Top of 2.5° Descent

We long ago figured out that 3 times your altitude (in thousands) gives a nice 3° descent from en route altitude. When the 60-to-1 gurus came out, they were convinced this was an offshoot of the flight levels to lose technique. In my Hawaii squadron our airplanes couldn't handle a 3° descent at some altitudes without losing pressurization or resorting to speed brakes. So we came up with 4 times your altitude (in thousands) gives a nice 2.5° descent. Are either of these due to the 60-to-1 relationship? No, but they do work. Here's why.

We'll define the rule, provide an example of its application, and then we'll provide a mathematical proof of the rule.

Definition

Start descent at four times your altitude to lose in thousands of feet to achieve a 2.5 degree gradient.

Example

Figure: 2.5° Top of Descent, from Eddie's notes.

Let's say you are cruising at FL410 and are planning a descent into an airport that is just over 1,000 feet MSL elevation. So you have 40 thousand feet to lose. If you want a gentle 2.5 descent, when should you start your descent?

Proof

If you multiply your altitude in thousands of feet by four, you will arrive at the ideal distance to start your descent and end up with about a 2.5 degree descent gradient. The 60-to-1 mavens will have you believe this is an offshoot of the earlier flight levels to lose technique:

If you do the math, that would mean:

Figure: Top Descent trigonometry, from Eddie's notes.

That works, but that isn't the "school solution." We can verify the technique using trigonometry. We will use K to represent the altitude in thousands of feet and D for the distance in nautical miles. Of course that means the formulas will require two conversion factors:

For a 2.5° angle, tan(θ) = 0.0437 and the math works out to:

That is an error of less than 6% from our time-tested rule of thumb.

Bottom Line

So what about the claims this rule of thumb is based on 60-to-1? My conclusion: It has more to do with trigonometry.

60-to-1 — Rule of thumb is based on the mathematical relationship of a 360° circle and/or 6076' to 1 nm.

Trigonometry — Rule of thumb is based on the relationship to a right angle and the derived trigonometric functions.

π — Rule of thumb is based on the relationship of a 360° circle, the number π, and/or 6076' to 1 nm.

Descent

Instrument Procedures

Top of Descent

Portions of this page can be found in the book Flight Lessons 2: Advanced Flight, Chapter 11.

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