Why is mixture lean recommended on the ground?

Question

Mixture lean is typically recommended during the ground operations at sea-level pressure in the POHs of most small aircraft equipped with a carburetor system. Why is it?

I heard some people saying it's because the engine requires less power for the ground operations, but that doesn't persuade me. If it's the amount of power output at stake, we just need to pull back the throttle and reduce power, which we exactly do for taxiing and holding short of the runway. Mixture is about controlling the ratio of fuel and air entering the cylinders, and we change it according to the variations in air density, not the power output required. If that's the case, then why do the POHs recommend a mixture lean for the ground operations at sea level?

Edit) I guess my question is put too poorly for some people to understand easily. Let me try again. Plug foulings happen when mixture is set too rich for the given density altitude. For this reason, many of the GA aircraft's POHs recommend pilots to start leaning mixture from 3000 MSL up. That means as long as the aircraft stay below 3000 MSL, the pilots have no reason to lean mixture, at least for the purpose of avoiding plug foulings. Yet, the same POHs recommend pilots to lean mixture for ground operations even when the elevation where ground operations are performed is below 3000 MSL. So my question is: "Why do the POHs recommend pilots to lean mixture for ground operations even when the density altitude of the ground does not require it?".

Answer 1

The new insurer for the Cessna 210 I fly is only charging \$300 to add me to the policy—instead of $1,300 the old one wanted—so I’ve been reading the 210 manual for the first time in several years. I noticed that the Owner’s Manual doesn’t mention anything about leaning on the ground and gives advice about leaning in the air that has been superseded by newer best practices. I looked in the manuals for the Cherokee and Cessna 182 and they don’t mention anything about leaning on the ground either.

One of the first things I learned when flying the Cessna 182 is to lean aggressively on the ground. There were several flights when I first started flying it that were not able to be started because, on runup, we couldn’t get the mags to clear. After a couple of expensive trips to the A&P, I started leaning aggressively on the ground. At the time, I was too busy with the firehose of information to even think about why I needed to lean, just that it saved a lot of money if I did. This post is an explanation of why you need to lean on the ground—especially older small-bore engines.

Fuel is different from the 1960s and 1970s

The POH for the Cherokee and 182 specify 80/87 fuel. The 210 specifies 100/130 and that 100/130 low lead is also approved. So what do these numbers mean?

Shell explains it really well, (bolding is mine)…

Avgas grades are defined primarily by their octane rating. Two ratings are applied to aviation gasolines (the lean mixture rating and the rich mixture rating) which results in a multiple numbering system e.g. Avgas 100/130 (in this case the lean mixture performance rating is 100 and the rich mixture rating is 130).

In the past, there were many different grades of aviation gasoline in general use e.g. 80/87, 91/96, 100/130,108/135 and 115/145. However, with decreasing demand these have been rationalised down to one principle grade, Avgas 100/130. (To avoid confusion and to minimise errors in handling aviation gasoline, it is common practice to designate the grade by just the lean mixture performance, i.e. Avgas 100/130 becomes Avgas 100).

Some years ago, an additional grade was introduced to allow one fuel to be used in engines originally designed for grades with lower lead contents: this grade is called Avgas 100LL, the LL standing for ‘low lead’.

All equipment and facilities handling avgas are colour coded and display prominently the API markings denoting the actual grade carried. Currently the two major grades in use internationally are Avgas 100LL and Avgas 100. To ease identification the fuels are dyed i.e. Avgas 100LL is coloured blue, while Avgas 100 is coloured green.

EAA explains the change as well…

Avgas 80/87 is used in low compression ratio aircraft engines, and contains little or no lead (up to .5 grams of lead per gallon is allowed, but none is required). It is red in color, and should not be used in any automotive engine due to a low motor octane number of about 80.

Avgas 100/130 can be used in some automotive engines. It has both research and motor octane numbers slightly over 100. Avgas 100/130 is green in color, contains four grams of lead per gallon, and is becoming hard to find.

Avgas 100 LL (the LL stands for “low-lead”) contains two grams per gallon, half the lead contained in the avgas 100/130 it was designed to replace. It has research and motor octane numbers very similar to 100/130 avgas. The color is blue. This product sometimes has a high level of aromatics.

Avgas 115/145 was developed for high performance piston aircraft engines used in World War II and the Korean conflict. It is very hard to find today due to lack of demand, and is usually only produced on special order. The color is purple.

Purvis Brothers has similar information in metric units:

Tetraethyl lead content:

• 80/87 Max of .14g/l
• 100/130 has 1.12 g/l
• 100LL has .56 g/l

So why do plugs foul?

The engines on the Cherokee and the 182 were designed for fuel that has little or no lead. 100LL has at least four times as much lead as 80/87. At annual there is a substantial amount of lead that needs to be removed from them. On the other hand, the 210 was designed for fuel that had twice as much lead. I don’t recall ever fouling the plugs and they don’t usually have much in the way of lead deposits at annual. I spoke to two other 210 drivers and they don’t recall fouling the plugs either—though they do lean aggressively on the ground.

As Steve Ells explains “The additive in Avgas that helps scavenge the lead in the fuel doesn’t do much until the combustion temperatures get up to around 1,400 degrees F.”

At idle, your engine isn’t generating much heat. If you remember from studying for your PP test, fuel is a coolant. So the more fuel you give an idling engine the cooler it runs. Leaning the engine makes it run hotter—which is what you want on the ground.

Leaning

None of the owner’s manuals I’ve looked at mention leaning on the ground. Probably because, when the planes were manufactured, fouling plugs wasn’t an issue. Fuel prices were low, so saving a half-gallon or so wasn’t that big of a deal. The truth is that a full-rich mixture combined with the low power setting is the perfect scenario for lead fouling your spark plugs and valve stems.

So why don’t instructors teach new students to lean aggressively? Maybe so low time pilots won’t forget to go full-rich before take-off. But if you lean appropriately, this will never be a problem. Lean the plane at taxi RPM (usually 1000 RPM) so that it starts to stumble. Then enrich just a bit. You’ll be able to taxi and do your runup at this mixture setting without worrying about taking off with less than full mixture. As soon as you advance the throttle a couple hundred RPM the engine will stumble. If you tried to advance to takeoff power, the engine would die from lack of fuel, before you even started your take-off run. To avoid the embarrassment of stalling when I leave the runup area, I put my hand on the mixture knob when I make my call to the tower or CTAF. And I don’t take it off until I’ve pushed the mixture to full rich for takeoff. (Or less that that for high-density altitude.)

John D. Collins explains his procedure for high density altitude:

At sea level up to a density altitude of approximately 3000 feet, I would push the mixture full rich for takeoff. At higher density altitudes, I would do a full power run up and set the mixture to achieve the highest RPM as this will give you the most power available. As you climb above 3000 MSL, I would lean the engine for the highest RPM.

How octane numbers are calculated.

From Shell:

Engine knock, which describes explosive detonation of the fuel/air mixture or preignition, can cause severe engine damage and subsequent failure in a short period of time. Anti-knock ratings are expressed as Octane Numbers for those of 100 or less and as Performance Numbers for those ratings above 100. These numbers relate the fuels performance compared to a reference fuel of pure isooctane. Because the anti-knock characteristics are influenced by the air/fuel mixture ratio, ratings are developed for both rich mixture performance and lean mixture performance. Rich mixture settings yield higher octane or performance numbers since the added fuel acts as an internal coolant and suppresses knock. Prior to 1975, both numbers were reported as the grade designation but current specification utilize only the lean mixture rating. Currently, ASTM (American Society for Testing and Materials) specifies five grades – 80, 82UL, 91, 100, and 100LL (low lead). In practice only 100LL is widely available. Production of Grade 80 has generally ceased due to small demand.

I don’t know when 80/87 started to become less available, but Service Letter L185B from Lycoming dated January 19, 1988 states that “Whenever 80/87 is not available you should use the lowest lead 100 grade fuel available.” It goes on to say that, “higher lead fuels can result in increased engine deposits both in the combustion chamber and in the engine oil. It may require increased spark pull maintenance and more frequent oil changes.…Operation at full rich mixture requires more frequent maintenance periods; therefore it is important to use proper approved mixture leaning procedures.”

The Service Letter doesn’t mention leaning on the ground, but it does mention that:

At engine speeds of 1000-1200 RPM on the ground, the spark plug temperatures are hot enough to activate the lead scavenging agents in the fuel which retards the formation of salt deposits on the spark plugs and exhaust valve stems.

Since the leaner the mixture, the hotter the cylinder temperatures, leaning aggressively would help with reducing lead deposits.

The letter provides a method for leaning in the air to reduce deposits.

Answer 2

This is done to prevent extended running with an overly rich mixture, which can result in fouled spark plugs, lead deposits, and rough engine running. It also consumes less fuel. Before take-off the mixture is moved to full rich to ensure no fuel starvation, and avoid power loss, but on the ground where even if it is accidentally leaned too far, the engine stopping would not be a major safety concern.

Answer 3

Your question appears to be Why should I lean my engine's mixture on the ground (when idling/taxiing)?

To get to the answer let's start with a few assumptions:
Assumption 1: The engine will be operating at 50% power or less.
Assumption 2: We're at or near sea level (Density altitude of 3000 feet or less).
Assumption 3: Full rich is VERY rich - Lots of extra fuel to cool the cylinders during takeoff.

All of these assumptions hold true for your typical GA piston aircraft during typical ground ops (idling/warm-up & taxiing).

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Why should you lean an engine for ground operations?

Running an engine at low power with a very rich mixture creates three problems:

1. Because you are dumping more fuel into the cylinders than can be burned you're getting incomplete combustion: This produces lots of carbon soot that will work its way past the piston rings and make your oil dirty.
2. Because you are dumping more fuel into the cylinders than can be burned you're blowing some quantity of unburned fuel out the exhaust. That's avgas you paid for that's effectively going to waste.
3. Because the engine is operating at a low power setting the internal combustion chamber temperatures are relatively low (and reduced further by the excess fuel), which prevents the lead-scavenging agents in your avgas from doing their job. That means lead ash will deposit on your spark plugs, and it will also work its way into your oil just like the carbon soot does.

None of these are particularly good things for your engine, and the solution to all 3 problems is the same: Lean the mixture so you're putting less fuel through the cylinders:

• The fuel you're putting in the cylinders will burn more completely, leaving less soot.
• You're burning all (or at least most) of the fuel, so less is being wasted out the exhaust.
• There is less excess fuel to cool the combustion chamber, so it will reach a temperature where the lead-scavenging agents in the avgas will bond with the lead and carry more of it away in the exhaust rather than depositing it in your engine.

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How should you lean an engine for ground operations?

Now that we covered the why it behooves us to think about how to properly lean on the ground.

The general rule is that if you are going to lean on the ground you want to lean as aggressively as possible: Remember the first assumption we made above is that the engine will be operating at 50% power or less, and at those power settings you can pretty much put the mixture knob wherever you want it as long as the engine is running smoothly: You won't hurt your engine.

Lycoming has some recommendations in SI 1497A which is specifically targeted at the IO-360 engines used in some Cessna aircraft (and more specifically toward flight training operations where you may spend a lot of time on the ground at low power), but the recommendations are generally applicable to carbureted engines as well.

In absence of more specific instructions from your engine or airframe manufacturer a simple way to handle leaning on the ground is to lean as aggressively as possible, to the point where the engine will stumble if you try to advance the throttle to takeoff or runup power. This prevents accidentally taking off with the mixture at an inappropriately lean setting, and because it's putting the minimum amount of fuel possible through the engine it minimizes the three problems we're concerned about.

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What about the "Never Lean On The Ground!" myth?

A lot of folks are taught in training that you should "never lean the engine on the ground", or that you should "never lean the engine below 5000 feet" - These are myths based on a common bit of advice you'll find in most POH or engine manuals, which usually reads something like this:

For 5000 ft. density altitude and above or high ambient temperatures, roughness or reduction of power may occur at full rich mixture. The mixture may be adjusted to obtain smooth engine operation.

This is not a prohibition on leaning below a given altitude, though it is often simplified to that level for the sake of students: It's easier for most of us operating at low-elevation fields to be taught "Run the engine full rich on the ground" than explain the inner workings of combustion and fuel mixture to a student who is already overwhelmed just trying to start the engine.

The fact that contradicts the "never lean below some altitude" myth is that both Lycoming and Continental advise leaning the engine during cruise flight (i.e. "extended periods of operation") when the engine is producing 75% power or less, and this recommendation is made regardless of altitude (cruising at 1,500 feet versus 15,000 feet makes no difference - what matters is the amount of power you're asking the engine to produce).
Operating on the ground is, as far as the engine is concerned, "cruise flight" (an extended period of operation with little power change) at a low altitude (sea level) and a low power setting (well below 75% power, typically well below than 50% power), so you can safely lean the engine for ground operations provided you remember to return the mixture to an appropriately rich setting before attempting a high-power operation (runup, takeoff, or getting "unstuck" on a soft surface).

In addition by leaning aggressively (to the point where the engine stumbles when you advance the throttle and cannot produce high power) you ensure that you cannot forget to set the mixture appropriately rich for those high-power operations: as the engine will not allow you to reach those power settings it can't damage itself and the worst it will do is stumble, or stop (starved from an overly lean mixture) if you don't retard the throttle.

Answer 4

At high power (takeoff) a richer mixture is needed to provide additional fuel for cooling, and to avoid excessively high internal cylinder pressures. The high power also leads to sufficiently high exhaust gas temperatures that carbon and lead deposits won't form, despite the excess fuel and incomplete combustion.

At low power (idle/taxi) a rich mixture will result in lower exhaust gas temperatures and produce carbon/lead deposits that foul plugs. Leaning the mixture encourages more complete combustion, this less unoxidised carbon to deposit, and higher exhaust gas temperatures, preventing the deposits from sticking and burning away deposits that are already there. It also saves fuel.

So the difference is not so much "on the ground" vs "in flight", but rather "low power" vs "high power".

Answer 5

The question has been answered, yet it is incorrect (and slightly silly if you think about it) to say that rich mixtures cool the cylinder because ‘fuel is a coolant’.

Rich mixtures burn slower, so the peak pressure inside the cylinder as a result of the expansion of the gas due to combustion occurs later when the piston is further from TDC. Therefore, there is a greater clearance volume above the piston at the point of peak pressure so the peak pressure value is lower, meaning the temperature is lower.

This is why the CHT steadily increases as the mixture is leaned from full rich - because the rate of combustion is increasing meaning peak pressure is occurring closer to TDC. At around ‘best power’ mixture, the combustion rate is fastest, meaning the CHT will be approximately maximum. If the mixture is further leaned, combustion slows again including at ‘best economy’, producing steadily decreasing CHTs.

Additionally, if peak pressure is delayed to around 16 degrees beyond TDC, the angle between the con rod and the crank web is closer to ideal giving greater mechanical advantage and less strain on the bearings.

Detonation results from mixtures not rich (nor lean) enough at high power because the peak pressure occurs too close to TDC when clearance volume is small, meaning pressure and temperature become too high. At high power, a very lean mixture would avoid detonation like full rich does because they both slow combustion so that the clearance volume is greater at peak pressure; however, a mixture lean enough to do so would not be possible to achieve with smooth running because of generally uneven fuel distribution systems, nor would the lean mixture contain enough fuel to actually produce full power. For these reasons full rich is used at high power.