How Do You Know When to Descend?

At a strange airport, the landmarks a pilot becomes accustomed to at home are missing.

Wednesday, May 08, 2024 at 02:34 PM ET

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Knowing how to figure out the top of descent is the key to finding pattern altitude for an unfamiliar facility. [FLYING file photo]

Key Takeaways:

For unfamiliar airports, pilots can calculate the "top of descent" to avoid relying on landmarks.
One method involves calculating the time needed for descent by dividing the total altitude to lose by the desired descent rate.
Another method estimates the distance from the airport to begin descent by multiplying the altitude to lose by three, then dividing by 1,000 to get miles.
Pilots should use VFR sectionals or GPS to identify the calculated descent point and practice these techniques for safe and efficient operations.

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Question: How do you know when to start your descent? At my home airport I plan my descents by using landmarks—be at this altitude over the water tower because that will put me at pattern altitude, etc. How am I supposed to do that when I go to an airport that I am unfamiliar with?

Answer: You have just described knowing the top of descent—that is, knowing how much time and distance it will take you to reach the predetermined altitude, such as the pattern.