How to get the correct speed/power on downwind?

Question

(Note: in this example I'm taking speeds from a Cessna 172)

I was taught that on downwind, you fly at pattern altitude at a normal power setting until you beam the numbers, at which you reduce power (to 1800 RPM.. ish) and start your descent. I was also told that the speed of each leg of the descent should be 80 (downwind), 70 (crosswind), and 60 (on final), and you add 10° of flaps each leg.

I'm running into two related problems:

1. On downwind, my airspeed can be 100-100 knots. This means I float for awhile at pattern altitude after beaming the numbers to get my airspeed down before starting the descent. I can't extend flaps until I'm under 80 knots and in the white arc.
2. My goal is not to add power after I've taken it out. Idea is to control the landing enough that I can still make the runway during an engine out. If I reduce the power a lot to hit 80 knots, I end up having to add a bunch of power on final to make the runway. If I keep the nose high and drop airspeed after beaming the numbers, I end up floating too far away before starting my descent, causing me to fly a larger pattern, and the need to add power to make the runway.

How do I control the speed and power setting to have a graceful pattern and smooth power setting while landing?

Answer 1

I spend most of my flying time in C172s of various models as well as 152s, PA-28s and the like. In most C172s you can extend 10° of flap at 100kts, or even 110kts in some models. The white arc is usually for 20° of flap and above and that's 85kts not 80kts, but you should check the POH to be sure or it may be written on the flap handle placard. 10° of flap will certainly help, but it's more your technique that seems to be the issue here.

Your goal to not add power after you've taken it out is what is the biggest reason for your float, and isn't really constructive. It's normal to reduce throttle to meet a particular airspeed and then throttle up again, if you don't you are relying solely on drag to slow you down, which as you have discovered takes time. Cessnas are draggy beasts because of their low wing loading, wing struts and big rivets, so you can kinda get away with it, but if you tried that in a PA-28 you'd float much, much farther as it is less draggy, so it's not a good habit to get into as you'll be carrying too much extra speed throughout the approach.

You're never going to hit on the perfect throttle setting that will carry you around the pattern and touchdown at just the right point, there are too many variables to factor in like wind direction, wind speed, rising or falling air, etc. Fixing your throttle forces you to fly a specific pattern for the specific conditions, which won't be the same pattern as everyone else, and it will be different every time. There's no benefit to doing it.

The moral of the story is don't be afraid to use the throttle as necessary to control your energy, that's what it's for. When you need to reduce speed throttle back more, then increase again when you get to your target airspeed. In the approach make throttle adjustments required to maintain the descent rate you need, if you start to get a bit low add some, if you get to high pull it, and practice practice practice.

Don't worry about engine failure in the pattern either, if your engine quits you most likely have enough energy to make the field, just clean up and fly your best glide speed. Try it with your instructor: throttle back to idle at downwind and fly a curved approach onto the runway, you'd be surprised how much energy you have at the end.

Answer 2

Part of your problem is being "behind the airplane", which is something that takes experience and practice to overcome, and part is it doesn't sound like you are being taught to fly very well.

It's key to understand the use of trim as your "hands free" speed control. Airplanes pitch to speed, and use power to determine climb, level descent at that speed (if you learn on gliders, this is easy because there's no engine to complicate things). For the pilot, trim determines the speed the airplane will seek when you aren't holding the controls, and the objective is to exploit this feature.

In general you should always be seeking a condition where the airplane maintains your speed without you having to continuously hold forward and backward force on the wheel. If you do that you invariably relax the force and your speed drifts.

So if you're going 100 and want to slow to 80 without climbing, you have to reduce power while you pitch to the speed, following up with trim once you're there. Before you start doing circuits/patterns, you should be trained to do this automatically during air work.

Once you have that automated you will be making power changes, pitch and trim-to-hands-off on speed pretty much in a single smooth action, all the time without thinking, and now your head is free to think beyond those basic actions and will be able to anticipate when to make changes in advance, and you'll have "caught up" to the airplane.

The other thing to keep in mind is with tractor airplanes with the tail in the slipstream, there are small changes to trim speed with power changes due to the slipstream "blowing" harder or less hard on the tail.

If you are on final at 70kt and get low and add a lot of power, your trim speed will go down a bit and the airplane will slow to say 67kt if left hands free. If the power application is temporary, you normally leave the trim alone and push to keep the nose from coming up, but if the power application is more long term, you will need to add a touch of ND trim to correct trim speed to 70kt. Vice versa for power reductions.

Answer 3

Stay high longer, then reduce power to idle. Fly your base at 65 knots.

With a power on approach in a single engine, if you lose power base to final you crash short of the runway. Trying to come in with extra speed (unless it is windy) is foolish because it will throw off your aim. (Energy is $v^2$).

There are many ways to land, pattern altitude around 1000 feet AGL at 100 knots is common. One technique is to cut power to near idle after passing the whole runway on downwind, add 10 degrees of flaps, and turn to base at around 70 knots. Trim to 65 knots and don't touch it. Look, you are now high, on, or low? (fast or slow is eliminated).

I liked this way because you could always make the runway, and adjust by adding drag. A bit more challenging with only 4 possible flap settings, so I learned the forward slip. These are easy, safe, and fun in a 172 at 65 knots.

Your concern about using throttle to control aiming point is well founded, however it works extremely well because throttle can be very finely tuned and is a valuable tool, especially in the wind. But speed control is critical. Don't wait until final to fool around with speed.

So, check your oil before you fly, keep up with maintenance, and remember carb heat on and you should be ok.