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Old 12th Feb 2011, 22:29   #1 (permalink)
 
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Cessna 172 landing techniques - what is the difference?

Hi guys,

I fly 172R and am a post (first) solo level student. I will ask my instructor next time I fly, but I want to hear what you guys think.

I learned to land my pulling the power when I am still pretty high on final, and gliding all the way down to the runway. I know that some instructors (I flew with one) teach that you keep power in all the way down to the runway, and then pull power just before the flare (basically).

What is the difference and pros/cons of each method? Any safety issues between the two? Which is more challenging?

Thanks a lot in advance.
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Old 12th Feb 2011, 23:55   #2 (permalink)
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In any case, in nearly all nosewheel planes, your objective for a normal landing will be to have the aircraft very nearly stalled just as you touch the runway, with good directional control, and a suitable length of runway ahead.

That said, you will be passing through the speed range (during the flare)from a suitable approach speed, to the stall speed of the aircraft for a nice touchdown. With skill, this can be neatly done power off, from however high you wish. If you're still developing that skill, you will find it easier to lengthen the time you have in the flare, to get everything just the way you want it to be for that nice landing.

With an extra long runway, it is actually possible to touch down with considerable power. Pundits will perhaps jump on me, suggesting this is pointless, but on a really cold day, when you don't want to shock cool an engine, it's appropriate to carry some power all the way in.

However, the opportunity to land with power is no excuse for not practicing the skill of a power off gliding landing.

There is no right or wrong way, as long as the end result is a neat, well controlled landing - but, if you're relying on power to carry you to the touchdown point, and it quits, you're going to look foolish if you don't make the runway - you should have been able to under normal circumstances....
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Old 13th Feb 2011, 00:33   #3 (permalink)
 
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I find that generally my landings are smoother if I fly the approach with a bit of power and only chop it in the flare. Unless it's a really tight field or a steep approach to clear obstacles.
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Old 13th Feb 2011, 01:00   #4 (permalink)
 
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I had the good fortune of finding an instructor who liked to fail the engine after the downwind checks were completed. In the C-172 with 40 flaps available, it was pretty simple to keep the approach tight and select 40 flaps when the runway appeared almost straight down. With 40 flaps, you need a steep descent to keep the speed up and have enough energy to flare -- especially with an upslope on the runway.

So if you are carrying power on the approach, if the engine becomes unhappy, you might not make it to the runway. Removing flap can cost you considerable altitude.

In cold weather, I do carry a bit of power so that the engine will not cool too rapidly; also a warmer engine will be useful if a go-around has to be done.
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Old 13th Feb 2011, 02:03   #5 (permalink)

 
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Idle descents to landing are good training, but generally poor practice.

Have a little more respect for your engine.
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Old 13th Feb 2011, 07:16   #6 (permalink)

 
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Since flying from smaller strips, I tend to drag the aeroplane in under power, so lots of drag and high angle of attack. The advantage is that you can more accurately determine your touchdown point, and can keep speed on the slow side of approch speed. You also minimize float, and maximize stopping distance.

Disadvantages include that if you engine stops on final, you won't make the runway (does this ever happen?).

This is not appropriate for all people though, the aeroplane I have flown most recently is designed for STOL operations and has leading edge slats, and at high AoA those slats pop out giving a nice stable approach and excellent short field.
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Old 13th Feb 2011, 08:54   #7 (permalink)

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Quote:
I learned to land my pulling the power when I am still pretty high on final, and gliding all the way down to the runway. I know that some instructors (I flew with one) teach that you keep power in all the way down to the runway, and then pull power just before the flare (basically).
Yes well, I use power as necessary to keep the approach path reasonable, and depending on all sorts of things (eg headwind and how competent I'm being that day) this might mean having the engine at idle at various points. One runway has a decided sink over the ponds in most weather conditions so what looks like a nice glide approach will almost always need some power as I cross the water.

Also if I follow the teaching "turn base, reduce power to 1500, remain level until below flap limiting speed, flaps 30, descend" I'm sometimes (again depending on wind etc) going to end up over the threshold at several hundred feet, so cutting the power to idle after the base leg turn to see how it goes today often works better for me particularly with a tailwind on base leg.

I've done these things with a variety of instructors sitting next to me on various lessons and check rides over the years and none has ever complained. Well, not about power handling on the approach, anyway.
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Old 13th Feb 2011, 10:25   #8 (permalink)
 
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As others have told you, there's something to say for both techniques and in the end we all use a blend of both. And practicing power-off landings is never a bad idea, as long as you have considerations for the engine (shock cooling) and the others in the circuit.

I think it's important not to focus on one or the other too much, but to understand some of the aerodynamics behind it.

First, what happens when you reduce the power to idle? Of course, thrust is reduced so you either maintain the pitch attitude and bleed off speed, or you have to lower the nose to keep the speed up. Trim plays a role here too. If the aircraft is trimmed correctly it will drop the nose by itself to maintain speed. But if the aircraft is not trimmed correctly you need to know exactly what control position/force is necessary to achieve the desired effect.

But that's not all. Due to propwash effects, p-factor and whatnot, reducing the power will induce yaw to the right (assuming the standard US Lycosaurus setup) which you need to counter with left rudder. For the exact same reasons that increasing the power, for instance at the startup roll, requires right rudder.

At your stage in training, do you find yourself intuitively controlling the rudder properly already so that the ball is centered? Or, in crosswind landings, can you keep the aircraft aligned with the runway intuitively while you play with the bank angle to induce a sideslip and maintain the centerline? Do you already know or feel the pitch attitude that's required to maintain speed with idle power, or to bleed off the correct amount of speed? If not, I can well imagine that your instructor is teaching you to land in stages, where you first reduce power to idle, have a second or so to consciously counter the yaw and pitch effects of this, and then start the flare.

And second, talking about the flare. This is not just a maneuver to bring up the nose so the nosewheel doesn't hit first. It's also a maneuver that's used to bleed off energy. After all, any movement of the controls, at any time, results in induced drag. But in the flare you are at the wrong end of the drag curve which multiplies this effect.

This is one of the two reasons we don't approach at stall speed. We need that extra energy for the flare. The steeper your approach is, the more energy you need. (The other reason is for protection against gusts.)

So again, your instructor will teach you to fly your final approach at a certain speed, and an approach angle that belongs to that speed. Eventually you should be able to fly to book (POH) numbers but initially your instructor may want you to fly a little faster than that. That gives you the ability to close the throttle a second or so before the flare, stabilize the aircraft again, maintain the pitch attitude so speed bleeds off, and still have enough energy left for the flare itself.

As you progress further, you'll fly the aircraft much more intuitively and you can combine all these actions. Heck, you could even extend the reasoning behind all this, and do something opposite if that's required. Pilot DAR has given you an example already: Leaving a bit of engine power on to prevent shock cooling. I'll give you another example. I was downwind 1000' in a fully loaded (4 adults) PA28 and needed to fly a tight circuit because of a 737 on final, which I had to get in front of (ATC orders). So I made a tight turn onto a tight final and approached the runway at a relatively high rate of descent. Instead of closing the throttle in the flare, I used a very small and short burst of power to cushion my ROD, and then made a very nice and controlled idle power touchdown. Without that burst of power I'm not sure if I would have gotten the energy to fully cushion the flare.
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Old 13th Feb 2011, 10:54   #9 (permalink)
 
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As the owner of a 172 I tend to pretty much glide the last 100ft or so just using trickles of power to correct for any turbulence etc. So pretty much a blend between glide and power.

The 172 needs to be put down at the correct speed or it will just bounce back into the air or wheelbarrow and shimmy down the runway. Carrying power to touch down will inevitably find you to fast and thats when things go wrong.

By using a blend of glide and power you control the approach speed exactly and thus the touch down speed.

Quote:
With an extra long runway, it is actually possible to touch down with considerable power. Pundits will perhaps jump on me, suggesting this is pointless, but on a really cold day, when you don't want to shock cool an engine, it's appropriate to carry some power all the way in.
I will respond to that. Maybe in the minus millions of Canada this could be a problem but the power requirements for a normal circuit have allowed the engine to cool down from cruise power and taking power off further for a glide approach is going to make little difference. Especially as the nature of a glide is no different from the taxi back to the ramp.

At the end of the day there are many ways to skin a cat. Look listen and learn and then use what is best for you. When you own your own aircraft you will use the technique that is not only safe and comfortable but also the kindest to your aircraft.
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Old 13th Feb 2011, 12:21   #10 (permalink)
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the power requirements for a normal circuit have allowed the engine to cool down from cruise power and taking power off further for a glide approach is going to make little difference. Especially as the nature of a glide is no different from the taxi back to the ramp.
On a warm summer day, agreed. Though, lets remind ourselves that the power to fly the "normal" circuit, could in many cases be about cruise power - which does not really afford the engine much opportunity to begin to gently cool down.

So, when it's minus millions out (well for my flying, I begin to think of things as getting cold at -25C), The nature of a glide is very much different to that of taxiing in as it affects engine cooling! The glide will be at let's say four to five times? the airspeed of taxiing? That's a lot of cooling in the glide, compared to the ground (depending upon how you taxi!). When the engine stops developing power, it stops developing most of it's heat. But if you're still flying, you're still cooling. Too much cooling all of a sudden is very bad for the cylinders.

On the 6000 foot runway, where I fly to work a few days a week, I amuse myself sometimes with partial power takeoffs, and high power landings, in the mighty C 150. With full power RPM being 2750, I have quite happly gotten airborne and "climbed out" with 2100 RPM, and landed with no less than 1900 'till I had all three wheels on.

Aside from skill development, there's no real need or benifit for these odd power settings. But I did find the skills useful when I was required to do partial power take offs in the Caravan for the purpose of takeoff distance data gathering, and a high power landing in it, so as to be extremely gentle with an unintended "object hanging loose beneath the aircraft" situation.
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Old 13th Feb 2011, 12:36   #11 (permalink)
 
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Though, lets remind ourselves that the power to fly the "normal" circuit, could in many cases be about cruise power - which does not really afford the engine much opportunity to begin to gently cool down.
Cruise power in my 172 is 23/2300 which gives 128kts. Circuit power is 15/2300 which allows me to get into the 85kt white arc for the second stage of flap. There is no way that I can fly a circuit at anywhere near cruise power and still get the speed back enough to get the flap in and land.

Maybe in some aircraft you need cruise power to fly an airliner circuit to a massive runway but to fly a normal circuit then you are a very long way from cruise power and thus a very long way from shock cooling when making power changes.

I have yet to fly any variant of 172 that required cruise power to fly a normal circuit.

I fail to understand how a glide after circuit power is going to shock cool. As explained the circuit power in a 172 is very low, the engine has already cooled during the circuit, taking the power off is not going to see much more of a change and certainly not enough to shock cool.
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Old 13th Feb 2011, 12:49   #12 (permalink)
 
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Your objective, in the end, is to touch down at the correct speed at +/- the correct point on the runway. This is valid for all aeroplanes: the C172 as much as any.

To achieve this, the power reduction becomes a factor of how close you are to your optimum path and speed, and what the trend of those two parameters are. It may sound obvious, but if you're slow and steep, you don't want to pull the power early, as you'll need the energy in the flare to arrest the descent before stalling. If you're fast and shallow, you need to bleed energy off, as you have plenty and little is required for the flare.

So it's all about judging how well your approach went by the time you approach/pass the threshold. A few extra knots? Pull the power early, anticipate the pitch down, and be prepared for a correction to this and then a flare on top of that.

Lagging behind the power curve a bit? Keep the power in in longer.
Anticipate the lessened need for pitch changes and don't over-flare as it could easily result in ballooning.

Steeper approaches are more difficult, and will bite you to a greater extent than a shallow approach, so be mindful of the PAPI or GS angle when going to different fields.

Being high or low to start with should really have been corrected earlier, although when you gain more practice, you can do "last-second" corrections to this too.

This judgment will take a while to develop, and mature with experience. However, it seems like most students and instructors like to fly the C172 on the conservative side of the book - ie with higher approach speeds than what the book says, and using these speeds there is normally sufficient energy to pull the power early (say, when the threshold disappears below the cowling).



So to sum up: be mindful of your planned approach speed and watch it as you get close to touchdown. Then decide on when and how fast to pull the power, and that in turn will dictate your flare technique. Nail it every time - and remember that a good landing is at the correct speed and at the correct position - sometimes it will be a greaser and sometimes... well, sometimes it's just another safe landing!
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Old 13th Feb 2011, 14:31   #13 (permalink)
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I think I see the problem....

Quote:
Circuit power is 15/2300 which allows me to get into the 85kt white arc for the second stage of flap. There is no way that I can fly a circuit at anywhere near cruise power and still get the speed back enough to get the flap in and land.

Maybe in some aircraft you need cruise power to fly an airliner circuit to a massive runway but to fly a normal circuit then you are a very long way from cruise power and thus a very long way from shock cooling when making power changes.
There I am in the mighty C 150, trying to be a courteous pilot in the circuit, fly the 85 knot speed of the rest of the circuit aircraft, and not slow up the works.... 'cause I know there's a pilot behind me flying the circuit at his circuit speed of 85kts with a "second stage" of flap (I'm not quite sure what a "stage" is relative to flap in a 172, the ones I fly are specified in degrees - I'll have to fly a UK one one day, and see the differences).

If I am to countinue my courtesy, and not slow down to my normal home circuit speed of 60 - 50 knots, I'll be flying with a power setting for my normal cruise speed of.... you guessed it.... 85 kts!

When I see how rapidly, even in the summer, my cylinder head temperature will drop off, when I throttle the engine, it reminds me to always treat it with easy power changes, and minimize cold weather gliding. I extend this attempt at courtesy, to the owners of the other aircraft I fly. I have never been accused of cracking a cylinder, and I'm going to try to keep it that way. If others choose to fly differently, that's between them and the aircraft owner - that's why I don't lend my plane to many people!
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Old 13th Feb 2011, 15:08   #14 (permalink)
 
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The question was about a 172, not the mighty 150.......

You keepmon flying in whatever way makes you happy in the 150.
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Old 13th Feb 2011, 18:50   #15 (permalink)
 
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If it's a 172 with 40 degrees of flap on it, and you're landing at a short strip, you'll need some power on to stop it falling out of the sky. You need a fair bit of power to keep 60 kts with the barn doors out on a normal approach.

Our '74 172 lands much better with a little bit of power on down to the threshold - makes it easier to nail the speed, and as has already been said, you want to nail the speed or it'll float a fair way, and if you put it down fast, you'll get a wicked nose gear shimmy.
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Old 13th Feb 2011, 18:57   #16 (permalink)
 
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That's what I love about the 172, it's massive approach envelope means you can try different ways to land and not bend it. My advice to the OP is, at this point in your training it is best that you do exactly what your instructor sitting next to you tells you to do, even if they contradict another instructor. Once you have more hours and are more confident in the aircraft you fly and your own ability you can experiment a little.
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Old 13th Feb 2011, 19:46   #17 (permalink)
 
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Thanks a lot for all the help. You guys are amazing with the responses. I haven't completely read them all, but I guess both ways are good and most people use a combination
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Old 13th Feb 2011, 19:56   #18 (permalink)
 
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ifresh21, what you have to realise is that your instructors are probably teaching you the safest, most straightforward way of landing at your particular airport. You are in your training phase and what you are taught is to give you a good general grounding and to get you through your flight test. Once you have your PPL, you get more confident and you start flying to loads of other airports you will find that you develop and use a number of different techniques to get back on the ground. There is no one right answer.
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Old 14th Feb 2011, 15:55   #19 (permalink)
 
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I teach on 172SPs and use both techniques. I start with the 'powered' approach because it helps to demonstrate that throttle isn't a 'speed controller'; it is an aircraft control and therefore has a number of interlinked effects.

Trying to steer clear of numbers, most approaches are flown with 1500-1700rpm although the precise figure doesn't actually matter. I sometimes demonstrate landing with this power setting if a student pilot has difficulty understanding what is happing in the flare (speed reducing, AOA increasing) as the technique gives more time in the flare to observe what the aircraft is doing (not good practice on a short runway )

I tend to stick with powered approaches until the student pilot is achieving a high success rate and then introduce the glide (from downwind). Once these two approaches are sorted, we then discuss how each type of approach has advantages/disadvantages. Both need to be part of a pilot's repertoire although the glide is the more difficult art.

One final point. "Chop(ing)" the throttle is not necessarily a good word to use. Throttle is a variable control and deserves to be treat with some care. I prefer to describe the technique as reducing power or retarding the throttle in a measured fashion. As already stated, there are a number of aerodynamic effects when you change power and good pilots want to carefully balance these effects.

Ultimately, you will become more comfortable with a particular technique (just as most pilots have their favoured x-wind technique). As long as you understand the pitfalls of each and the times where you may need to use an alternative, you will be fine.
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Old 14th Feb 2011, 18:30   #20 (permalink)
 
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I teach on 172SPs and use both techniques. I start with the 'powered' approach because it helps to demonstrate that throttle isn't a 'speed controller'; it is an aircraft control and therefore has a number of interlinked effects.
CowsgettingBigger.

Here I am going to disagree with you

Both throttle and column are energy controllers.

One taps into energy from the engine the other taps into potential energy in the airframe.

A glider has no engine as such so its throttle (if you like) is the stick and in still air is the only link to tapping into the potential energy of the airframe.

Add the engine and you now have another source of energy available.
Both are sources of energy so ignore one at your peril

We are back to the old arguement of pitch for speed or power for speed neither which is totally correct as it should be pitch for energy power for energy.

pace

Last edited by Pace; 14th Feb 2011 at 18:49.
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