What is the purpose of the “3-2-1” rule in approach planning?

The “3-2-1” rule in approach planning serves the purpose of initiating descent based on a specific distance-to-altitude ratio. This rule provides a simple guideline for pilots to determine when to begin their descent so that they can reach the runway at the proper altitude and time for landing. According to the rule, a pilot should begin a descent when they are approximately three nautical miles from the runway for every 1,000 feet of altitude. Therefore, if an aircraft is at 3,000 feet, the descent should begin about 9 nautical miles from the runway.

This method helps in maintaining a controlled and steady descent profile, allowing for adjustments as necessary while ensuring that the aircraft can arrive at the threshold of the runway at the correct altitude and configuration for landing. Understanding this rule is critical for effective approach planning and execution, contributing to safer landings.