We know that what goes up must come down, but when I was in flight training, I always had a hard time determining exactly when to come down. I’d tend to begin my descent to an airport too late, when it was in clear view, leaving me with a more rapid descent than ideal. On the other extreme, a descent that is too early has you flying low for a few miles. This leaves the potential for terrain, obstacles and limited options for any engine troubles. Let’s run through a scenario to give you an idea of how to choose when to start that cruise descent when inbound into an airport.
The most memorable time that this descent planning became a reality for me was on my first solo cross country. It was my first time to a new airport without an instructor to give the gentle reminder that it’s time to start down. I was on my own with a lot to think about. Let’s revisit that flight. I’ll throw out some numbers and then walk you through it. The route was from Athens, GA (KAHN) to Greenwood, SC – about 61 miles. My cruise altitude was 3500 feet and the traffic pattern altitude at Greenwood was 1431 feet. Was cruising around 110 knots. So how far out do I start a descent? Sounds like a word problem to figure out on a math test. Let’s begin by organizing our thought process.
The Thought Process
There are three big variables you need to know for this calculation; the altitude to lose, the descent rate, and how fast you are going. For the altitude to lose, subtract the pattern altitude from the cruise altitude (3500-1431= roughly 2000 feet). Use the pattern altitude instead of the field elevation because the descent ends at the traffic pattern, not the runway. The speed can be ball-parked using your airspeed (110 knots in this case), but know that you will technically use groundspeed. While descent rates can vary, a 500 foot per minute descent works well for basic flights like this. It’s both comfortable for the passengers and easy to use in calculations. Now that we have what we need, let’s look at two ways to solve it.
Method 1: Mental Math
Our altitude to lose is roughly 2000 feet. Using the 500 foot per minute descent rate, it’ll take four minutes of descent. (2000/500). Remember, what we are looking for is how far out we need to start our descent. To allow four minutes of descent, we need to use our speed to figure out how far out four minutes of flying is. In a Cessna 172, cruising somewhere around 110-120 knots means we are traveling roughly two miles per minute. This means that every minute we spend descending, we’ll travel two miles. So four minutes of descent will require 8 miles of travel (4 x 2). If we start our descent about 8 miles out, we’ll be down to pattern altitude at the right time. Build in a little extra margin so that you don’t arrive at pattern altitude over the airport. In this scenario, I’d use about 10 miles out to start my descent.
Method 2: Using An E6B
Because we had nice round numbers, the mental math worked out pretty well. But what if the numbers weren’t as round? Let’s say we are descending at a speed of 100 knots, at a rate of 700 feet per minute and had 4000 feet to lose? I’m a big fan of the manual E6B. We can use the old “wiz wheel” to calculate this – or even to verify our basic mental math. If you need a refresher, read about how to use a manual E6B. To solve, we need the same variables as before – the altitude to lose, the descent rate, and the speed (4000 feet, 700 feet per minute, 100 knots).
First, figure out the time needed to descend. Line up the rate needle to 700 feet per minute (the 70) on the A scale. Look to 4000 feet (40) on the outer A scale to read the time in minutes of just under 6 min from the inner C scale.
Now we need to calculate how far we will fly in those 6 minutes going 100 knots. Line up the rate arrow with 100 knots (the 10). Then look for 6 minutes on the inner B scale to read your distance of 10 miles on the outer A scale.
With practice, either mental math or E6B calculations can be used effectively in flight. At first, it can seem like taking a math test while flying the plane. But a little forethought can let you figure it out on the ground and plan a point for your descent. On my solo cross country, I ran these numbers and planned on beginning my descent over the town of Abbeville, which was along my route and 12 miles away from my destination. Planning ahead meant that I didn’t have to do the math in the air. Once I saw Abbeville down below, I began my 500fpm cruise descent. You can read my journal entry to see how it worked out. Even when using an iPad to fly, spending the time on the ground studying the route and planning ahead for decisions like a descent point can help you stay well ahead of the airplane.
Do you have any other methods or rules of thumb for descent planning? Share them with us!
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