April 2014 -
Prior to my first cross-country flight from Seattle (KBFI) to Arlington, Wash. (KAWO), I was told that we were going to lean the engine when we got up to cruise altitude. I was instructed to pull the mixture knob slowly aft until the engine started to get rough, then to push it back in until the engine smoothed out.
Since those early days I've learned quite a bit more about leaning. The following is a general discussion on the basics of leaning; it is intended only to be educational. Always refer to your Flight Manual or POH for specific leaning instructions.
Red knob basics
Leaning seems simple: since air density decreases as air temperature and altitude increases, and since the carburetors and fuel injection components on our airplane engines don't adjust for these density changes, pilots need to manually reduce the amount of fuel delivered to the engine combustion chambers to maintain the most efficient and economical fuel/air ratio.
Pilots sometimes hear opinions that leaning burns exhaust valves, and that fuel is less costly than a top overhaul. Based on these convictions they may conclude that leaning is bad. However, real-world testing of engines—in what's probably the world's most sophisticated aircraft engine test cell—have proven those claims to be false.
The main reason you should learn to lean correctly is to maintain the health of your engine. A second reason is to save money.
An airplane engine can be leaned whenever it's running. All engines can be safely leaned during taxi—in fact, it's a good practice to lean during taxi to cut down on the possibility of spark plug fouling due to carbon or lead contamination. Just remember to always richen the mixture to adjust for the airport density altitude before turning onto the runway for takeoff.
A rich mixture means there's a surplus of fuel compared to the amount of air. This excess fuel slows combustion. Excess fuel is also detrimental to long-term engine health for at least two reasons. Carbon created during incomplete combustion, along with tetraethyl lead (TEL), a fuel additive that reduces the possibility of uncontrolled combustion (detonation), is deposited on valve stems, piston crowns and piston ring lands. A richer than necessary mixture wastes fuel and has the long-term effect of lessening engine efficiency.
Landmarks on the mixture map
As can be seen in "Landmarks on the Way to Understanding Piston Engine Powerplant Management" illustration (photo 01, page 28), as the red knob is pulled aft (leaned), every important engine parameter—except fuel consumption—initially rises.
CHT, EGT, internal combustion pressures (ICP) and horsepower (HP) all increase predictably; then they decrease predictably. These landmarks are true for every piston aircraft engine in the fleet.
Only brake-specific fuel consumption (BSFC) decreases. BSFC is a meaningful number depicting the number of pounds of fuel burned per hour for each horsepower produced at the propeller shaft (Lb/BHP/Hr). This number for aircraft reciprocating engines varies between 0.35 and 0.6 depending on the engine specifications, atmospheric conditions and leaning practices.
Peak EGT is the zero point milepost for all leaning. It's where all the fuel molecules and all the oxygen molecules are consumed in combustion.
It takes approximately 15 pounds of air and one pound of fuel for complete combustion. It's called peak EGT because this fuel-to-air ratio produces the hottest combustion temperature.
On the rich side of peak EGT, excess fuel slows and cools combustion; on the lean side of peak EGT, a deficit of fuel slows and cools combustion.
When the mixture is leaned beyond peak EGT, all the landmarks begin to decrease. It makes sense: as the amount of fuel in the fuel/air mixture decreases, so does the amount of heat energy created during combustion. Leaning beyond peak EGT is referred to as running lean of peak (LOP).
Operating LOP dramatically reduces carbon and TEL deposits in high temperature parts of the engine such as the exhaust valve stem, the piston crown and the combustion chamber. LOP operations are much "cleaner" and also dramatically reduce the amount of carbon monoxide in the exhaust stream.
There are two vertical dashed lines on the roadmap. The one to the left side of the peak EGT dotted line indicates a mixture that is 50 degrees rich of peak, or ROP. The dashed line to the right of the peak EGT line indicates a mixture that is 50 degrees LOP. (For readers' ease, all references to degrees in this article are in Fahrenheit (F) unless otherwise marked. —Ed.)
The CHT and ICP lines are relatively flat from approximately 80 degrees ROP to just before peak, then both begin to drop off; the HP line rises and peaks between 80 degrees and 50 degrees ROP and then drops off; while the fuel consumption (1/BSFC) "peaks"—if you will— between peak EGT and 50 degrees LOP.
Referring again to the roadmap, notice that only EGT is equal at the Rich and Lean dashed lines; CHT, EGT, ICP, and HP are all lower on the lean side. BSFC is at its maximum on the lean side.
As a number, peak EGT has zero value to the pilot since variables such as the positioning of the probe in the exhaust tube and distance from the cylinder exhaust port affect this number. Yet, no matter what the number, peak EGT is the critical reference point when leaning.
Peak EGT is the first signpost. This critical leaning signpost is always defined by the first cylinder to peak; that's the leanest cylinder in the engine at that fuel/air ratio. Learn to ignore EGT numbers; you're looking for the first one to peak, not the one with the hottest EGT number.
Remember these two definitions. Best Power is the fuel/air ratio at which the engine produces maximum power for a fixed mass of airflow. We have already said best power is obtained at around 80 degrees rich of peak. All GA engines can be leaned to best power.
Best Economy is the fuel/air ratio at which the engine produces the maximum power for a fixed mass of fuel flow. At low power (65 to 70 percent, or below) this is 15 to 40 degrees lean of peak. At higher power settings, the point is 40 to 90 degrees rich of peak. A great many GA engines can't be successfully operated at these Best Economy settings.
Best Power is on the rich side of peak EGT; Best Economy is on the lean side.
The development and installation of EGT probes—and more recently, very sophisticated engine monitors—have provided pilots with tools that simplify leaning.
Fixed pitch propeller, no EGT instrument
One leaning tool for a "no-instrument" airplane consists of leaning until engine roughness is felt and then richening until the engine smoothes out. Another is leaning for the highest airspeed.
Since horsepower peaks at approximately 80 degrees ROP, this will also show up as a maximum indicated rpm. Leaning to maximum rpm is difficult with basic instrumentation due to the inherent inaccuracy of an analog tachometer.
The drawback of this method is that the CHT and the ICP will be near, or at, the maximum values.
Single cylinder or single point (manifold) EGT
In the mid-1960s the EGT gauge was introduced to General Aviation by Al Hundere of Alcor Inc. Soon Cessna began to offer EGT gauges as an option. The tool used a single EGT probe that was installed in a single exhaust pipe downstream from a single cylinder, or in an exhaust collector that combined exhaust gases from two or three cylinders.
A single EGT probe is better than no EGT indicator in that it gives the pilot a rough idea of peak EGT indication. However, due to something called EGT spread, pilots are often puzzled while leaning to peak with a single-probe system when the engine starts to feel a little rough before or when the peak EGT is indicated on the gauge.
When roughness is felt, that means that the probe is not installed on the first cylinder (leanest) to peak. Due to EGT spread (which I'll explain soon) the power output across the cylinders is mis-matched; this creates a noticeable vibration. When this occurs, the onset of vibration must be used as the peak EGT point.
A single EGT gauge was a big step forward in mixture management, but there was still more needed.
All-cylinder engine monitors
In 1981, John Youngquist of Insight Instrument Corp. expanded our understanding of leaning when Insight introduced a small panel-mounted instrument called the Graphic Engine Monitor (GEM).
The GEM showed EGT and CHT for all cylinders in a graphical presentation and instantly revealed a little-known fact to aircraft owners: as pilots leaned the mixtures, no two cylinders peaked at the same point in leaning.
In the ideal internal combustion engine, each cylinder generates an identical power pulse. This requires that the fuel/air ratio delivered to each cylinder of an engine be evenly matched—but this ideal is unobtainable in many, if not most piston aircraft engines due to inefficient engine induction systems.
Six-EGT-displays-in-one instruments revealed a characteristic known as EGT spread. As we learned more about how individual cylinders react when leaning, we realized that leaning to peak EGT by referencing a single EGT probe is a crapshoot since pilots had no way of knowing if the single probe is installed on the first cylinder (leanest) to peak.
And if the probe isn't on the first cylinder to peak, one or more of the other cylinders could have already peaked and be burning an LOP mixture. While this isn't harmful to the engine at power levels below 60 percent, it results in a less-than-smooth engine.
Lean of peak (LOP)
Since we know now that operating an engine with a mixture that's on the lean side of peak EGT results in lower EGT, CHT and ICP, why doesn't everyone fly this way?
Operating LOP does lessen airspeed, but it also greatly lessens fuel flow. Usually speed loss is about 10 percent; range increase is about 20 percent. But the main reason most pilots don't fly LOP is they can't; their engines won't let them.
EGT spread is caused by inefficient fuel distribution due to nonadjustable engine factors such as rudimentary induction systems, inefficient fuel/air mixing at carburetors and the simple continuous-flow fuel injection systems that are the rule in the majority of Cessna airplanes.
I installed a switchable six-cylinder EGT gauge in my 1966 Cessna 182J then tried leaning to get all six cylinders LOP. During my attempts, I concluded that it was impossible to operate that carbureted engine LOP since the amount of fuel/air mixture entering the combustion chamber of each cylinder varied widely. The EGT spread was often 150 degrees. (Refer to photo 02, page 31.)
Is fuel injection the answer?
Engines equipped with fuel injection systems do a better job of distributing fuel since both the Bendix-type system used on Lycoming engines and the system used on Continental Motors engines deliver fuel via individual tubes to fuel injection nozzles near the intake valve of each cylinder.
Yet even with this advantage there's still an EGT spread, since engine induction manifolds cause variances in air delivery from cylinder to cylinder. In order to reduce the EGT spread in a fuel-injected engine, the fuel nozzles have to match the airflow variances at each cylinder.
LOP flying—but only if your engine is fuel injected
General Aviation Modifications Inc. (GAMI) of Ada, Okla. has the most sophisticated engine test cell in the country. GAMI has used data from test cell engine runs to develop sets of fuel injection system nozzles—called GAMIjectors—that compensate for the variances and tighten the EGT spread.
GAMIjectors make LOP operations a reality for pilots, and the product is made for almost all fuel-injected engines in the GA fleet.
GAMI has also developed engine mixture leaning suggestions based on data gathered during engine testing. This has resulted in what GAMI calls its "red box" leaning suggestions. (See photo 03, page 32.)
The idea is simple: leaning to stay out of the red box keeps ICP below 750 psi, reduces the possibility of CHTs climbing above 380 degrees and lessens stresses on the main and connecting rod bearings by moving the highest ICP toward the ideal point in crankshaft rotation after top dead center (ATDC).
Ideal ICP timing
GAMI's red box testing has shown that the ideal timing for peak ICP is 16 to 18 degrees of crankshaft rotation ATDC. The timing of peak ICP can be controlled on either the ROP side or on the LOP side; the combustion event is slowed on both sides.
As can be seen on page 32 (photo 03), the outer edges of red box leaning boxes move farther and farther away from peak EGT on the rich side with each higher power setting.
Does this mean that every pilot of every airplane would be wise to cleave to the red box suggestions for leaning? It depends on the engine in your airplane. I asked the head of GAMI if there was a red box for the 180 hp Lycoming engine in my airplane; he said no.
Red box suggestions are important considerations for owners of airplanes with higher power and higher compression engines.
The quote at the beginning of this article provides a guideline for leaning non-fuel injected engines. Leaning to peak EGT at the power settings recommended by the engine manufacturers (75 percent power or below for Lycoming; 65 percent or below for Continental) does result in high CHT and ICP and does produce less-than-ideal peak ICP timing.
Leaning to Best Power (80 to 100 degrees ROP) will provide the highest speeds. It also produces high CHT, but because it's richer than peak EGT, the ICP peak will occur closer to the ideal time.
Red box theory suggests any engine at 75 percent power be leaned to nearly 150 degrees ROP; at 65 percent richen to 80 degrees ROP or richer. The red box mixture settings are much more conservative than the engine manufacturer's leaning suggestions.
One tool developed to aid in red box leaning is the APS Power Wheel from Far West Aviation. This tool, shown in photo 04 on page 32, was developed in conjunction with GAMI recommendations.
Users adjust a clear Lexan scale to align their cruise altitude with engine manifold pressure; then OAT above or below the ISA standard are factored in for that altitude. Scanning across on the engine rpm scale reveals the percent of power and the red box-recommended mixture setting. (An electronic version of this tool is available for Apple iPhone and iPad users at the App Store; search under "Power Wheel." —Ed.)
High altitude airport leaning tricks
Since air density decreases as we climb above sea level, mixtures go richer. Savvy pilots lean for takeoff when the density altitude is greater than 3,000 feet.
What's the best way to lean for performance when operating from high density altitude airports? No-EGT-instrumentation airplanes should lean to maximum rpm at full power before turning onto the runway for takeoff. Then richen the mixture slightly for additional cooling.
Leaning for a high density altitude takeoff is much easier (and quieter) for airplanes with EGT instrumentation. During the initial takeoff roll, pilots using a single-probe system should lean until the pointer is at the same place on the gauge scale as it is when taking off from a sea level airport.
Leaning for all cylinder systems is simple if pilots note the full-throttle takeoff EGT number (at a sea level airport) on one cylinder—it doesn't matter which one—and lean to that number on that cylinder for takeoff.
Pilots flying airplanes with turbocharged or turbo-normalized engines don't lean during high density altitude takeoffs.
Lastly, there's one more engine operation number that must always be considered during all leaning procedures: the cylinder head temperature. The science that predicts metal fatigue (as well as extensive test cell monitoring) has shown that cylinders live long and prosper when CHTs are kept below 380 degrees.
Whenever CHTs go above 380 degrees, it's a damn good idea to do whatever it takes to reduce these temperatures. Actions to reduce CHTs include reducing power, opening cowl flaps, flattening the climb angle and richening the fuel/air mixture.
Want to learn more about leaning and the science of operating your airplane engine? I recommend that you read "Basic and Advanced Light Plane EGT Systems" authored by Kas Thomas and the editors of Light Plane Maintenance. Used copies are available through eBay and Amazon.
If you want to go to "grad school" on engine operation, consider enrolling in Advanced Pilot Seminars' "Engine Management Made Easy" online course; you can complete the seminar in the comfort of your own home. This course is especially useful for pilots learning to manage high-powered fuel-injected turbocharged engines.
Steve Ells has been an A&P/IA for 43 years and is a commercial pilot with instrument and multi-engine ratings. Ells also loves utility and bush-style airplanes and operations. He's a former tech rep and editor for Cessna Pilots Association and served as associate editor for AOPA Pilot until 2008. Ells is the owner of Ells Aviation (EllsAviation.com) and lives in Paso Robles, Calif. with his wife Audrey. Send questions and comments to .
Engine monitors, CFA supporters
Electronics International, Inc.
Insight Instrument Corp.
JP Instruments, Inc.
General Aviation Modifications Inc. (GAMI)
Power management computer
Far West Aviation
Further reading and study
"Basic and Advanced Light Plane EGT Systems" by Kas Thomas
Belvoir Publications, Inc., 1989
"Engine Management Made Easy"
Advanced Pilot Seminars