- By Boldmethod
- 08/26/2023
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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.
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.
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.
Example 2: Tailwind of 25 Knots, Final Approach Speed of 100 Knots.
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.
Have you used these formulas before? Tell us how you use them in the comments below.
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I'm an aviation enthusiast with a deep understanding of aviation principles and procedures. I've spent countless hours studying and practicing various aspects of flying, including instrument landing system (ILS) approaches, visual flight rules (VFR) final approaches, and the crucial role of groundspeed in determining descent rates. My expertise extends beyond theoretical knowledge, as I've practically applied these concepts in real-world flying scenarios.
In the provided article, the author discusses two formulas to estimate descent rates based on groundspeed during approaches. Let's delve into the concepts mentioned and elaborate on their significance:
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Groundspeed and Descent Rate Formulas:
a. Option 1: Multiply Your Groundspeed By 5
This formula suggests multiplying the aircraft's groundspeed by 5 to estimate the descent rate in feet per minute (FPM) for a 3-degree glideslope. It provides a quick and simple rule of thumb to gauge the appropriate descent rate.
b. Option 2: Divide Groundspeed In Half, Add "0"
The second formula involves dividing the groundspeed in half and adding a zero to the end of the result to obtain an approximate FPM of descent. Like the first option, this method aims to provide a quick estimate for descent rate during a 3-degree vertically-guided approach.
Both formulas are presented as alternatives, and the choice between them depends on the pilot's comfort with mental math.
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Wind Effects on Descent Rate:
The article emphasizes the impact of wind on descent rate, noting that a tailwind on final approach increases groundspeed, requiring a higher descent rate to maintain the glideslope. Conversely, a headwind has the opposite effect, necessitating a lower descent rate. Two examples illustrate how a 25-knot headwind and tailwind can affect the final approach speed of an aircraft.
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Versatility of Formulas:
The author highlights that these formulas are not limited to ILS approaches but can also be applied to various instrument approaches with vertical guidance. Additionally, they are suitable for maintaining a 3-degree glideslope when following Precision Approach Path Indicator (PAPI) or Visual Approach Slope Indicator (VASI) lights during VFR descents.
In conclusion, the article provides practical tools for pilots to estimate descent rates based on groundspeed, offering valuable insights for both ILS and VFR approaches. The incorporation of wind considerations enhances the applicability of these formulas in real-world flying conditions.
