To a non-flyer taking a ride in an older small plane, there is one moment early in the flight that may well shift the relaxed passenger’s demeanor into one of startled attention. When the pilot-in-command levels off, pulls on the big red knob and makes the engine skip a beat, the non-flyer’s heart is apt to do the same. What the pilot is doing is leaning the fuel/air mixture ratio that is entering the cylinders. Here’s a few considerations regarding such action.
Operating an aircraft engine lean-of-peak (LOP), without adequate instrumentation, used to be considered a certifiable way to ruin your powerplant. Do it, pundits would say, and risk cracked cylinders and burnt exhaust valves owing to overly high cylinder head temperatures. Training flights – given the comparative lower altitudes at which they take place as well as their shorter duration – are often conducted entirely full-rich exposing students to few opportunities to practice LOP techniques. Abusive and unpracticed use of the mixture knob, schools might say, can lead to premature cylinder overhaul. It’s better and more cost-effective, goes the thinking, to burn more fuel than to replace an engine before its time has passed.
There are, however, good reasons to fly LOP. Doing so allows the engine to run cooler while drinking less fuel. More power and higher airspeeds will favour your flight too. Furthermore, LOP offers cleaner combustion thus minimizing the threat of fouled plugs, or stuck rings and valves. Working against operating LOP are manufacturer’s concerns that (1) fuel distribution is not balanced enough across cylinders to allow engines to operate smoothly, and (2) aircraft lacking appropriate engine instrumentation do not permit pilots to set their mixture controls to optimal LOP positions. True enough, ideal LOP settings are hard to hone without multiprobe digital equipment to show you where to place the mixture knob. At best, it’s always a close approximation.
LOP can be set in different ways. Most commonly, pull on the mixture control until (1) exhaust gas temperature (EGT) peaks then starts down on its lean side; or (2) you hear or feel the engine start to lose an itsy-bitsy bit of power. Changes in cylinder head temperatures (CHTs) result from these changes in mixture setting. (CHTs are also affected by rpm, manifold pressure settings, outside air temperatures, and cowl flap positioning.)
If you’re in a typical normally-aspirated single-engine piston aircraft driven by ubiquitous power sources from either Lycoming or Continental, CHT readings of 380EF should be considered your upper operational boundary when LOP. 25EF to 50EF past peak is considered the optimal range for LOP using your EGT. Since engine cylinders don’t all operate at the same temperatures, LOP settings should always be fine-tuned according to the hottest cylinder’s CHT readout. (The front cylinders tend to run cooler than those tucked behind given their immediate exposure to cool air flowing into the engine’s cowling.) Also, any mixture setting ultimately chosen according to the EGT readout should ensure the engine runs smooth and vibration-free.
If you’re wanting to go fast in cruise and your hottest CHT readout rises above 380EF, you can lean the mixture slightly from its LOP position to maintain a desirable CHT. If the hottest CHT readout drops below 375EF, enrich the mixture slightly from its LOP position. If range and economy are your flight mission’s goals, lean the mixture as much as possible but not so much that your engine runs rough. You’ll gain fuel economy and range by doing so. Leaner is cooler when on the lean side of peak, and richer is cooler when on the rich side of peak. Remember, too, that the position of the mixture control should always be rechecked with changes in operating conditions, most notably altitude.
The above merely scratches the surface of the LOP subject. LOP in climbs and descents, on turbo aircraft, on multi-cylinder engines, on constant and variable speed props, all have further considerations to the above. Whatever settings you chose, it’s always best to operate your aircraft in accordance with manufacturer’s recommendations.