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Two Easy Rules-of-Thumb For Calculating a 3-Degree Glide Slope

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Have you ever found yourself chasing the glideslope on an ILS approach? How about the VASI or PAPI on a VFR final approach?

There's an easier way to do it. Groundspeed has a significant effect on descent rate, and there's a formula you can use to ballpark your feet per minute (FPM) descent, even before you get on glideslope.

The following formulas are a great way to get yourself on glide, and your GPS ground speed will make these rules-of-thumb extremely easy to use...

Option 1: Multiply Your Groundspeed By 5

If you're flying your aircraft on a roughly 3-degree glideslope, try multiplying your groundspeed by 5 to estimate your descent rate. The result will be a FPM value for descent that you should target. As you capture the glideslope, make adjustments as necessary.

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Option 2: Divide Groundspeed In Half, Add "0"

Divide your groundspeed in half, add a zero to the end, and you'll have an approximate FPM of descent. This is another easy way to target an initial descent rate for a 3-degree vertically-guided approach, or a VFR descent into an airport.

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Both formulas leave you with the same result. Choosing which formula to use comes down to which mental math you're more comfortable with.

How Wind Affects Descent Rate

A tailwind on final will result in a higher groundspeed, requiring a higher descent rate to maintain glideslope. The opposite is true for headwinds. Let's take a look at a few examples:

Example 1: Headwind of 25 Knots, Final Approach Speed of 100 Knots Indicated Airspeed.

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Example 2: Tailwind of 25 Knots, Final Approach Speed of 100 Knots.

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Useful For More Than Just ILS Approaches

Looking for a good way to plan out your 3 degree glideslope? These formulas are great references for most instrument approaches with vertical guidance, as well as following any 3-degree PAPI or VASI lights.

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Have you used these formulas before? Tell us how you use them in the comments below.

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