Do you know what you are doing when you are moving that big red knob in a Cessna 172 or Diamond DA-20, and many other General Aviation aircraft? Do you know what it means when someone says “go full rich”, or “Let’s lean for Cruise”? Other than pushing the big red knob all the way in to start our engines and pulling it all the way back out to shut down the engine, what is really going on?
So what are we physically controlling when we are adjusting the mixture knob?
The “Mixture” control knob found on almost all G.A. aircraft Is controlling how much fuel we are allowing into the system. A “rich” mixture refers to a higher amount of fuel relative to air (the fuel / air mixture is “RICH” with fuel), while a “lean” mixture contains less fuel and more air (Leaning the engine).
“Leaning” and “richening” are terms used to describe the process of adjusting the fuel-to-air ratio in an aviation engine to achieve optimal performance and efficiency during flight.
During the flight, a pilot may adjust the fuel mixture to optimize engine performance depending on various factors, such as altitude, temperature, and humidity. Generally, at higher altitudes, the air is less dense, which can cause the engine to run too rich if the mixture is not adjusted. A rich mixture can lead to incomplete combustion and fouling of spark plugs, which can result in reduced power output, increased fuel consumption, and potential damage to the engine.
On the other hand, a lean mixture may be used to reduce fuel consumption and increase range during long-distance flights. However, running the engine too lean can cause overheating and potential engine damage due to higher combustion temperatures.
Pilots are trained to monitor the engine parameters, such as cylinder head temperature and exhaust gas temperature, to ensure the fuel mixture is adjusted correctly for the specific flight conditions.
This is a 10,000 Ft view (Pun intended), of the Mixture knob. This one simple control has resulted in many, many hotly debated discussions in the G.A. community. If you want to see for yourself, just Google “Lean of Peak”. I prefer to be “Rich” myself.
In one case, a too too lean mixture cooked the valves from being over lean and the exhaust valve on the hottest cylinder, snapped off its stem and the valve face went right through the crown of the piston and down into the crankcase oil pan. Instantly the engine was running on 3 cylinders. And only putting out 1400 rpm at max power… Lucky we were mid downwind and all we had to do was descend and land. After the roll out the engine came to an abrupt shutdown.