First Church of Combustion

Never operate your airplane engine lean of peak exhaust gas temperature. These guys aren’t buyin’ it

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IT HAPPENS EVERY TWO MONTHS. AT ABOUT NOON on Friday people begin to arrive at the little airport that serves Ada, Oklahoma, population 16,000, lying roughly 80 miles southeast of Oklahoma City. Alternately griddle-flat prairie and gently rolling hills, Ada probably has a little picture of a longhorn cow and an oil well next to its name on those schoolbook maps. But the people flying in here today don't deal in cattle or petroleum.

They come from across the nation, many of them flying their own aircraft. Beech Bonanzas and Barons are the most numerous types out on the parking ramp. But here's a single-engine Cessna on stork-tall amphibious floats, its registration and little red maple leaf indicating that it flew here all the way from Canada. These visitors share one thing: They're pilots who fly aircraft powered by reciprocating engines. General aviation aircraft. The little guys.

Some of them are here because they've heard that they've been running their engines wrong all these years and they want to learn how to do it right. Some have thousands of hours, and others are barely starting out. Some are openly skeptical, and some may even harbor a private urge to unmask all that is shown and said here as fraud and sham so they can depart vindicated. Some of these people will have a hard time accepting what they will hear because it is hard to admit you've been doing something the wrong way for a very long time, especially if you are a pilot.

They have come to a class with the vaguely worded title "Advanced Pilot Seminars." The session opens on Friday evening and ends on Sunday afternoon, and the lessons are delivered in intense doses. But compared to how airmen and -women have been trained in the past, what goes on here is really closer to the founding of a religion. Call it the First Church of Combustion.

Its bishop is George Braly (pronounced BRAW-ly), an aeronautical engineer and attorney with a Wilford Brimley mustache and a booming voice cultivated in the courtroom. John Deakin and Walter Atkinson have signed on as disciples since being converted in the mid-1990s, when they were the first, aside from Braly himself, to test the tenets of the new gospel in their own airplanes. Deakin has the wise look of a wood owl. He retired in 2001 as a captain with Japan Air Lines, and the Boeing 747 time in his logbook adds up to more than four years. Atkinson's day job is dentistry, but he is also rated as an airline transport pilot, airframe-and-powerplant mechanic, and flight instructor. In the right light, he's a pretty good double for actor Fred Willard.

Braly and the two disciples promise the converted a life of airplane engine happiness, with cooler operating temperatures, fuel savings on the order of three gallons per hour for a typical six-cylinder engine in a Beech Bonanza, and reduced life-sapping carbon deposits on the valves and pistons. All they ask is that the believers ante up for precision engine monitoring systems.

The three pilots became friends while they were exploring the same subject they are about to preach in the classroom, which has filled with 36 students, each leafing silently through a fat three-ring binder. On the binders' covers is the name of the course: "Engine Management Made Easy." The $995 tuition covers all meals (except one on Saturday night), which are taken on site to cut down on travel time to restaurants. And the students will find that they need every minute of classroom time they can get. Here is some of what they'll learn:

All reciprocating engines that burn gasoline are ruled by the incontrovertible laws of chemistry and physics. They produce power by drawing air into a cylinder, mixing it with a combustible amount of gasoline, sealing the cylinder, compressing the mixture, and igniting it at just the right moment with an electric spark. Most modern engines use some method of fuel injection to mix the gas and air. What's different about aircraft engines is that they operate at widely varying altitudes: As the airplane climbs, the air becomes thinner. With less air to support combustion, the amount of gasoline to be mixed with the oxygen molecules must be reduced accordingly. Which is why airplanes have an engine control you'll never find in a car: the mixture control. Whether it's a knob or a lever, the mixture control adjusts the flow of fuel to all the engine's cylinders.

Student pilots who train in general aviation aircraft have traditionally been taught that at some altitude during the initial climb (typically 3,000 to 5,000 feet), they should move the mixture control from full rich, the setting for takeoff, to lean, then even farther to lean after they pull back the throttle to the cruise-power setting. While leaning at cruise, they learned to keep a sharp eye on an instrument that displays the temperature of the engine exhaust gas. The instrument, the exhaust gas temperature (EGT) gauge, sometimes uses a graphical bar display. (Older gauges used a needle on a dial.)

When the gasoline-air ratio is such that combustion has used up both fuel and oxygen, combustion occurs at the highest possible, or peak, temperature. (This mixture is described by chemists as "stoichiometric.") If the mixture is rich, containing an excess of fuel, or lean, containing an excess of air, the temperature of the combustion process drops. In managing mixture control, it is not a matter of what the absolute temperature of combustion is but where the mixture is relative to peak temperature, which serves only as an easily measurable reference point. Aircraft engines are not operated at peak EGT despite the apparent chemical perfection of combustion there. Cooler operating temperatures are desirable; on that all agree.

About George C. Larson

George C. Larson served as editor of Air & Space from 1985 to 2005. He is currently an inactive pilot, but holds a commercial pilot's license, with instrument and multi-engine ratings. He is between airplanes at this time, but has owned or operated a Grumman American AA-5B Tiger and a Mooney 201. He has been writing about aviation since 1972, when he joined the staff of Flying Magazine.

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