What Do Those Little Numbers Mean?
or
The Truth About Leaning
There are few subjects in general aviation that can evoke more differing opinions and create livelier discussion that the subject of mixture leaning. Each participant in the industry has a priority concern, and develops a mixture-management procedure that addresses that concern. The end user, the aircraft owner who views fuel consumption as the major expense incurred in the operation of piston-powered aircraft, may favor aggressive leaning. The pilot of rental aircraft (including the flight instructor), who considers per-hour fuel consumption as a fixed cost, may not lean the mixture at all. The A&P mechanic, who has to replace the burnt exhaust valves and clean the fouled spark plugs, will be a proponent of an individual leaning technique that he or she thinks will minimize the workload and ease the unappreciated technician’s life. The owner of rental aircraft who walks the tightrope, balancing fuel cost against maintenance cost, may favor different techniques that change with the wind. The manufacturer, torn between the desires for customer satisfaction, simplicity of operation, and limitation of liability may publish a compromise procedure.
Every pilot of piston-powered
aircraft, regardless of the level of participation, should be directly
concerned with proper leaning technique. Fuel economy and mixture-related maintenance
costs have a direct impact on the cost of aircraft ownership. If these costs increase, they must be
reflected in the rates charged for rental aircraft and the compensation to
professional pilots of those aircraft.
Since avgas is the only fuel remaining in the
A complete understanding of effective leaning practices requires an examination of the multifaceted role of tetraethyl lead in aircraft engine dynamics.
The first reason to include lead as an additive to avgas is to raise the “octane rating.” Tetraethyl lead raises the flash point of gasoline, allowing it to be compressed to a higher pressure without detonation. Without lead acting as predetonation suppressor, the air-fuel mixture would self-detonate (as a result of the heat of compression) before the piston reaches the top of the compression stroke, reducing power and possibly damaging the engine. Higher compression makes it possible to produce more power from an engine with smaller displacement. A high power-to-weight ratio is the goal of every designer of aircraft engines.
Lead also plays the part of coolant in the exhaust stroke of the piston aircraft engine. Since metal is a more efficient conductor of heat that the exhaust gasses, it conducts more specific heat out the exhaust and away from the combustion chamber and, more specifically, the area of the exhaust valve and valve seat.
When lead is precipitated in solid form during the combustion process, a certain amount of it is deposited on the exhaust valve seat. When the exhaust valve closes, this lead film acts as a cushion in the seat, lessening the impact of the steel-on-steel contact and reducing wear.
Finally, the film of lead deposited on the exhaust valve and seat acts as a sealant in that area, becoming a self-replacing gasket that allows greater pressure to be maintained during the next compression stroke.
During takeoff, climb, and go-around, when maximum power is required, the sealing effect of lead maximizes its availability. Since the RPM is allowed to reach its maximum allowable level, the cushioning effect of lead works to decrease the wear incurred during this period of rapid action of the exhaust valve. The extreme heat generated during this phase of flight requires that predetonation be avoided so that proper combustion timing will allow maximum power to be generated. These heat levels can be reduce by employing precipitated lead to aid in the conduction of heat from the combustion chamber. Since these positive effects can be maximized by maintaining a surplus of tetraethyl lead, a full rich mixture is normally recommended, even at the risk of lowered fuel efficiency.
Cruise, descent, and ground operations require less power and generate less heat. During these operations, the demand for lead is reduced, allowing the pilot to lean the mixture for more fuel efficiency. But this action should not be viewed as strictly an economy measure. The levels of heat and RPM experienced in these power regimes are insufficient to scavenge the excess lead from the combustion chamber when the mixture is allowed to remain at the full-rich setting. The most noticeable result is the formation of lead deposits in the spark plugs (usually the lower ones) to such a degree that the lead will form a “bridge” from the body of the plug to the electrode, causing a misfire. These misfires can only be eliminated by aggressive leaning (which may not be advisable) or by removing the spark plug and cleaning it. Either way, the time and money spent is inconvenient and counterproductive.
So what guidelines should a pilot use for leaning?
Manufacturers of low-end general aviation aircraft have traditionally written their operation manuals for the lowest common denominator: the pilot who has no knowledge of the technical aspects of airframe and powerplant operation. Usually this has taken the form of admonitions against leaning the mixture below 3000 feet or 5000 feet. These vague limitations are often presented to student pilots as concrete values. The logic of this appears to be: if the novice pilot avoids leaning in all but the most obvious cruise scenarios, then engine damage that can occur as the result of overly aggressive leaning will naturally be avoided.
Higher-end general aviation aircraft are provided with detailed power charts, carefully outlining the proper leaning procedure. Apparently, pilots who are experienced enough to operate these advanced machines have the superior technical understanding necessary to lean the mixture responsibly.
Piston aircraft powerplants are actually designed to be operated with a lean mixture at power outputs less than 75%. The published altitude “limitations” on leaning reflect the concept that power outputs greater that 75% cannot be achieved above those altitudes. So, if the novice pilot always operates full rich below the published altitude, then the engine cannot be run lean at power settings in excess of 75%. The educated pilot, however, could make use of the cruise performance table in the performance section of the pilot operating handbook to make informed decisions regarding leaning.
What about the little numbers? Refer to Figure 1.
Figure 1
In the late 1970’s, the Cessna Aircraft Company began to take a more considered approach when providing pilots with information on mixture management. On tachometers installed in Cessna 152’s and Cessna 172’s the normal operating range is depicted with step-downs at the top of the green arc, accompanied by small markings. These markings are altitude/RPM combinations that correspond with 75% power. On the illustration above, representing the tachometer of a Cessna 172, 2600 RPM corresponds with the 75% power setting at 5000 feet of altitude. As long as the RPM remains below 2600 at 5000 feet, then the mixture can be leaned prudently.