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

Verified

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:

Pilots can calculate the "top of descent" for unfamiliar airports by first determining the altitude to lose to reach a predetermined altitude like the traffic pattern altitude (TPA).
A time-based descent can be calculated by dividing the total altitude to lose by your planned descent rate (e.g., 500 feet per minute) to determine the time required for descent.
A distance-based descent can be approximated using the "rule of 3": multiply the altitude to lose (in feet) by 3, then divide by 1,000 to get the approximate distance in miles from the airport where descent should begin.
These calculations should be applied using VFR sectional charts or GPS to identify the descent start point, with pilots adjusting their descent rate as needed and practicing these methods for safe and accurate pattern entry.

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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.