Mixture, Oil and Fuel

Three engine housekeeping systems deserve attention on every flight. The mixture control sets the ratio of fuel to air. Because air thins with altitude, a mixture set for sea level runs too rich as you climb, so leaning removes the excess fuel to restore efficiency and prevent spark plug fouling; pilots use full rich for takeoff at low elevations and lean per the flight manual. The oil system does more than lubricate: it also cools, cleans, seals, and cushions, and the pilot judges its health from the oil pressure and oil temperature gauges. Fuel matters just as much. Aviation gasoline is color-coded by grade so a pilot can confirm the correct fuel — 100LL is blue, grade 100 is green, and the older grade 80 is red — while turbine jet fuel is clear or straw-colored and must never go into a piston engine. Using the wrong grade or type can damage the engine, which is why sampling and confirming fuel color is part of every preflight. Fuel and mixture also drive two abnormal-combustion hazards. Detonation is the uncontrolled, near-explosive burning of the charge — brought on by too low a fuel grade, an over-lean mixture at high power, or high manifold pressure with low RPM and hot cylinders — and it shows up as high cylinder head temperature, roughness, and power loss. Preignition is different: the mixture lights early off a hot spot such as a glowing deposit before the plug fires. Both overheat the engine and can occur together, and both are countered the same practical way: use the correct fuel grade, keep the mixture appropriate for the power setting, and reduce power or enrich to bring temperatures back down.