Flying Techniques: Descent Planning, The Basics

Flying Techniques: Descent Planning, The Basics

Effective descent planning is essential for ensuring a smooth and comfortable approach to landing. One of the most useful tools in aviation for calculating descent profiles is the"Three-to-One Rule." This simple yet effective rule helps pilots determine the optimal point at which to begin a descent, known as the Top of Descent (TOD).

What is the Three-to-One Rule?

The Three-to-One Rule is a guideline used by pilots to estimate the distance needed for a comfortable descent when transitioning from cruising altitude to landing. It is based on a general approximation that, for every 3 nautical miles (NM) traveled, an aircraft will descend by 1,000 feet. This ratio offers pilots a quick and practical way to plan a descent without needing complex calculations or instruments.

In essence, the Three-to-One Rule states that:

• It takes 3 NM of horizontal distance in order to descent 1,000 feet.

For example, if a pilot needs to descend from an altitude of 30,000 feet to reach the airport (assuming at SeaLevel), using the Three-to-One Rule, the calculation would be as follows:

• 30,000 feet ÷ 1,000 feet = 30, and then multiplied by 3 NM = 90 nautical miles.
• So, the aircraft should start its descent 90 nautical miles away from the airport to achieve a smooth, comfortable descent.

How Fast Do You Need to Descend?

The Three-to-One Rule allows a pilot to determine where to descend, but now they need to determine how fast to descend. A Boeing 737 and a Cessna 172 can both use the Three-to-One rule to determine their descent point, but they will not have the same descent rate when they reach theTop of Descent.

There is another rule of thumb pilots can use to determine the descent rate, and that is multiplying your groundspeed times 5. For example, A Boeing 737 traveling at 400 kts groundspeed would need a descent rate of 2000 feet per minute when it reaches its Top of Descent. If math is hard, multiplying by 10and then cutting the number in half will work as well, (400 times 10 is 4,000, then divided by 2 is 2,000).

What Makes a Descent Comfortable?

Aircraft get the best airspeed and fuel efficiency at higher altitudes, so pilots prefer to stay higher as long as possible to take advantage of these benefits. However, if an aircraft descends too quickly, the change in cabin pressure may be uncomfortable for the passengers, resulting in discomfort or pain on eardrums or sinuses. Therefore, there is a balance between descending early for passenger comfort and descending later for speed and fuel efficiency. In general, a cabin descent rate of 500feet per minute (fpm) is comfortable for passengers.

Pressurized vs Unpressurized Aircraft

Pressurized aircraft have the ability to control the amount of air flowing into/out of the aircraft cabin. This allows the aircraft to climb or descend quickly, usually several thousand feet per minute, while the aircraft cabin pressure climbs or descends at a much slower rate, usually around a comfortable 500 feet per minute. However, for unpressurized aircraft, the aircraft cabin will climb and descend at the same rate that the aircraft itself is climbing/descending. Therefore, in these types of aircraft, we often have to descend earlier than what would be required by the Three-to-One rule, in order to maintain a comfortable descent rate.This would the case in an aircraft like a Cirrus SR22, which cruises around 165 kts. If a pilot tried to descend at 5 times the groundspeed, this would yield an 850fpm descent, beyond our 500 fpm goal, and likely uncomfortable for pilot and passengers. For faster, unpressurized aircraft, many pilots will instead use a “Five-to-One” Rule, choosing to start the descent 5nm out per every thousand feet of altitude.

Conclusion

Descent planning is a vital part of flight safety and passenger comfort. The Three-to-One Rule is an invaluable tool for estimating when to start a descent, ensuring that pilots have the distance needed for a smooth, gradual approach. However, understanding how to adjust the descent rate based on aircraft type—whether pressurized or unpressurized—further enhances comfort and efficiency.

‍