There are precious few circumstances in which pilots can cut their fuel burn, reduce costs, and extend the lives of their piston engines at the same time—but lean-of-peak engine operations is just such a rarity.
A decade ago George Braly, founder of General Aviation Modifications Inc. (GAMI), began teaching his then-heretical lean-of-peak (LOP) engine operating philosophy for some piston aircraft engines.
Engine manufacturers decried the practice as dangerous, and overhaul shops anticipated a steady supply of top overhauls from cooked cylinders damaged by the extreme temperatures they were sure would result from pilots over-leaning their engines at high power settings. Environmental benefits such as reducing fuel burn up to 20 percent barely entered the equation.
More than 19,000 sets of GAMI’s matched fuel injectors have been installed on a wide range of aircraft engines during the past decade and they’ve logged hundreds of thousands of LOP flight hours; air taxis have flown tens of thousands of flight hours LOP; airframe manufacturer Cirrus Aircraft now requires that pilots operate its turbocharged aircraft LOP; and engine manufacturers have changed their guidance or muted their previous criticisms.
Despite such real-world experience, however, some pilots and mechanics still regard LOP as a risky, unproven gimmick. They see a conspiracy between engine manufacturers who want to sell and overhaul more powerplants, and airframe manufacturers who use LOP to cynically pad range and performance specs for their aircraft.
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But what about the engine shops that commonly see the insides of engines run both rich of peak (ROP) and LOP? Do they see any evidence of LOP engine damage? And how about air taxi operators that depend on reliable fleets and seek to keep both fuel and maintenance costs as low as possible? Would they fly LOP if it saved fuel but shortened engine life?
“This discussion about lean-of-peak operations has been going on an awfully long time,” said Braly, who began evangelizing on its benefits 10 years ago at the Advanced Pilot Seminars he taught at the company’s Ada, Oklahoma, headquarters. “We’ve got some NACA research from 1943 that was classified as top secret at the time because they wanted to know if they could run lean of peak to extend the range of bombers and other U.S. military aircraft. The data was very clear and it always has been. Lean of peak is cooler and cleaner.
“But some pilots continue to resist on an emotional level,” he said. “They just can’t believe they’ve been so wrong for so long.”
Using extremely precise lab test equipment, GAMI showed that—with precise fuel/air metering systems and graphical engine monitors—high-compression and turbocharged piston aircraft engines could run safely and reliably LOP with significantly reduced fuel burn. But Braly’s suggestions required pilots to lean far more aggressively than engine manufacturers recommended.
There was a great deal of reluctance at first. But when pilots saw others reducing their fuel burns about 20 percent (and their airspeeds about five percent) in typical six-cylinder engines, they took notice. And when those LOP engines kept running year after year, the evidence became even more compelling.
Not all GA engines can run safely LOP. Carbureted engines, for example, lack precise fuel/air metering systems and typically run rough and lose power LOP. And electronic engine monitors that show cylinder head and exhaust gas temperatures for every cylinder are necessary for safe LOP operations. Graphic engine monitors are even better.
The danger of flying LOP with a single-cylinder CHT or EGT probe is that some rich-running cylinders could become too hot, damaging internal valves and guides, and causing a loss of engine compression that would require a top overhaul.
“The problem isn’t running too lean,” Braly said. “The problem is running too rich without knowing about it.”
Bill Middlebrook, president of Penn Yan Aero, a New York company that performs about 400 engine overhauls annually, said properly equipped engines are operating lean of peak safely and reliably. But carbureted engines, and those without engine monitors, can and are being damaged from pilots running them too lean. “We’ve never had any issues with lean-of-peak operations in engines equipped with GAMI injectors and graphic engine monitors,” Middlebrook said. “In fact, we’ve got them on our own aircraft fleet. But we’re also seeing burned valves and cylinder damage in carbureted engines or those with single-point EGTs trying to fly lean of peak. If you don’t have matched fuel injection nozzles and a six-point engine monitor, you just don’t have enough information for lean-of-peak operations.”
Bill Cunningham, founder of Powermaster engines in Tulsa, Oklahoma, said the key to keeping cylinders healthy is keeping them cool. That can be accomplished with a rich mixture setting that pours unburned fuel through them, or a very lean setting that restricts fuel and creates less heat in combustion.
“I test fly each airplane and fly with the owners,” Cunningham said. “I show them how to set up properly rich of peak and lean of peak. Virtually every one of them starts flying lean of peak. Their engine monitors prove the benefits.”
Cunningham tells pilots to keep CHTs below 380 degrees at all times. That can be done in a climb by enriching the mixture, reducing the climb angle, decreasing the power, or any combination. In cruise, pilots of properly equipped airplanes can lean the mixture well beyond peak to accomplish the same thing.
“I disassemble a lot of engines and the differences are obvious,” he said. “With an engine flown rich of peak its whole life, you can take a knife and cut out the buildup on the exhaust ports in sheets. With engines run lean of peak, there’s less residue, less buildup on the pistons and in the combustion chamber. The components of the engine are cleaner. We hardly ever see warped valves on a lean-of-peak engine—and we get them on rich-of-peak engines all the time.”
As precise fuel/air metering systems, engine monitors, and pilot training in LOP operations becomes even more widespread, Cunningham said he expects GA pilots and aircraft owners to benefit from better engine reliability and longevity.
“We show owners how to keep their cylinders cool,” Cunningham said. “Cool temperatures are the whole story in extending engine life.”
Monty Barrett, a veteran aircraft engine builder and modifications expert, said he got interested in LOP operations during a flight with Braly from Oklahoma to Florida in Braly’s turbocharged Bonanza.
“The cylinders were running cool, in the 300s, and we did 190 knots true airspeed all the way to Pensacola,” he said. “We got there with enough fuel to fly home nonstop—and that’s when I really started paying attention.”
Barrett said high-time engines run LOP look dramatically different inside.
“They don’t carbon up and they’re not full of abrasive crud,” he said. “I just looked at a set of cylinders from an IO-550 with 1,300 hours on them. The engine had been operated lean of peak from the start, and the bores, guides, and valves were within new-parts limits.”
Rick Beach bought his Cirrus SR22 as a new private pilot with just 65 hours flying experience. He flew ROP during the engine break-in period and LOP during the subsequent 2,500 hours. The airplane’s six-cylinder Continental IO-550 required a top overhaul at 2,200 hours because of excessive oil consumption. But Beach said none of his engine problems have been related to LOP operations.
“I operate my engine lean of peak about 95 percent of the time,” Beach said. “I’ve had [oil-] fouled spark plugs, clogged fuel injectors, and magneto problems. But I’ve never had any engine problems associated with lean-of-peak.”
Steve Hanvey, chief executive at SATSair, an air taxi firm with a fleet of 26 SR22s based in Greenville, South Carolina, said his fleet has logged more than 50,000 flying hours using both ROP and LOP engine operating techniques.
SATSair tells pilots to run ROP at the engine’s best-power setting when confronted with strong headwinds and LOP with calm winds or tailwinds. That way, customers using the air taxi service can rely on groundspeeds of about 170 knots. SATSair operates a Cirrus Service Center that maintains SR20s and SR22s for the air taxi business and retail customers.
Hanvey said he flies the oldest airplane in the SATSair fleet, an SR22 with more than 2,800 flight hours, and flies both ROP at 75-percent power for top speed and LOP at 65-percent power for greater range and economy—whatever the situation calls for.
“I’m an analytical test-pilot type,” said Hanvey, a former Navy aviator and Beech test pilot and executive. “We perform engine trend monitoring on a daily basis, and we fly consistent profiles and operate each aircraft the same way. We see nothing from lean-of-peak operations that would shorten engine life in any way. The data on this subject is very clear and convincing.”
Ken Tunnell, a principal at Lycon Aircraft Engines in Visalia, California, said he was hostile to the idea of LOP operations at first—but he’s open to it now under carefully controlled conditions.
In addition to having precise fuel/air metering systems and engine monitors, Tunnell said pilots should only fly LOP at 65-percent power or less. That reduces or eliminates the danger of damaging engines through pilot inattention, faulty data, or other errors. He sees cylinder valves and guides destroyed by excessive heat— but it’s impossible to know exactly how or when the damage was caused.
“Some of these engines were set up so that they ran too lean at takeoff with the mixture full rich,” he said. “Or they could have been run too lean in cruise at high power settings. It’s impossible to tell from looking at them.”
Tunnell recommends 150 degrees ROP at high power settings. And sees no harm in flying 25 to 50 degrees LOP at 65-percent power or less. “Years ago, I was totally against lean of peak under any circumstances,” he said. “Now, I regard lean-of-peak operations as acceptable under certain circumstances.”
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