Leaning the Mixture: Why and When

The mixture (fuel-air mixture) is the ratio of the weight of fuel to the weight of air in the charge that is burned. You influence it with the mixture control. This page explains why the mixture gets richer with altitude and what leaning does.

Mixture	When it happens	Effects
too rich	at altitude, if you do not lean	rough running, loss of power, fouled spark plugs
too lean	in a descent from high altitude, if you do not enrich	detonation, rough running, overheating, loss of power

Why the mixture gets richer with altitude

Carburettors are normally calibrated at sea-level air pressure. There the mixture ratio is correct when the mixture control is set to FULL RICH. As altitude increases, the density of the air entering the carburettor decreases, while the density of the fuel stays the same. The mixture therefore becomes progressively richer.

What a mixture that is too rich does

A mixture that is too rich can cause engine roughness and a noticeable loss of power. The roughness is normally due to fouled spark plugs, because too much carbon builds up on them. The carbon forms because the rich mixture lowers the temperature inside the cylinder and prevents complete combustion. This can happen during the run-up before take-off at high-elevation airfields, and also in the climb and in cruise at high altitude.

What leaning does

Leaning reduces the fuel flow. This compensates for the lower air density at altitude.

Some exhaust systems have an EGT probe. It sends the exhaust gas temperature to an instrument in the cockpit. This temperature varies with the ratio of fuel to air and can serve as a basis for regulating the mixture. If you use the EGT display for leaning, fuel consumption can fall.

Enriching again in the descent

According to the FAA handbook, you must enrich the mixture in a descent from high altitude, or it may become too lean. A mixture that is too lean causes detonation, which can lead to rough running, overheating or loss of power. The handbook says the best way is to monitor the engine temperature and enrich as needed.

Detonation

Detonation is an uncontrolled, explosive ignition of the mixture in the combustion chamber. It produces excessive temperatures and pressures. If not corrected, it can quickly lead to failure of the piston, cylinder or valves. It is most likely at high power settings. A common cause is high power with an excessively lean mixture. To prevent it, the handbook recommends, among other things, an enriched mixture and a shallow climb angle during take-off and the initial climb, so that the cylinders are cooled better.

Carburettor heat and the mixture

With carburettor heat on, engine power falls, sometimes by up to 15 percent, because the heated air is less dense than the outside air. This makes the mixture richer.

Fuel injection and FADEC

On fuel-injected engines the fuel-air control unit replaces the carburettor. It meters the fuel according to the position of the mixture control. The handbook names precise control of the mixture as an advantage of fuel injection. On engines with FADEC, a digital engine control, you need no mixture control: you do not have to monitor or control the mixture yourself.

What the EASA syllabus covers

The EASA syllabus for the PPL(A) theory lists “Mixture” under piston engines, with definition, characteristic mixtures, control instruments, and the associated control levers and indications. It also names “Engine handling: power and mixture settings during various flight phases and operational limitations”. PPL(A) flight training covers operating the mixture control in Exercise 4 (“Effects of controls”).