02/14/2019
![](https://www.boldmethod.com/images/blog/quizzes/2018/11/do-you-know-these-six-common-approach-symbols/primary.jpg)
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. In most glass cockpit aircraft, wind vectors and ground track diamonds mean you'll have a easily visible references to use. GPS groundspeed will make the following equations 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.
![](https://www.boldmethod.com/images/learn-to-fly/performance/use-this-formula-to-calculate-a-3-degree-descent-rate/gsx5.jpg)
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 precision approach, or a VFR descent into an airport.
![](https://www.boldmethod.com/images/learn-to-fly/performance/use-this-formula-to-calculate-a-3-degree-descent-rate/divide-in-half.jpg)
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.
![](https://www.boldmethod.com/images/learn-to-fly/performance/use-this-formula-to-calculate-a-3-degree-descent-rate/Example-1.jpg)
Example 2: Tailwind of 25 Knots, Final Approach Speed of 100 Knots.
![](https://www.boldmethod.com/images/learn-to-fly/performance/use-this-formula-to-calculate-a-3-degree-descent-rate/Example-2.jpg)
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 guideance, as well as following any 3-degree PAPI or VASI lights.
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