Why is mixture lean recommended on the ground?
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Manual doesn’t mention anything about leaning on the ground and gives advice about leaning in the air that has been superseded by newer best practices.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&amp;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.