AOPA Pilot Magazine - May 2011 - 90

Continued from page 88

Continued from page 89

Rich-of-peak operations mean safe margins from detonation, running hot enough to keep lead deposits to a minimum, and yet cool enough to prevent damage to exhaust valves and other top-end components. Now the lean-of-peak (LOP)-EGT bandwagon seems to be in vogue. This tactic uses mixture settings that rely on the air in the fuel-air mixture to do the cylinder cooling. It requires precisely calibrated fuel injectors (General Aviation Manufacturers Inc.’s GAMIjectors being the most popular) that create evenly distributed cylinder head temperatures (CHT), the afore-mentioned engine analyzing gear, and no small amount of finesse in the leaning process. Here’s where problems can crop up. Sure, you can run lean of peak, but you’d better know what you’re doing. (Maybe you should hire a flight engineer?) Stories abound of pilots misinterpreting CHT, EGT, fuel flow, or other indications. Some analyzers are more intuitive than others, so the leaning sequence can be confusing. I’ve seen some pilots referring to a “cheat sheet” every time they go LOP. Instant recall eludes them, and they can’t quickly recall the steps to get to the LOP promised land. For many, the procedure is simply counter-intuitive and slightly unsettling. Does air really do a better job at cooling than fuel? Is there enough fuel in my fuel-air mixture to prevent detonation? Thoughts like these cross your mind when you go to LOP-land. And understandably so. That’s because there’s a danger in misinterpreting temperature indications—and running at a mixture setting that will damage your cylinders. So what’ll it be for you? The comfort of ROP operations, or the complexity, potential confusion, and risk of damage that can come with running LOP? Yes, lean-of-peak operation promises lower fuel burns and less fouling of spark plugs and valve assemblies. But is it worth it? I once asked a mechanic, and owner of an engine shop, what he thought of cruising at lean-of-peak mixture settings. “I think it’s great!” he effused. “My top-end business is shooting out the roof!” Makes you wonder. E-mail the author at tom.horne@aopa. org.
AOPA PILOT •

it happen during full-power takeoffs and climbs when the mixture knob was in the full rich position but fuel flow to that cylinder was somehow restricted? Eichhorn was too savvy to take the bait. He said diplomatically that there was no way to tell from the information given. Even though many pilots would consider 1,300 hours from a set of IO-550 cylinders a moral victory, let’s assume for the sake of argument that lean-ofpeak operation was to blame for this cylinder’s woes. An IO-550-powered Bonanza flying rich of peak typically consumes about 18 gallons of fuel an hour. Lean of peak, that number is reduced to about 13 gph. If this engine operated 500 hours lean of peak (and that number is probably low), the five gph fuel savings at $4.50 a gallon was $11,250—more than enough to pay for the required valve work. On a larger scale, the numbers are even more impressive. More than 20,000 GA engines are equipped to run LOP, and assuming just half of them do so for 125 hours a year at a fuel savings of 2.5 gph per engine, they save 3.1 million gallons of fuel (and $14 million) annually. But the real reason to operate lean of peak isn’t economy, or range extension, or saving the planet (although those things are nice!). The real reason to do it is to make your airplane fly safer and longer. Ask the people who make their living overhauling and maintaining piston aircraft engines (Bill Cunningham and Monty Barrett in Oklahoma are good places to start) and they will tell you that engines operated properly LOP are cleaner internally with less damaging crud in the case or residue in the cylinders. The whole story in keeping cylinders healthy is keeping them cool (below 380 degrees F), Cunningham says. Whether that’s done by enriching the mixture and pouring unburned fuel through them, or a very lean setting that restricts fuel and creates less heat from combustion, is up to you. Questions about the benefits of lean-of-peak operations have been asked and answered. E-mail the author at dave.hirschman@ aopa.org.

90 • MAY

2011

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AOPA Pilot Magazine - May 2011

Table of Contents for the Digital Edition of AOPA Pilot Magazine - May 2011