Correct procedures for leaning an aircraft engine are a bit of a mystery to many pilots. Leaning is not always well taught during Private Pilot training and many pilots settle for the information they gained during their initial training as sufficient to carry them through a lifetime of flying.

I received a very nice email message from a pilot who wrote:

“I took off from Banff a few years ago, and, unsure of how to lean, found myself uncomfortably close to the trees at the end. (Now, of course, my Cherokee goes up like an Avro Arrow, due to the hot pursuit of the Parks personnel behind me). The reason I was reluctant to lean too much arose from a previous time waiting in line to depart from Victoria. I had leaned to avoid fouling the plugs and, when I started my roll, the performance was anaemic at best until I went full rich halfway down the runway.”

This is a good story and one that might be typical for many people. Our friend shows he has an idea about leaning his engine but isn’t quite sure of all the details. The good news is he has the sense to know he doesn’t know it all and the interest to look more deeply into the problem.

Fuel burns in air at a fixed ratio: about 1 to 15. To burn one pound of fuel requires approximately 15 pounds of air. The key idea here: there is a given quantity of fuel that can be burned in a given amount of air or, specifically, in a given weight of oxygen in the air. If the proportionate weight of fuel exceeds the available proportionate weight of oxygen, excess fuel will be left unburned.

Some excess or unburned fuel does serve a useful purpose particularly in a climb or when operating the engine at high power settings, above 75% rated horsepower. A small amount of extra fuel helps with engine cooling and adds additional lubrication to the valves, rings, and cylinder walls. This is the good news.

The bad news arrives when we attempt to operate an engine with either too much or too little fuel: a mixture that is too rich or too lean. Too much fuel, too rich a mixture, produces too much cooling and excess unburned fuel may accumulate and foul spark plugs, valve guides and other important parts. Running the engine too cool or allowing fouling to occur reduces power output.

Too little fuel, too lean a mixture, may result in the engine running too hot causing damage to valves and other internal parts and a reduction in power output. Complete engine failure is also a possibility. This condition can become very expensive very quickly.

Neither too much nor too little fuel is ideal either for engine longevity or for achieving the power your engine is capable of producing.

Our calculations are further complicated by altitude and temperature. As we climb, the air “gets thinner”. The density of the air decreases; the distance between individual molecules of oxygen increases. For a given volume of air the actual weight of available oxygen decreases with altitude. Temperature has a similar effect on air density for the same reason: increased temperature results in reduced air density.

To produce best power from our normally aspirated engine, we would like to keep the fuel/air mixture quite close to the ideal ratio so we have neither too much nor too little fuel mixing with the available air.

As they say, if all else fails follow the directions. Engine manufacturers have worked with the issue of leaning quite closely and are happy to provide operators with their findings and recommendations. For example, Textron-Lycoming has published Service Instruction No. 1094D, dated March 25, 1994, available on line, that provides recommendations for proper leaning procedures of all Textron Lycoming opposed series engines.

They suggest four general rules for successful leaning:

1) Always observe the red line temperature limits during take-off, climb, and high performance cruise power operation. (We don’t ever want to overheat the engine),
2) Whenever mixture is adjusted, either rich or lean, it should be done slowly. (Change may be a good thing, but changing things too quickly puts unwanted stresses on the machinery),
3) Always return mixture slowly to full rich before increasing power settings (once again, we want to give the machinery a chance to catch up with the changes we are inducing), and
4) Use caution to prevent shock cooling of cylinders. A maximum temperature change not exceeding 50ºF. per minute is recommended. (Easy does it).

A good look at your aircraft’s Pilot Operating Handbook and at the appropriate service bulletins for the type of engine you operate is an excellent plan. You may want to discuss your particular situation with your AME. Some generic procedures for successful leaning are:

1) Use full rich mixture for take-off and climb for density altitudes below 5000’.
2) For take-off or climb at a density altitude above 5000’, adjust mixture at full throttle to produce smooth operation and maximum RPM. Try to keep full throttle operation on the ground to minimum time.
3) In cruise at power settings at or below 75% rated horsepower, slowly lean for maximum RPM. This will give you approximately best power or the setting at which your engine will produce the most miles per hour airspeed for a given power setting.
4) Slowly and gently return mixture setting to full rich as or before you descend and before you increase power settings. It may be an excellent plan to gradually adjust mixture as you descend so engine temperatures do not drop too quickly, say every 1000’.

So, what useful advice could we pass along to our pilot friend? At Banff (CYBA) the elevation is 4583’ ASL. If the temperature were, say, 20º C and the altimeter setting were, say, standard so pressure altitude was the same as absolute altitude, the density altitude would be, by quick E6B calculation, just over 6000’. Leaning the engine on the ground prior to take off would be sound practice.

At Victoria International (CYYJ) elevation is 63’ ASL. Ground leaning for taxi would certainly be a good plan, but, unless it was an extremely hot day, leaning for take-off would not be recommended. However, on reaching cruise altitude, once power was set at or below 75% rated horsepower and engine temperatures had stabilized, leaning would be an excellent plan both to save fuel and to allow the engine to operate at best efficiency.

A generic method for leaning your engine without a cylinder head temperature gauge (CHT) or an exhaust gas temperature gauge (EGT), once the aircraft is levelled off and stabilized in cruise, is to gently come back on the mixture control in stages until the engine begins to run just slightly rough then ease mixture back in until the roughness smoothes out and maximum RPM is achieved. Some pilots like to add a couple or three “clicks” richer from there “just for mother”. Be sure to monitor oil temperature and pressure gauges to ensure you are not running too lean.

From The Ground Up provides some excellent insight: “Engines that are operated at too lean a mixture for considerable period of time will undoubtedly sustain more wear and will cost more to overhaul than will those engines operated with correct mixture selection. Pilots should realize that, in the long run, it is usually far cheaper to use a little more fuel during the running time between overhauls than to pay the extra overhaul costs.”

To maximize the service life of your engine, Textron Lycoming recommends, during cruise flight, the engine be operated at 65% rated horsepower or less, the cylinder head temperatures be held at or below 400ºF, and the oil temperature be maintained within the 165ºF. to 220ºF temperature range.