How Do Pilots Decide When To Begin Their Descent?

Simple Flying

The typical descent for an airliner is performed at a 3-degree angle.

Photo: Angel DiBilio | Shutterstock

Pilots use trigonometry to calculate the top of descent (TOD) point for efficient descent and landing.
A shortcut for calculating TOD is to multiply the altitude (without zeros) by 3, giving a close approximation.
The ground speed of the aircraft is used to calculate the descent rate, with a shortcut of multiplying by 5.