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Wednesday, June 10, 2015

Goldilocks and the Three Descent Rates: Too much, too little, and just right

Recently, I created a video where I mash up Grand Theft Auto: San Andreas and Microsoft Flight Simulator X into a lesson on how to figure out where you're going to land, and whether you're going to be too high, too low, or just right. A viewer over at the Larry the Flying Guy YouTube channel asked what is too much of a descent rate in a small plane. Let's answer this, but first, the video:

I make it a point to reply to comments on the channel (just as I do here), and in the course of replying, it became obvious that this was going to be too long a subject to fit into one comment.

First, let's figure out what a proper descent rate is. Sounds simple enough, right? Well, it is and it isn't. It's incredibly easy if you're doing an instrument approach or a long straight-in. Ironically, it's more complicated if you're out flying on a clear blue day just puttering around in the pattern.

Let's get the easy one out of the way first. If you're on an instrument approach or tower tells you to make a five-mile straight in for the runway, 99% of the time you're going to be coming in on a 3° glidepath. It doesn't matter if I'm flying the 172 into old Lorain County Regional Airport or the Dash-8 into Newark or Washington-Dulles International Airport, I'm still going to be coming in at the same angle.

(That's nice, but what does a "3° glidepath" mean? In its simplest terms, it means you go down 300 feet for every 1 nautical mile you go forward. If you're on a 3° glidepath for 3 miles, you'll lose 900 feet of altitude. 5 miles? 1500 feet. 10 miles? 3000 feet. And so on.)

Since you're going to be doing the same thing every time, it sure would be nice if there was a rule of thumb you could use to get the descent rate you'll need quickly. There isn't one. There are two! Both of them are simple, so use whichever one you find easier.

You'll need to know your groundspeed for either one of these. If you've got a GPS, all it knows is groundspeed, so whatever it says it is what it is. If you don't, then you'll need to know your wind components and subtract the headwind from your airspeed to get a precise answer. This can get messy (although there are some rules of thumb to help you out), and the winds you're probably to be doing your private or instrument training in aren't likely to be strong enough to make more than a 10-20% difference in the final answer anyway. This is why we're just going to assume your groundspeed is the same as your airspeed from here on out.

Method one: Take half your ground speed and add a 0.

If I'm flying the 172 at 80 knots: 80/2 = 40. Add a 0 for 400 feet-per-minute (FPM).

If I'm flying the Dash-8 at 110 knots: 110/2 = 55. Add a 0 for 550 FPM.

If I'm flying the Space Shuttle at 300 knots: 300/2 = 150. Add a 0 for 1500 FPM.

Method two: Multiply your groundspeed by 5.

If I'm flying the 172 at 80 knots: 80 x 5 = 400.

If I'm flying the Dash-8 at 110 knots: 110 x 5 = 550.

If I'm flying the Space Shuttle at 300 knots: 300 x 5 = 1500.

This rule of thumb is actually how I calculated the descent rate during the planning for the River Visual 19 into Washington-National  videos. Using this in reverse is why I picked 90 knots to fly it at, since 90 x 5 = 450, which is a manageable rate that also gets me there in a reasonable amount of time. The 300 feet per nautical mile rule is why the chart designers picked what they did when they created the approach:

Now that's all well and good, but when you're flying the pattern, you're not going to be coming in on a 3° glidepath. You're going to be coming in at closer to a 5° glidepath when you're working the pattern. This is a nice balance between being close enough to the runway to still make it if the engine quits and not being too steep.

Does this mean you have to throw Method 1 and Method 2 out the window? Not really. Since 4.5 is 50% more than 3, you can adjust them to fit. (Unlike the first example, they don't give the same result, but they're close enough. They will equal each other if you change the second method to 7.5, but if you can multiply things by 7.5 in your head, you're a better mental math wizard than the rest of us.) There's also a third method in this case, which is the one I use.

Method one: Take half your ground speed and add a 0. Take half of the result and add it to itself.

If I'm flying the 172 at 80 knots: 80/2 = 40. Add a 0 for 400 feet-per-minute (FPM). Half of 400 is 200. 400 + 200 = 600 FPM.

If I'm flying the Dash-8 at 110 knots: 110/2 = 55. Add a 0 for 550 FPM. Half of 550 is 275. 550 + 275 = 825 FPM.

If I'm flying the Space Shuttle at 300 knots: 300/2 = 150. Add a 0 for 1500 FPM. Half of 1500 is 750. 1500 + 750 = 2250 FPM.

Method two: Multiply your groundspeed by 8.

If I'm flying the 172 at 80 knots: 80 x 8 = 640.

If I'm flying the Dash-8 at 110 knots: 110 x 8 = 880.

If I'm flying the Space Shuttle at 300 knots: 300 x 8 = 2400.

Method three: Forget about your vertical speed indicator and use your eyeballs.

In the pattern, all this math is too complicated because you're changing airspeeds and flap settings at the same time. If you learned (or are learning) in a 172, you probably hear "85, 75, 65" in your head to this day, meaning drop a notch of flaps and pitch for 85 knots. Drop another notch and go to 75. Drop the third and go to 65. Do you want to do arithmetic or do you want to fly? You probably can't do both at the same time, especially since your descent angle is also changing every time you add flaps—after all, that's one of the reasons airplanes have flaps.

If you covered my vertical speed indicator (VSI) and asked me what my descent rate is, two things would happen. First, I would have no idea you covered it up, because I don't look at it in the pattern. I'm looking at the runway numbers at that point in the flight, not the instruments. Second, I'd answer you not with a number but with one of three answers: "Not enough", "Too much", or "Just right".

Don't believe me? I take this to the ultimate test in my very first YouTube video ever, where I fly around the entire pattern with the glass panel completely off, so there are no instruments to look at whatsoever!

How do I know if I have the correct descent rate without the VSI? As I look at my aiming point, one of three things is happening:

1. It's moving down my windscreen = "Not enough."

2. It's moving up my windscreen = "Too much."

3. It's staying at the same point in my windscreen = "Just right."

Each of these has a fix:

1. "Not enough" = reduce power some.

2. "Too much" = add some power.

3. "Just right" = don't touch anything.

Again, if you're in a 172, you're in luck because you have a nice rule of thumb for how much to change the power to start a descent. For every 100 FPM more of descent you want, reduce power by 100 RPM. If you want to reduce your descent rate, add 100 RPM for every 100 FPM you want to reduce it by. But what if you don't have a VSI or you had me as an instructor so you had to fly with no instruments at all before you were allowed to solo?


1. "Not enough" = reduce power some. If your aiming point is still moving down after you've given the plane a few seconds to respond, reduce a little more. If it's going up now, you reduced too much. Add about half of what you just reduced it by. (In other words, if reducing the power by 100 RPM made the aiming point start moving up, add about 50 RPM back.)

2. "Too much" = add some power. If your aiming point is still moving up after you've given the plane a few seconds to respond, add a little more. If it's going down now, you added too much. Take out about half of what you added.

3. "Just right" = don't touch anything.

By "some" I mean "a little". Don't make massive power changes or your descent will look like a camel's back. If you're in a 172, "a little" means about 100 RPM. Don't add or subtract much more than that unless you're obviously way too high or way to low—and in that case, you'll probably want to consider going around instead of trying to salvage an approach that is likely to end up unstable.

I use this same process even when I'm flying the Dash-8. On a normal approach, around 20% power tends to work well. If I'm a little low, I'll add about 5%. If I'm too high, I'll reduce it by about 5%. Just that much of an adjustment almost always fixes things, but if it doesn't, I'll add or subtract another 3% or so. Smaller corrections sooner are better than big corrections later.

I've answered the question of why descent rates matter, but I still haven't answered the commenter's main question. How much of a descent rate is too much in a small plane?

There isn't a hard-and-fast rule. The big answer is that if you have to dive to catch the proper glidepath, just go around and try it again rather than force an unstable approach. Good landings come from good approaches, and greaser landings start a mile before touchdown. The biggest problem most students have is being too high on final, which means more power needed to be reduced abeam the numbers. Fix it then and you won't have to fix it later.

A more specific answer is that anything more than 1000 FPM once you've put the first notch of flaps in is getting close, and it's definitely too much when you've turned final. Once you've put the first notch in and slowed to 85, 1000 FPM is approximately a 7° glidepath. That's steeper than necessary, but at this point in the pattern, you're probably not much below 1000 feet in altitude, so if you bump up against 1000 FPM for a few seconds, you've still got time.

However, once you're on final and slowed to 65 knots, 1000 FPM is closer to a 10° glidepath! If you rolled out on final at about 400 feet and 1/2 mile out, which is right about where you should be, a descent rate of 1000 FPM will kill you in 20 seconds. Our airline's procedures prohibit descents greater than 1200 FPM when we're below 1000 feet. For the same margin of safety in a 172, this would mean no more than about 700 FPM on final. If you have no wind, that's pretty close to what you'll be doing. The more headwind you have, the less FPM you'll need, since your groundspeed will be slower.

Learning to judge whether your spot is moving up or down or not at all is a skill that is perfect to practice in a flight simulator. You can create a scenario where you're lined up on final and practice it several times in less than 10 minutes. In a flight simulator, you don't have to worry about getting dangerously low and you don't have to pay a lot of extra money if you're too high and have to go around, since you can just reset and try again.

Once you get good at judging your height on a straight-in, you can progress to creating a flight at your home airport where you're on midfield downwind and getting ready to start the full descent profile. This is much harder in a flight simulator than it is in real life, since it's harder to look out the side windows in a sim. If you can master this, your training (or your approaches in general) will go much more smoothly, and your investment in time will be repaid by much less money on lessons!

This technique is something I wish someone had told me when I was learning to fly. It wasn't until I read Wolfgang Langewiesche's classic Stick and Rudder (the book that gave Keyboard & Rudder its name) and became a flight instructor that I found out there was an easy way to figure out whether I was on the correct glidepath.

Happy practicing, and see you next Wednesday!

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The author is an airline pilot, flight instructor, and adjunct college professor teaching aviation ground schools. He holds an ATP certificate with a DHC-8 type rating, as well as CFI, CFII, MEI, AGI, and IGI certificates, and is a FAASafety Team representative and Master-level participant in the FAA's WINGS program. He is on Facebook as Larry the Flying Guy, has a Larry the Flying Guy YouTube channel, and is on Twitter as @Lairspeed.

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