A safe, precise landing is the goal of every pilot at the end of their flight, and one of the keys to nailing that perfect landing is approaching it at the proper rate of descent.
Compared to some of the other types of pilot math, rate of descent calculations are very quick and easy. They can be performed mentally or by hand as you set up your approach.
What is a rate of descent?
An aircraft’s rate of descent, or descent rate, is the rate of decrease of vertical height per unit of time. In the United States, descent rate is measured in feet per minute.
Why is knowing your rate of descent important?
A consistent descent rate during landing allows IFR pilots to stay on the glideslope and VFR pilots to avoid chasing the VASI or PAPI during final. Descend too quickly, and our descent angle will be dangerously steep. Descend too slowly, and the shallow approach may cause us to run out of runway prior to touchdown. Rate of descent calculations give us a target rate of descent that will set us up to land safely in the touchdown zone.
How to calculate rate of descent
During an approach, we need a way to calculate our descent rates quickly and easily. The most common IFR glideslope and VFR VASI or PAPI-guided approaches are set up for a 3-degree descent. Although more precise mathematical rate of descent calculations can be done, approximating using one of the two rule-of-thumb methods below is sufficient. These methods can be used to calculate the approximate rate of descent needed to achieve a typical 3-degree descent angle.
Rate of descent calculation method 1
The first method of calculating necessary rate of descent is to use the following equation:
Groundspeed x 5
The resulting number is our approximate descent rate in feet per minute. For example, if our groundspeed is 100 knots indicated airspeed (KIAS), and we multiply it by 5, that would equate to a 500 FPM descent rate to achieve a 3-degree descent angle.
Rate of descent calculation method 2
Necessary rate of descent for a 3-degree angle can also be calculated with the following formula:
Groundspeed/2 + add one decimal place
After dividing groundspeed in half, add one decimal place to the end of the answer to get your target rate of descent. For example, if we take our same 100 KIAS groundspeed and divide it in half, the answer is 50. Adding another decimal place to the end provides the same 500 FPM descent rate as method 1.
How do headwinds and tailwinds impact descent rates?
As you may realize, descent rates are not static. They are based on our groundspeed, and both headwinds and tailwinds impact that groundspeed. Our descent rate will be less with a headwind than it will be with a tailwind because our groundspeed is slower with a headwind than a tailwind.
For instance, we can go back to our previous examples using 100 KIAS. Now let’s add a 25-knot headwind. We must subtract the headwind speed from our indicated airspeed, giving us a groundspeed of 75 knots. Using method 1 of calculating our rate of descent gives us 75 x 5 which comes out to a 375 FPM descent rate.
If we were traveling at the same 100 knots indicated airspeed but this time, we have a 25-knot tailwind, we would add the tailwind speed to our indicated airspeed giving us a groundspeed of 125 knots. Using method 2 of calculating descent rate, we divide 125 in half to get 62.5. When we add a decimal place to our answer, it gives us a 625 FPM rate of descent.
Notice that there is a 250 FPM difference in descent rate based on the addition of a headwind vs a tailwind. Due to the significance a headwind and tailwind can make, we calculate our descent rate each time we approach the airfield even if we make this landing frequently and know what our typical descent rate would be in calm air.
How to calculate top-of-descent
We now know how rapidly to descend based on our descent rate calculations, but how far out from the airport should we start our descent? For this, we need to do a top-of-descent calculation. Like the rate of descent calculation, top-of-descent can be found using a rule-of-thumb method. Note that both rule-of-thumb methods below assume a 500 FPM descent rate.
Top-of-descent calculation method 1
The first way to find a ballpark top-of-descent distance is to use the following series of equations:
The answer provides us with the approximated distance in nautical miles from the airport at which we should start our descent.
For example, let’s run through the above equations assuming our current altitude is 10,000 feet MSL and the airfield altitude is 1,000 feet MSL.
According to our calculations, we should start our descent when we are approximately 27 nautical miles from the airport assuming our calculated descent rate is 500 FPM.
Top-of-descent calculation method 2
Another way to calculate our top-of-descent is using a rule-of-thumb method based on time. For this method, assume 2 minutes of descent time for each 1,000 feet of altitude loss required.
Using the same altitudes as our above example, our calculation would be:
The difference between our altitude and the airfield altitude is 9,000 feet. We divide that by 1,000 and then multiply by 2 minutes to get an answer of 18 minutes. We should start our descent when we are 18 minutes away from the airport. Again, this is assuming a 500 FPM rate of descent.
Takeaways
As we set up for our approach to landing, we need to know how rapidly we should descend so that our angle is not too steep or too shallow. Our targeted rate of descent is based on our groundspeed and can be calculated quickly and easily using one of several simple formulas. Another calculation that is often done in tandem with rate of descent is top-of-descent. A top-of-descent calculation tells us how far out from the airport we should begin that descent so we make a smooth and safe landing on the runway.
Review these and other rule-of-thumb calculations in your copy of Pilot’s Rules of Thumb.