This glossary is written for virtual airline and flight simulation use within Virtual Air Canada Airline. It is not intended as a real-world aviation reference.
TOD
Top of Descent
Top of Descent is the calculated geographic point along a flight’s route where the aircraft should begin its descent from cruise altitude in order to arrive at the destination at the correct altitude, speed, and configuration for the approach. The FMS calculates TOD based on the cruise altitude, the required altitude at the first constraint on the STAR or approach, the aircraft’s speed and weight, and the descent profile programmed into the flight plan. In managed descent mode, the FMS will initiate the descent automatically at TOD and follow the computed vertical profile.
Passing TOD without beginning the descent - flying level past the calculated point - results in being “high” on the descent profile, which can lead to arriving at the initial approach fix too high and fast. Recovery from a high situation often requires requesting an amended clearance from ATC, using speed brakes, or accepting a longer approach routing. In modern glass-cockpit aircraft, the TOD is displayed as a downward arrow symbol on the ND and PFD, and crews are trained to initiate descent at or before this point. On flights using VNAV, the FMS will provide a continuous advisory of descent path deviation - typically shown as a deviation scale on the PFD - to help crews maintain the computed profile.