Some people might wonder why different pilots use different techniques or ways of thinking. In reality, they are essentially the same—they lead to the same result. It just depends on which perspective you choose, based on what is easier and more convenient for you.
For example, comparing Bill Fly’s method and mine:
At an altitude of 6,000 ft and a distance of 20 NM, that equals 300 ft/NM.
This corresponds to a 3-degree path, because 1 degree is approximately 100 ft/NM (more precisely about 106 ft/NM).
Or you can think of it as FL60 divided by 20, which also gives 3 degrees—same as Bill Fly’s method.
For rate of descent:
• My method: GS × (300/60)
• Bill Fly’s method: TAS / 600 × 3 (mach x degree)
and also same as Mach x feet/nm
( SS is about +/- 600)
Both give very similar results.
There may be slight differences when using Mach instead of GS, especially in strong headwind or tailwind conditions(same M but not same GS).However, it is just a rule of thump and pilots already have ways to compensate for that. In the end, you can use whichever method works best for you.
PS# Alternatively, another method I use is to take the time remaining to reach a point and divide the altitude (in feet) to get the rate. For example, if point A is 10 minutes away and the altitude is 20,000 feet to go, then 20,000 ÷ 10 gives 2,000 ft/min.
Last edited by Noknoipobin; 14th April 2026 at 10:40.