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Descending to land—The 3:1 Rule

Glenn Carlson has a post on descending along the ILS glide slope. It explains how to be sure that you are on the glide slope and not a false glide slope. The same rule of thumb can be extended for any descent to land. There is a 3:1 ratio of height above the airport and distance to the airport, where HAA is thousands of feet and distance is nautical miles.

As an example, our Class D airport usually requests that you “Report 4 mile final.” The rule says that you should be at 1200′ above the airport. In this case, you should be at 1,409′ MSL. If you are flying at 120 kts (2 nm per minute), the 4 miles will take 2 minutes. Descending at 600 fpm is comfortable rate of descent. Faster speeds will require faster descent rate.

The AIM says that you should make a position call to non-towered fields or to the tower at Class D fields when you are 10 miles out. Unless there is terrain that makes it impractical, you can make a comfortable descent if your HAA is 3,000′.

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