What is Full Mixture Forward on Cessna 172? This question often arises among new pilots as they familiarize themselves with the aircraft’s engine controls. Full Mixture Forward, commonly referred to as “Full Rich,” is a critical setting for the engine’s fuel mixture control. It involves pushing the mixture control lever completely forward to provide the maximum amount of fuel into the engine’s combustion chambers relative to the amount of air. Understanding this control is essential for efficient engine operation and optimal aircraft performance.
The Cessna 172, powered by a Lycoming O-320 or O-360 series engine depending on the model, uses a mixture control to manually adjust the fuel-to-air ratio supplied to the engine. Setting the mixture to Full Mixture Forward means delivering the richest possible mixture, which is necessary in certain phases of flight, including starting the engine and operating at low altitudes. This article explores in detail the importance, technical aspects, and operational guidelines surrounding Full Mixture Forward on the Cessna 172.
Understanding Full Mixture Forward Control
Full Mixture Forward on Cessna 172 controls the fuel-to-air ratio by increasing the amount of fuel delivered to the engine. The mixture control is typically a red knob located on the cockpit’s right console. Pushing it fully forward results in a fuel-rich mixture, which is necessary for certain flight conditions, such as engine start-up or low altitude operations below 3000 feet density altitude.
Specifically, in the Lycoming O-320 engine that powers many Cessna 172s, Full Mixture Forward adjusts fuel delivery to approximately a 12:1 or richer fuel-to-air ratio, compared to a leaner ratio of about 15:1 used at higher altitudes. This richer mixture ensures the engine runs smoothly by preventing pre-ignition and providing adequate lubrication and cooling. It also reduces the risk of engine roughness or stoppage during start and high power settings. Pilots must realize that although Full Mixture Forward increases fuel consumption, it is necessary in certain phases for engine safety and performance.
When to Use Full Mixture Forward on Cessna 172
Pilots should use Full Mixture Forward on Cessna 172 in specific situations. One primary instance is during engine start on the ground. At sea level or low altitudes, pushing the mixture fully forward provides enough fuel to start and run the engine effectively. During this phase, the mixture should remain Full Rich until the engine reaches operating temperatures. This procedure is critical for both cold and warm starts to avoid engine hiccups or flooding.
Another circumstance for Full Mixture Forward is during takeoff and climb at lower altitudes. At altitudes below 3000 feet density altitude, maintaining Full Rich mixture ensures maximum power output from the engine. Since air density is higher at low altitude, the engine demands a higher volume of fuel to maintain the proper stoichiometric mixture. Operating Full Rich in these phases helps the engine develop maximum horsepower, typically around 160 hp in the O-320 engine, thereby optimizing aircraft performance and climb rate.
Technical Implications of Full Mixture Forward in Flight
Flying with Full Mixture Forward has significant technical implications for fuel consumption, engine temperature, and emissions. With the mixture fully forward, the engine receives a higher quantity of fuel, which increases fuel flow rates substantially. At maximum power settings, the Lycoming O-320 can consume approximately 10-11 gallons per hour of avgas with Full Rich mixture. This increased fuel flow provides adequate cooling for the engine’s pistons and valves, reducing the risk of detonation and excessive cylinder head temperatures.
However, prolonged use of Full Mixture Forward at high altitudes leads to inefficient fuel use and potential fouling of spark plugs due to excess fuel. As altitude increases, air density decreases, and the fuel mixture must be leaned accordingly to maintain engine efficiency. Pilots flying the Cessna 172 above 3000 feet density altitude must adjust the mixture control aft of Full Rich to achieve the best power-to-fuel efficiency ratio. Failure to lean the mixture properly can result in increased exhaust gas temperature (EGT) without an increase in power, which can damage the engine over time.
For detailed operational guidance on mixture control and leaning, the FAA’s Airplane Flying Handbook provides extensive resources. Pilots are encouraged to review materials such as the handbook available on FAA.gov to better understand mixture management principles, especially for aircraft like the Cessna 172.
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