Leaning Aircraft Engines

Aero engines usually have a mixture control, its the red knob. It is used to control the fuel air ratio in the carburettor and to stop the engine at the end of a flight. Some engines have an automatic mixture control unit in the form of a pressure sensor (Rotax/Bing) or a sophisticated FADEC engine controller (Aerosance). These engines normally do not have a manual mixture control and must be stopped by switching the ignition off.

In a carburettor fuel is metered on basis of a volume of air and fuel. As altitude is gained, the volume of air the engine intakes remains the same but its density decreases however. This is the reason we need to use the mixture to lean the fuel as to keep the engine running smoothly. Fuel savings up to 20 - 25% can be reached by leaning the engine properly.

Leaning Procedures

The engine is normally operated with a slightly richer mixture to safeguard it against detonation, pre-ignition and possible overheating of exhaust valves. This small amount of extra fuel cools the cylinders by evaporation and a richer mixture burns also at lower temperatures. Leaning by the pilot is normally done below 75% power.

If the aircraft is equipped with exhaust gas temperature (EGT) and cylinder head temperature gauges (CHT) preferable for each cylinder, it then becomes possible to lean very precisely. Keep in mind that with a carburetted engine there always will be a difference in temperatures among the cylinders. With a balanced fuel injected engine (Gami) this will be minimized. Installing a FADEC is a real advantage making sure that the engine is leaned properly on each cylinder and is not running too rich will save fuel and money at the end of the day.

Fuel air ratio

The fuel / air ratio for best power is around 1:13 and for best specific fuel consumption (SFC) it is 1:17. Peak EGT occurs at the chemically correct ratio of 1:14.7 and peak CHT at a slightly richer ratio.

Mixture too lean

Combined with a high power setting this will result in high cylinder temperatures, possible detonation, very hot or burnt exhaust valves and maybe damage to the pistons resulting in a power failure and a possible off airport landing.

Mixture too rich

Causes rough running, loss of power, fouled spark plugs, lead deposits and lower cylinder and exhaust temperatures. A good example is if the primer pump not locked properly (usually just after engine start) which results in these symptoms acompanied with black puffy exhaust, rough running engine and possibly backfiring.

Ground operations

Idling the engine during a long taxi and waiting for takeoff with the mixture in the full rich position results in carbon deposits on the spark plugs and the engine will start to run rough when power is applied. It helps to lean the mixture and increase the combustion temperatures so the spark plugs stay clean. Especially so with AVGAS 100, which has twice the amount of lead compared with AVGAS 100LL.

At low power settings during idling and taxi, exhaust temperatures, CHTs and detonation will not be a problem. You will do no damage to the engine by leaning. Notice, however, that the mixture control needs to be pulled out considerably before any leaning will occur.
Do not forget to set the mixture to fully rich just before takeoff. Run-up is a really good time to check this.

Takeoff and climb

Mixture must be fully rich before takeoff. EGT for most non-turbo engines will range from 1100 - 1250°F and for most turbo engines from 1200 - 1350°F. Make sure to stay below 1350°F at all times.
If climbing is continued over 3000 ft start reducing the mixture control to keep the mixture from getting to rich. If the mixture gets to lean the RPM will start to drop and the engine runs a little rough. This rough running is caused by mixture differences among the cylinders (carburetted engines) and not by detonation as some may think. Enrichen the mixture gently until you get to the point where the engine runs smooth again. Do keep an eye on EGTs.

High altitude takeoff

A reduced air density will lead to a richer mixture. Leaning is therefore needed to keep the engine running smoothly during these takeoffs and climbs.

Leaning during cruise

This is where leaning can really save fuel. And we have a number of leaning methods available: best power, peak EGT, lean of peak (LOP) and with recovery.

Best power (ROP)

Set the mixture so that the leanest cylinder runs at 100°F rich of peak EGT. This ensures that the exhaust valves stay cool. Maximum power is produced along with the highest airspeed, fuel consumption and CHTs.

This method is know to cause engine trouble over time, problems as sticky valves, fouled sparks plugs, carbon deposits to name a few. See mixture too rich above.

The way to achieve this without a EGT gauge is to lean the mixture until the engine start to run rough and then enrichen until it runs smooth again. Keep in mind to move the mixture knob slowly.

Peak EGT

Set the mixture so that the leanest cylinder runs at peak EGT. The EGTs will be 100°F hotter than the best power method and heat damage is possible to exhaust valves. The engine will produce some 4% less power and fuel consumption will be 14% lower than with best power. CHTs are aproaching the highest values.

The engine is now run on a chemically correct mixture, stoichiometric, and the fuel burn is at it hottest. EGTs and CHT are peaking.

Lean of Peak (LOP)

Set the mixture so that the richest cylinder runs at 30-80°F lean of peak EGT. Now the CHTs will be at their coolest and the EGTS will be 20-70°F hotter than the 100°F rich of peak (ROP) method. We now have reasonable cool exhaust valves with the engine producing 7% less power and fuel consumption will be 20% lower than the best power method.

If you want to "recover" the power, increase MAP with an inch or so and make sure that the hottest cylinder doesn't get hotter than 30°F rich of peak EGT, do maintain LOP operation.

Big radial engines like Pratt&Whitney Wasp / Wasp Major and the Wright Cyclones were all run at LOP, ensuring reliable long service life and without the problems caused by running rich of peak.

Descent

Maintain the same mixture level as during cruise, just reduce the throttle (reducing MAP) to maintain the same power levels. Before entering the circuit set the mixture to fully rich, just in case a go-around is needed.