Pilots frequently ask us for information and guidance concerning landings and takeoffs from high elevation airports. Our reference point in this discussion will be based on density altitude. The discussion also requires that we treat separately operation of normally aspirated, turbocharged, and supercharged engines at high elevation airports.
The normally aspirated engine performs and reacts to density altitude. As an example, this type powerplant at takeoff from an airport with an indicated altitude of 3,000 feet, but with an ambient temperature at 85oF, would have a density altitude of more than 5,000 feet. The engine would lack some 20 to 25% of its power and also probably run rough because of a rich mixture on the ground at full rich. Therefore, the typical normally aspirated direct drive engine requires the mixture be leaned on the ground for efficient takeoff performance where airports are 5,000 feet (density altitude) or higher. The overrich condition is something the pilot can compensate for by leaning. However, the higher density altitude with its thinner air cannot be compensated for with a normally aspirated engine unless a supercharger or turbocharger unit is added to the powerplant. Thus, at density altitudes of 5,000 - 6,000 feet, the pilot of a normally aspirated engine has available to him approximately 75% of the engine power, and must plan his takeoff accordingly after setting the mixture.
PROCEDURE FOR LEANING
1. The fixed pitch propeller - lean to maximum RPM at full throttle prior to takeoff where airports are 5,000 feet density altitude or higher. Limit operation at full throttle on the ground to a minimum time.
2. The direct drive normally aspirated engine with a prop governor but without a fuel flow gage, set throttle at full power and lean mixture at maximum RPM with smooth operation of the engine as a deciding factor.
3. With fuel injection, if the powerplant has a marked fuel flow gage, then set mixture in accordance with instructions on the fuel flow gage and/or in accordance with the airplane Pilot’s Operating Handbook.
4. Pressure carburetor - All Lycoming engines equipped with Bendix PS carburetors have an automatic mixture control which does not require leaning for takeoff.
5. Turbocharged and supercharged engines - All takeoffs must be at full rich mixture because the engine is brought back to sea level horsepower which does not permit leaning.
Regardless of the field elevation where the pilot intends to land, the descent from cruise altitude to traffic pattern altitude should be made with the engine leaned for smooth engine operation. Low elevation fields (below 5,000 feet density altitude) will require that the mixture be moved to full rich in the "before landing checklist". Landing at airports above 5,000 feet density altitude, the mixture must be leaned to smooth engine operation during traffic pattern flight and landing; otherwise, the engine may stop on the runway because of excessive richness.