise flight changes to the descent .
At this point the planned descent up to the approach altitude must begin.
The Top of Descent is mostly from the flight management system calculates (onboard computer) and on the navigation screen (Engl. Navigation display ) is displayed. The type of display on the navigation screen is not standardized, for example the TOD is displayed as a green arc. The top of descent is calculated in such a way that it allows the most economical descent to the approach altitude. Sometimes, however, the top of descent is not optimized for profitability, but for other criteria, for example the fastest descent or the greatest range.
The top of descent can also be calculated manually by the pilot, but this requires considerable effort.
Since a jet aircraft uses much less fuel at high altitudes than at low altitudes, the pilots endeavor to stay at cruising altitude as long as possible. However, the aircraft must not remain at its cruising altitude beyond the top of descent , as it would then have to descend too steeply. However, if the aircraft descended too steeply, it would become too fast, which in certain circumstances would cause it to exceed the load limits of its structure. The increasing speed during descent can be reduced by extending spoilers ; however, energy is wasted in the process, so that the aircraft does not fly energy efficiently. Flying additional loops or detours would also allow a further decrease in altitude, but would be very uneconomical. The most economical descent is carried out with minimal fuel consumption, with the engines being reduced to flight idle . The top of descent is calculated and displayed by the flight management system. If this is not available in the aircraft, it is an important task of the pilot to calculate or at least estimate this point.