Back to the Future
The Wright Aeronautical Division (WAD) Recommended Leaning Procedure
by George W. Braly
Other than "downwind turns" and "logging time," there is no other subject in general aviation that is as likely to cause a serious debate among a handful of pilots and mechanics, as is the subject of "leaning" aircraft piston engines.
A review of recent articles in various publications on the subject of leaning piston aircraft engines, and extensive conversations with pilots and mechanics, continuously reveals more misinformation, consistently presented as "gospel," than any subject this writer has observed in some 30 years as a pilot, flight instructor, and aeronautical engineer.
Let us start at the top. In a recent FAA written exam for the Air Frame and Powerplant certificate, the FAA asked:
Q. What happens when an opposed piston engine is run lean of peak?
The FAA's "right" answer to this multiple choice question was "The cylinder head temperatures get hotter."
That answer is, of course, simply false. My sympathies to the test applicants, both those who got it "right" and those who got it "wrong", but were really "right". How many A & P's, pilots and others would have given the same "right" answer?
During the past six months, in connection with General Aviation Modifications, Inc.'s (GAMI) daily business, my associates and I have talked at length to well more than a thousand pilots and mechanics flying or maintaining high performance single and twin-engine piston powered aircraft. From this experience it is fair to say that well more than 75% of those pilots and mechanics have only the most basic understanding of this subject. Even that basic understanding is clouded by misinformation, often strongly believed, --- but still wrong or misleading. Frankly, this lack of knowledge extends into many, but not all, of the premier engine rebuilding shops around the country. It turns out, this is one of those areas of knowledge, where a "little bit of knowledge" can be an expensive or even dangerous.
A large part of the problem in understanding how and why to properly lean your engine is rooted in the gradual erosion of the language and terminology we use in the discussion of operating and leaning aircraft piston engines.
Before taking you "Back to the Future," let us begin by defining some terms so that we can all read the rest of this article using the same understanding of the same terms.
- Lean and Rich
Leaner, a comparative term, means little, unless you relate it to something else. Richer; is the same. If someone says the engine was "run too lean," please ask: "Too lean? Compared to what?" In short lean is a relative term; so is rich. For the purpose of the following discussion, the word lean or rich will always be referenced to the peak exhaust gas temperature, or peak EGT.
- Peak EGT
Peak EGT is the highest temperature that is reached by the first cylinder to reach its peak exhaust gas temperature, during a full or partial "lean cycle." In an engine with excellent balance among the cylinder fuel/air ratios. Peak EGT will occur for all cylinders at the same, or nearly the same fuel flow, and will occur at the same fuel flow as the Peak Turbine Inlet Temperature (T.I.T.) We have found only two factory stock engines currently on the market (the Teledyne Continental IO-550-G and the TSIO-520-BE) that consistently come close to even moderately good "balance" among the cylinders' fuel/air ratios. Even those two engines can stand significant improvement.
- Lean Cycle
A full "lean cycle" refers to the process by which the pilot progressively leans the mixture. Starting at a mixture setting that is 100F or more rich of peak EGT, the pilot leans the mixture until all cylinders of the engine have reached their individual maximum EGT values. The leaning cycle continues further until all exhaust gas temperatures are at least 50F lean of their respective maximum EGT values.
- Hot/Cold cylinder
Some pilots confuse a "hot" cylinder with a cylinder that has a high EGT reading. The cylinder with the hottest EGT is not necessarily, or even often, the cylinder with the hottest cylinder head temperature (CHT). That usage is very confusing. When reference is made to a "hot" or "cold" cylinder, in this article, it refers to the temperature measured by the thermocouple embedded in the aluminum cylinder head, and it has nothing to do with the temperature of the exhaust gas flowing out the exhaust stack.
- Power and Best power
Most pilot's think of "power" in terms of a percent of the maximum power the engine is capable of producing. Thus, when a pilot says, "I run my 300 HP IO-550 engines in my Baron at 65% power," what he should be thinking is that he runs his engine at a manifold pressure (MP) and RPM that will produce 196 (0.65 X 300) horsepower, when the mixture is adjusted to "best power." Best power is usually found somewhere around 75 F rich of peak EGT Because horsepower varies with mixture, this terminology becomes "fuzzy" and unclear. In the remainder of this series of articles, reference will be made to a certain number of horsepower; i.e., 237 HP, or 196 HP, rather than a percent of power, unless the context is otherwise clear.
- Brake Mean Effective Pressure (BMEP)
This is a somewhat artificially defined engineering term. It is defined to be equal, to a theoretical mean pressure inside the cylinder during a combustion cycle that would produce the same horsepower as would be measured directly on an engine test stand. If you know the torque of an engine at the crankshaft, the RPM, and the displacement, then you can calculate the BMEP and the horsepower directly. There are simple equations for this, but they are not necessary for our purposes.
Now.... Back to the Future!
Most "modern day" general aviation pilots are horrified at the thought of running their expensive engine "too lean." Probably the most common "false statement" heard about "leaning" from most general aviation pilots is, "I won't run my engine lean, because I will burn it up!" Running an engine lean of "Best Power" may cause a significant temperature increase, but running an engine lean of peak will never cause heat related damage to an engine. Perhaps pilots' real concern ought to be that they don't run their engines nearly lean enough!
Brake Mean Effective Pressure (BMEP) gauge.
The instrument indicates that the current BMEP is approximately 143 psi and the engine is producing approximately 940 horsepower at 2000 RPM. These guages measured crankshaft torque by measuring oil pressure in the engine planetary gear drive system. This type of gauge was the primary tool used to lean the large radial engines in service late in the piston-powered airliner era.
In any event, let us now borrow a Hollywood phrase, and go "Back To The Future" -- back to the end of the era of big piston-powered DC-7s and Lockheed Constellations, in the early 1960s. Have you ever asked yourself how those engines were leaned? Did they run those big, super-charged or "turbo-compound" engines 50, 100 or 150 F rich of peak exhaust gas temperature? Did they even have an EGT probe in those engines? "Inquiring minds want to know!"
A bit of history
Approximately one year after American Airlines began operating the Wright R-3350 Turbo Compound Engine, one Norman Rice, Supervisor of Operating Manuals, gave a series of lectures to Flight Crews and Maintenance Personal.
The lectures were based on material supplied by the Wright Aeronautical Division (WAD) of the Curtiss-Wright Corporation. From that lecture material, written manuals were developed. Those manuals survive to this day in old file cabinets and attics around the world.
There is a wealth of forgotten knowledge in those materials -- knowledge that clearly condemns the typical current "conventional wisdom" about leaning practices as being, for the most part, either misunderstood, or simply "wrong".
What, Exactly Was The WAD Procedure?
The WAD Recommended Leaning Procedure was a method used to lean those engines to a point on the lean side of what we now commonly refer to as peak EGT This mixture setting also happened to provide the very best brake specific fuel consumption, coupled with very cool cylinder head temperatures.
How Successful Was It?
Isn't it true that those big radial engines were always failing and causing trouble? Not at all. On the contrary. After the bugs were ironed out, and the crews came "up to speed "operating those engines, they ran very reliably. This information comes from a number of people with personal experience with the long-term maintenance and operation of those engines. By the end of the "piston" era, those engines were routinely operating, depending on the specific model, to TBOs of more than 2,000 hours. Some airlines consistently operated more than 3000 hours between overhauls. Most or all were using some variation of the WAD Recommended Leaning Procedure.
Most of us would give some part of our anatomy to achieve routine TBO's out of our IO -470s - 520s, -550s,-540s, etc, of 2,000 hours. We would be especially happy to get 3,000 hour TBOs out of our "TSIO" versions of those engines. The bottom line : The "WAD Recommended Leaning Procedure" demonstrated its operational merit over millions of hours of successful engine operation. Now that is some pretty serious experience.
Different Instruments -- Then and Now.
Figure 1 is a picture of an instrument that you have never seen in your Bonanza or Baron. It is a "BMEP" gauge. It is really only a simple torque meter that measured the torque in the planetary gear box. Space does not permit a detailed explanation of how it worked, but suffice it to say, it measured the torque on the crank shaft and converted that number to BMEP. One could then rotate a dial on the face of the instrument and manually set in RPM.The instrument would directly read "horsepower".
We do not have a BMEP gauge in our aircraft, in this "modern" day. We do have something that ultimately will accomplish the same purpose, if we just learn how to correlate its use to achieve the same engine operating conditions that were obtained by using the BMEP gauge. That instrument is the ubiquitous, but commonly misunderstood, EGT gauge.
The WAD Recommended Leaning Procedure -- On Your Engine
How did they do it? Figure 2 is a simplified example of the WAD Recommended Leaning Procedure.
Instead of using the power charts from a Wright R-3350 Turbo Compound Engine, I have adapted the procedure to the power chart from a Teledyne Continental IO-550. For the moment, just assume that you, too, have BMEP gauges for your TCM IO-550 engines in your Baron (if you only fly a single engine aircraft, smile and take the "upgrade"!) You be the pilot. Let me be the flight engineer, along in the right seat, to manage the power.
Look at figure 2. Starting at the top of this graph, there are curves representing EGT, CHT, horsepower, and brake specific fuel consumption (BSFC). These curves are all plotted against fuel flow. There are two power curves plotted. The red curves represent the noted engine parameters during a lean cycle for the engine MP and RPM settings that give 237 horsepower when the engine is leaned to its best power mixture.
The blue curves represent the engine parameters for the same engine, but at lower MP, and which produces just 210 horsepower when the engine is again leaned to its best power mixture. I have not included the exact MP and RPM values, because they are really rather unimportant to this discussion. Pilots tend to get distracted by those details as they try to compare those numbers to their own operating practices. However, please understand, the data is real data, digitized from official Continental power charts, and simply re-organized for this presentation.
The first point  represents the engine operation after our Baron is leveled off in cruise flight. Under the WAD procedure, the engine would be run for a short time in the "auto rich" condition, and certain basic engine operating data would be recorded by the flight engineer. This is the equivalent of our hypothetical Baron reaching cruise altitude at, for example, 4,500', and letting the engine run at a nominal 70% cruise power at 150-200F Rich of Peak EGT.
Your flight engineer is going to lean the mixture to point , which represents best power, or, in the WAD terminology, the peak BMEP for the given MP and RPM.
Referring to our hypothetical BMEP gauges, we would record a peak horsepower of approximately 210 horsepower (See the blue brake horsepower curve in figure 2. ) Then your flight engineer (probably to your horrified reaction) will continue to lean the engines until the BMEPs on our imaginary gauges have fallen to a value of 10% LESS than the maximum BMEP (i.e., 10 % less than best power). In this case, that would be point , at approximately 189 horsepower.
Now, you say, "Wait a minute! This is just trickery. I bought my whiz-bang-go-fast aero-machine to GO FAST! I want my 210 horsepower back!"
At this comment, your trusted flight engineer says, "OK, Captain. Stand by, because we aren't finished yet!" Now the flight engineer in your Baron is going to reach over to the throttles and simply add back approximately 2" of manifold pressure. This takes you to a place on the chart that is marked as point . At this point, we are, once again, producing 210 horsepower (actually, a bit more, around 215 horsepower, but the difference is unimportant). Notice, the mixture was not adjusted, just the throttle position.
After a few minutes, you will notice that the airspeed has settled down and you are cruising where you always have cruised, when you were running at "best power" at your original manifold pressure and RPM.
But what about the Cylinder Head Temperatures? The EGT's?
So here we are, all set up according to the WAD Recommended Leaning Procedure in our Baron. But what are the CHTs doing? How hot are the cylinder heads? Take a look. Just follow the green line in figure 2 upwards until you intercept the red cylinder head temperature curve. Continue further up the green vertical line, and you will find your exhaust gas temperature. Your EGT is now running about 40-45F on the cool and lean side of peak. And your cylinder head temperatures? Your cylinder head temperatures are now running about 35F cooler than they were running over at "Best Power"!
What happens to the fuel flow?
Good question. Follow the same vertical green line down to the bottom of the chart, and you will see two sets of numbers. The blue fuel flow numbers correspond to the blue power and engine operating curves. The red fuel flow numbers correspond to the red power and engine operating curves.
When we were running the engine at 210 horsepower, at point  our fuel flow was approximately 93 pounds/hour (blue fuel flow numbers). But when we moved over to point  with the mixture control, and moved up to near the red curve at point , our fuel flow, at the same or even a bit more horsepower is now down around 84 pounds/hour (red fuel flow numbers below point ). And it all happens while the engine runs at dramatically cooler operating temperatures.
Think about this for a while, then ask yourself three questions:
- Is the engine running cleaner, with fewer residual hydrocarbon deposits?
- Is the engine running cooler?
- Are cleaner and cooler better?
But fuel is cheaper than engines. I don't want to burn up my engine!
Good!. By using the WAD Recommended Leaning Procedure, you will be running your engine cylinder heads about 35 F cooler than you would otherwise, using your traditional leaning procedure. Go back and re-read the three questions, above.
Ok, maybe running the engines the "WAD" way is better, but how do I do it without the BMEP gauge?
Go back to the chart. While you do not have the BMEP gauge, you do have something much better, more accurate, and very repeatable. It is something that the airlines and their pilots and flight engineers did not have on their engines. That instrument is your common EGT gauge. And, your EGT will let you set up and run your engine exactly like the WAD Recommended Leaning Procedure, only you can do it much more directly and more precisely.
If you look again at figure 2, you will see that when you get to the final cruise setting following the WAD Recommended Leaning Procedure, you are also running your engine about 40 F on the LEAN SIDE OF PEAK. That observation, is one of two key insights that gives you the ability to lean and get to the same final cruise mixture setting as is accomplished with the WAD Recommended Leaning Procedure. You can reliably get to the same final cruise mixture setting, as is accomplished with the WAD Recommended Leaning Procedure, if you just lean your airplane to approximately 30 F to 45 F lean of peak EGT.
Over the past two years we have done a considerable study, including substantial flight test data collection with a sophisticated 64 channel data acquisition system over the last two years. All of the data we have collected at GAMI confirms what is obvious from looking at the engine power charts. You can accomplish the same goals as the WAD Procedure, just by leaning to a point that is approximately 30F to 50F Lean of Peak. For turbocharged engines, that point will be shifted a bit further lean of peak, to approximately 40F to 60F lean of peak. For normally aspirated engines operated at lower power settings, that point will be as little as 15 F to 30F lean of peak EGT./p>
Why haven't I heard about this already and why doesn't everyone do this?
Again, good questions. The good news is that the WAD Procedure is a well-proven procedure. It has been proven in several hundred millions of hours of high powered piston engine operation.
The bad news? It simply won't work well with most of our current generation of horizontally opposed piston engines, at least without some modification of the fuel systems to balance out the fuel/air ratios among the cylinders.
Our research has shown that the fuel/air ratio in the typical Teledyne Continental IO or TSIO -470, 520, and 550 engine varies from cylinder to cylinder by as much as 8-12%, from leanest to richest. The Lycoming engines, do not appear to be consistently better. Lycoming engine cylinder to cylinder variations in fuel/air ratios apparently happens due to a combination of different reasons.
Ultimately, a major portion of the answer to the question as to "why" everyone, including Continental and Lycoming do not routinely recommend the WAD Leaning Procedure is that if you try to do so, your engine will run rough. It does not run "rough" because of the reasons you have probably heard. It does not run rough due to "lean misfire" or anything like that. It runs rough on the lean side of peak only because of the variation in fuel/air ratio from cylinder to cylinder in your engine. This variation in fuel/air ratios causes cylinder to cylinder variations in horsepower on the lean side, but not on the rich side of peak.
In part two of this article You will see why this problem exists, explore the manufacturers' leaning recommendations as they relate to the WAD Procedure, look at more graphs, and find some solutions to let you take advantage of the WAD Recommended Leaning Procedure, at least for most of our engines.