Full Rich for landing?

[MRC01]

... Leaning during taxi at high elevation is required unless you want to foul plugs ...

This (leaning during ground ops) is recommended even at sea level. During taxi the power output is so low you can lean almost all the way to the cutoff. You'll keep your spark plugs & valves cleaner, avoid fouled plugs, and waste less fuel.

 

[Pinecone]

I don’t worry about mixture for going around. The engine makes plenty of power. Having carb heat on may kill you. Mixture won’t. I never touch down with carb heat on.

Hmm, ever heard of detonation? And blowing a hole in a piston so you now only have 3 cylinders?

 

[Stewartb]

Hmm, ever heard of detonation? And blowing a hole in a piston so you now only have 3 cylinders?

Yes. It isn't like I've done it on purpose, but I can admit to taking off and doing go-arounds with the mixture leaned several times. Both with carb'd and fuel injected engines. I didn't know until I adjusted power after the initial climb. No damage. I was taught that the takeoff fuel flow is set high to combat heat. I wouldn't have had enough time to cook anything. I'm not talking about 15 minute climbs.

 

[Dan Thomas]

I always wondered why the very first item on a carbureted airplane manufacturer's emergency engine out checklist wasn't "Carb heat --- HOT."

172N POH:



Second item, probably should be first, or concurrent with pulling up a little to turn the excess airspeed into a bit of extra altitude.

 

[MooneyDriver78]

Yes. It isn't like I've done it on purpose, but I can admit to taking off and doing go-arounds with the mixture leaned several times. Both with carb'd and fuel injected engines. I didn't know until I adjusted power after the initial climb. No damage. I was taught that the takeoff fuel flow is set high to combat heat. I wouldn't have had enough time to cook anything. I'm not talking about 15 minute climbs.

I don’t think you can have detonation if mixture is lean of peak, and engine runs cool as well…the problem is going from rich to lean. Of course if leaned you won’t be making maximum horsepower.

 

[Bill]

In Cessnas I was taught that whenever the rpms are below the green arc on the tach, carb heat is to be on.

 

[MRC01]

In Cessnas I was taught that whenever the rpms are below the green arc on the tach, carb heat is to be on.

Yep, that's in the POH, and I still follow that advice. They are prone to carb ice.

 

[Stewartb]

I don’t think you can have detonation if mixture is lean of peak, and engine runs cool as well…the problem is going from rich to lean. Of course if leaned you won’t be making maximum horsepower.

Lean mixtures fire faster. Pre-ignition (backfiring through the carb) at startup is an indication of an excessively lean mixture, usually from an induction leak.

Lean mixtures make more power assuming they aren’t lean of peak. If you do a static run up on a summer day you can usually demonstrate it for yourself. If an airplane fuel system is set appropriately to fly in winter, most summer ops benefit from leaning. Where I fly we want 200° of leaning authority. That is, we can lean 200° before engine stumble or peak EGT from warm engine full rich cruise. Engine manufacturers tell us max power happens near peak EGT. At full throttle there’s more fuel delivery than just the main jet, and the main jet is the only one metered by the mixture knob.

 

[Dan Thomas]

Lean mixtures fire faster. Pre-ignition (backfiring through the carb) at startup is an indication of an excessively lean mixture, usually from an induction leak.

Nope. Lean mixtures fire slower. How do you get the fire back into the intake when the intake valve is closed if the lean mix fires faster? It can burn slowly enough that there is still flame in the cylinder near the top of the exhaust stroke, when the intake is opening. That's when you get backfire (as opposed to afterfiring out the exhaust, usually, and wrongly, called backfiring). Over-rich mixtures will also burn slowly, such as too much prime on startup.

Severe carb ice has sometimes been dislodged by leaning until backfire occurs. I was taught that long ago. 50 years ago last month, when I started flight training.

Lean mixtures can cause detonation by their slowness. It takes time for the complex, detonation-resistant fuel molecules to break down into autoignitable molecules, and lean mixtures are a contributing factor.

 

[Stewartb]

In the context of airplane engines, particularly carbureted engines, most run too rich in summer temps, and many run too lean in winter temps. As I said, best power mixtures are easy to figure out. In my history of taking off with the mixture leaned? Never any hiccups. In winter? I rarely lean. Winter ops need all the fuel I can get and even then, I manage throttle carefully.

 

[Dan Thomas]

In the context of airplane engines, particularly carbureted engines, most run too rich in summer temps, and many run too lean in winter temps. As I said, best power mixtures are easy to figure out. In my history of taking off with the mixture leaned? Never any hiccups. In winter? I rarely lean. Winter ops need all the fuel I can get and even then, I manage throttle carefully.

Yes. A common mistake made by mechanics is setting the idle mixture to the conditions that are in play at the time he's doing this. If that airplane is based at that same airport, and the weather is colder, it should be OK. If it is based at a lower airport, and it is summertime when he's setting the mix, it will be way too lean once winter comes and won't idle properly.

Density altitude matters. At a 1000' ASL airport on a -20°C day, the DA is -2900 feet. You need a lot of fuel to mix with air as dense as that.

 

[Pinecone]

Yes. It isn't like I've done it on purpose, but I can admit to taking off and doing go-arounds with the mixture leaned several times. Both with carb'd and fuel injected engines. I didn't know until I adjusted power after the initial climb. No damage. I was taught that the takeoff fuel flow is set high to combat heat. I wouldn't have had enough time to cook anything. I'm not talking about 15 minute climbs.

If you are leaned, you are limiting the fuel flow.

A LOT depends on what engine. Many engines were designed to run on 80/87 or 91/96 fuel. If so, and you are running 100LL you have a lot more detonation margin. Try that with a high compression or turbo engine, and you may end up with detonation.

 

[Pinecone]

Lean mixtures fire faster. Pre-ignition (backfiring through the carb) at startup is an indication of an excessively lean mixture, usually from an induction leak.

Pre-ignition is NOT backfiring through the carb. Pre-ignition is ignition prior to the spark plug firing, but WELL after the intake valve closes.

 

[Magman]

So I have a choice?

Slight possibility of fouling a plug immediately before landing

or

Possibility of destroying all 4 or 6 or 14 cylinders in seconds


Let me think about that?

 

[Stewartb]

If you are leaned, you are limiting the fuel flow.

A LOT depends on what engine. Many engines were designed to run on 80/87 or 91/96 fuel. If so, and you are running 100LL you have a lot more detonation margin. Try that with a high compression or turbo engine, and you may end up with detonation.

Like my 10-1 IO-390? I’ve taken off leaned in that plane a couple of times. Ran fine. Again, I didn’t know until I leveled off.

 

[JAWS]

Good discussion.

One thing not mentioned so far is fuel enrichment at full power. Carbs and fuel injection have extra fuel added at full throttle to aid in cooling and prevent detonation.

 

[MRC01]

So I have a choice?
Slight possibility of fouling a plug immediately before landing
or
Possibility of destroying all 4 or 6 or 14 cylinders in seconds
Let me think about that?

That mis-states the risks. The actual choice is:
1. Full Rich: Risk loss of power if you go around, due to fouled plugs.
2. Leaned: Risk loss of power if you go around, due to lean mixture.
Choose your poison.

It's worth mention that (1) is always a risk even if you do it right, and fouled plugs are not easily cleared. But (2) is only a risk if you do it wrong (forget full rich on go-around), and it can be immediately corrected by pushing the mixture forward.

Whatever your choice, put it in your checklist and train it repetitively and it won't be a problem no matter which choice you make.

 

[Dan Thomas]

That mis-states the risks. The actual choice is:
1. Full Rich: Risk loss of power if you go around, due to fouled plugs.
2. Leaned: Risk loss of power if you go around, due to lean mixture.
Choose your poison.

If the plugs are fouling up just because you're at full rich on final, you have some major engine problems.

The worst engine for fouling plugs that we had in the flight school fleet was the O-235 in the Citabria 7ECA. And that's because those engines run fairly cold, and even with hot plugs they'll foul up. Pattern work would foul them in less than 50 hours, cross-country would take longer. None of the other engines fouled plugs like that. I used the REM37BY plug in all the engines for which they were certified, and those plugs are pretty much foul-proof. They sure fixed the O-235.

Long glides at idle power and full rich will more often suck lots of oil past the rings and foul plugs faster than any fuel-carbon fouling. Worn engines are worse, and the small Continentals are bad this way.

When that aircooled engine gets cold, its piston/ring/cylinder clearances open up and more oil is sucked past the rings on every intake stroke due to low manifold pressure. Oil also get pulled past the intake valve stems and into the cylinders. Leaning the mixture does absolutely nothing for that. So don't do long power-off glides without occasionally "clearing" that engine by running it up to 1800 or whatever for ten seconds or so to burn accumulated oil and fuel out of it, and that includes fuel that condenses on the induction system walls and puddles in there when the airflow is low, and can cause some serious hesitation when you open the throttle and suck all that fuel in at once. The O-320-A2B we had in the Citabria 7GCBC was famous for that. It made a practice forced landing turn into the real thing once. No damage. Clearing the engine also warms the carb heat and cleans out any accumulating ice. But it takes time; two seconds won't help.

 

[MRC01]

... When that aircooled engine gets cold, its piston/ring/cylinder clearances open up and more oil is sucked past the rings on every intake stroke due to low manifold pressure. Oil also get pulled past the intake valve stems and into the cylinders. ...

If you're sucking that much oil past the rings, then you have some major engine problems.

 

[Fearless Tower]

If the plugs are fouling up just because you're at full rich on final, you have some major engine problems.

Thank you.

Dan’s right - if you are fouling plugs in the short time between pushing the mixture in on the downwind and exiting the runway, then you have bigger problems to worry about.

 

[Pinecone]

Agreed. And most times, you are not flying your pattern at idle, you have some power on. So it is not like taxiing.

 

[Dan Thomas]

If you're sucking that much oil past the rings, then you have some major engine problems.

The small continentals are famous for it, usually when they're not at operating temps. Their bottom sparkplugs are very close to the bottom of the bore in the cylinder head, and oil that gets past the rings accumulates right there and flows into the plug well and shorts it, and it cokes in there. Fouled bottom plugs in O-200s, for instance, are way too common. It's one reason we got rid of the 150s in the flight school; too many instances of having to pull the bottom plugs at unscheduled times to clean them off. Very often they'd foul up just when warming up the engine before taking off. In a long glide they'll also try to do it.

By their design, these things are not car engines. They can't be operated like car engines. The pilot needs to know that they are different, and has to operate them properly. Aircooled engines have their quirks. Liquid-cooling is nice, but it adds weight and expense, and in the 1930s and late '40s when private aviation was becoming popular, the direct-drive aircooled engine was chosen because it was light, simple, and cheaper to build, and now we're pretty much stuck with it.

 

[MRC01]

The small continentals are famous for it, usually when they're not at operating temps. ...

It's interesting how different engines, sometimes the same engine as installed in different airframes, have different idiosyncrasies. The O-320 engines in the 172s are more prone to icing than most other airplanes, yet the problem seems to disappear when the same engine is installed in a Piper.

... By their design, these things are not car engines. They can't be operated like car engines. The pilot needs to know that they are different, and has to operate them properly. ...

Of course, but it's not just cars to airplanes. Each airplane has different quirks and operating requirements, and those requirements can change when the same engine is put in a different airplane.

 

[Dan Thomas]

It's interesting how different engines, sometimes the same engine as installed in different airframes, have different idiosyncrasies. The O-320 engines in the 172s are more prone to icing than most other airplanes, yet the problem seems to disappear when the same engine is installed in a Piper.

Sometime, take a look at a 172 with its cowl off, then go look at a Cherokee with its cowl off. Look at the exhaust system on both of them, and see that the 172's is mostly up front, with the muffler shielded by the cabin heat shroud. Then see the Cherokee's (Archer, Warrior as well) and see the exhaust system surrounding that carb, front, sides and rear. That carb gets a lot of warm air flowing around it, and radiated heat right at it.

Anyone studying this stuff figures it out. Pilots who see little more than the cowling and propeller don't see it or understand the reasons for the differences.

The Citabria with the O-320 ices up easily. Its muffler is also up front, and shielded, but it's a much smaller muffler.

Typical 172 exhaust system. The carb is behind that muffler (#1).



Typical PA-28 exhaust system. The carb is in the middle of all that. Those two unshielded pipes in front radiate a lot of heat.

 

[MRC01]

Sometime, take a look at a 172 with its cowl off, then go look at a Cherokee with its cowl off. Look at the exhaust system on both of them, and see that the 172's is mostly up front, with the muffler shielded by the cabin heat shroud. Then see the Cherokee's (Archer, Warrior as well) and see the exhaust system surrounding that carb, front, sides and rear. That carb gets a lot of warm air flowing around it, and radiated heat right at it. ...

Yep. Done that, seen that, which confirmed my in-flight observations.

 

[Stewartb]

So the idea is that a warm pipe warms the carb and prevents ice? I seriously doubt that. Not with as much air flowing as most cowls have. Cubs are like most other types. Some make ice while other identical planes don’t. My Skywagon doesn’t make ice but I know very similar planes that do. I’ve searched for answers for a long time. I haven’t found any.

 

[MRC01]

So the idea is that a warm pipe warms the carb and prevents ice? I seriously doubt that. Not with as much air flowing as most cowls have. Cubs are like most other types. Some make ice while other identical planes don’t. My Skywagon doesn’t make ice but I know very similar planes that do. I’ve searched for answers for a long time. I haven’t found any.

The observation that Pipers are less prone to carb ice than Cessna 172s having the same engine, seems well established. I'm not the only one who has noticed it.
As for why, that's a different question. The exhaust configuration is the most plausible explanation I have heard.

 

[Dan Thomas]

So the idea is that a warm pipe warms the carb and prevents ice? I seriously doubt that. Not with as much air flowing as most cowls have.

Heat is transferred in three ways: Convection, conduction and radiation. Convection is typically by air heated and then transferring its heat to a third party. Conduction is typically through a solid medium, like aluminum or steel. Radiation is by infrared light, through space, and that is what a hot exhaust pipe does: it radiates terrific heat. Putting more exhaust piping all around the carb is going to irradiate the thing from all sides. The air flowing around that exhaust and carb will not stop that, though it will cool both of them somewhat, as long as that air is cooler. Radiated heat will pass right through any air, whether it's moving or not.

When you sit at a campfire, you're not getting convected heat unless the smoke is blowing right at you. You're not getting conducted heat, either. You're getting radiated heat, and you feel it.

 

[Gary Ward]

Closing the throttle restricts the airflow, alright, and it does that by making a much smaller area for the air to pass the throttle plate into the induction system. The air, in squeezing past that plate, accelerates a whole bunch, and if we listen to Bernoulli, as the speed of a fluid increases, its static pressure decreases, and then the Gas Laws show us that as pressure decreases, so does temperature.

So the biggest temperature drop in a carburetor is not in the venturi. It's in the throttle plate/carb bore wall area, and that's why we get the carb icing charts looking like this:

View attachment 117874

Now, see and understand: the biggest risk of carb ice occurs at glide power, or throttle closed or nearly so. You can get ice at ambient temperatures up to 100°F that way, and serious ice up to 90°F, and in a very wide range of dewpoints. The cruise power ice risk is far smaller.

So closing the carb heat on final, especially when the temp and dewpoint aren't far apart, introduces a risk that you need to understand.

Now, that chart is pretty general. Different engines and airframe installations have different risk factors, with carbed Continentals being, generally, more ice-prone than Lycomings. But Lycs will still ice up, contrary to popular belief. I've had it several times in Lycs, and seen it on the ramp many other times. Students and instructors very often did not recognize it.

Another popular belief is that carb ice is a wintertime thing. False, absolutely false. That carburetor is an efficient little refrigerator, and makes ice the same way a freezer does: using a pressure drop.

View attachment 117876

Winter time around here I am full rich landing in a 172, hot summer not so much. Usually stay 1/4" lean all the time until I climb higher then leaner during the summer. Not that big of deal on 172, IMO.
OT
I have experienced what had to been carb ice in my 172 in all phases of flights and on the ground.
Early on I was not sure what it was, called my mechanic the first time.
-Now I have 1000 hrs in it and I am more sure the 6-7 events that I experienced was carb ice. First I would shove in the mixture thinking it was lean? Didn't think about carb heat until after a few events. They would only last 3-5 seconds and it would clean up.
Learned to fly in 172S models with fuel injection so no carb heat training.
Got my own carb 172 and put a carb heat placard on the panel to remind me.

Then I started flying with my buddy in his cheerokee and he never used carb heat. It was a bad influence on me at first.
Now I use carb heat year round any time out of the green.
Especially in descents through clouds even if I am still in the green rpm band.

My mechanic thinks my carb icing is amplified by the powerflow exhaust that is on my 0-320 lycoming. Do you think that comes into play?

 

[Tarheelpilot]

Only if appropriate.

We try so hard to make hard and fast rules to follow the K.I.S.S. principle but sometimes it actually makes it more complicated.

I like to put the mixture at the approximate setting for go around on the approach and then fine tune on the go if necessary.

 

[Stewartb]

My old Cub didn’t make ice. I sold it to a friend after he crashed his similar Cub due to carb ice. In the Cub world it’s about 50/50 for making ice. Same or close in the Skywagon world. Most guys think Lycomings don’t ice because the induction goes through the sump. That’s downstream of the carb and doesn’t add up, either. All the Lycoming induction does is warm the air and make the mixture rich, and it robs power, which is why some of us use cold air induction. If there was hard evidence that one condition or model makes ice more than others, it would have been addressed and corrected long ago. The root cause probably has more to do with the operator than the machine.

 

[MRC01]

I think it's both: some engine/airframe setups are more prone to icing, and some operation methods are more prone to icing.

Rich mixtures are more prone to icing, since you have more liquid being vaporized. Lean mixtures are less prone to icing. All else equal. This suggests there should be a correlation between pilots who use full rich mixtures when they aren't needed (below 70% power), and those who encounter carb icing.

 

[Dan Thomas]

I have experienced what had to been carb ice in my 172 in all phases of flights and on the ground.

We can learn to expect carb ice. It's not difficult. Look at the METAR for Vancouver, BC, right now:



Air temp 20°C, dewpoint 14. Six degrees apart. A typical carb ice chart:


20 degrees and dewpoint 16 puts us right in the green band, where we can expect moderate icing in cruise or serious ice in descent. That will depend on the engine and installation quirks, as well, and some POHs will have more applicable charts.

The METAR for Regina, SK, right now:



Temp 28, dewpoint 10. On that chart, it's on the line between the orange and yellow bands, a lot less ice risk. If we were in Phoenix, AZ, the likelihood of carb ice would be very small.

My mechanic thinks my carb icing is amplified by the powerflow exhaust that is on my 0-320 lycoming. Do you think that comes into play?

I don't know. I haven't seen a Powerflow installation.

Most guys think Lycomings don’t ice because the induction goes through the sump. That’s downstream of the carb and doesn’t add up, either. All the Lycoming induction does is warm the air and make the mixture rich, and it robs power, which is why some of us use cold air induction.

The Lyc is more ice-resistance because of conduction. That oil sump gets hot, and heat is conducted through the aluminum sump and into the carb body. The only insulation there is a relatively thin gasket. The Continental O-300 was built the same. The rest of the Continentals didn't have that.


The sump-heated induction tubing is relatively short. The cooling airflow off the heads and radiated heat off adjacent exhaust piping will heat the external induction sections at least as much. And Continentals suffer from that, too, unless it's a top-induction engine like a 550:

 

[Magman]

No one has mentioned Reid Vapor Pressure (RVP) yet. It is the tendency for fuel

to evaporate and is a major player with both Carb Ice and Vapor Lock.

Generally av- gas has a lower RVP than mo -gas. Mo - gas had the RVP blended

for high RVP in cold weather to promote starting and lower RVP in summer for

reduced Vapor lock tendencies. This also varies by local climate. Av-gas is

constant all year.


My bud “ Crude Dude”; has done some demos with a Bell Jar , vacuum pump

and samples of both fuel types. All quiet until things really boil over.

Impressive.

I don’t recall altitude but I’m sure it was above 10 K.


You can observe the phenomenon on engines such as O-200 and O-470’s with

the cowling off. A brief run will show you the manifold near the carb forming

frost. Even on an 85 degree day. Warm air holds more moisture than cold

so when it goes in the fridge you have ice. Even on a warm day.

 

[Piper Dreaming]

Rarely do I land full rich.
Adjust accordingly for altitude.

 

[Jeff767]

Go full throttle with carb heat and the engine blubbers and doesn’t spin up with authority. Mine doesn’t miss a beat with it leaned at go-around. I dragged my gear through trees once when the plane wouldn’t climb as expected when going around. Carb heat was on. Since then I push carb heat in on final and may or may not adjust mixture. It depends on the day.

Most engines make decent power regardless of how lean they are. The problem is you might put the engine into a regime where detonation can occur. It’s a factor of air temp, fuel quality, CHT and most important mixture. Detonation can destroy an engine in 30 seconds. Best practice is to never run the engine at full power leaned at lower altitudes. Below 65% you can’t make a lycoming detonate so you can run the mixture anywhere you want.

 

[MRC01]

... Below 65% you can’t make a lycoming detonate so you can run the mixture anywhere you want.

True, though the manuals for the O-320 and O-360 say it's a bit higher, 70%.

 

[Jeff767]

Like my 10-1 IO-390? I’ve taken off leaned in that plane a couple of times. Ran fine. Again, I didn’t know until I leveled off.

It’s also quite possible you were getting some light detonation. This will certainly affect the lifespan of the engine but would not necessarily be noticeable in the cockpit. I ran a IO360 with 10 to 1 and currently run a 540 with 9.5 to 1. Be very careful! Detonation is a combination of many factors. When the factors line up and you leave it lean it can be a extremely expensive mistake.

 

[Wagondriver]

I have had carb ice on downwind once, and numerous times shortly after takeoff. I also have a JPI 830 with carb temp. It is interesting to watch the temperature, with the throttle closed all the way, carb temp goes way up, like up to 60 or 70 degrees on days with ambient in the 20's or so. With carb heat on as suggested, with closed throttle and a warm engine, carb temp will get over 100, I have seen 140. Full throttle on a zero degree day with just about any humidity will create ice after takeoff. Full throttle dumps so much fuel, enough it seems to really help cool off and create ice. This last winter was the first winter after having my carb flow checked and a larger jet installed, it nearly shut off on me twice just after takeoff. I use carb heat on takeoff in those situations now, and also have tried using only about 3/4 throttle.
I pretty much never run carb heat, UNLESS, there is high humidity or visible moisture AND carb temp is less than 38. I may pull some carb heat now and then on decent, just to see if there is much exhaust heat to work with.

 

[MRC01]

... Full throttle on a zero degree day with just about any humidity will create ice after takeoff. Full throttle dumps so much fuel, enough it seems to really help cool off and create ice. ...

That's unusual. Do you have a MP gauge? It sounds like the carb throat isn't big enough for the engine, so at full throttle the restriction is creating too much vacuum. An MP gauge would show how much pressure drop you have at full throttle.