Mixture Control
by LIGHT PLANE MAINTENANCE staffPart 1 - Mixture Control Systems
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Here we go again, another article on mixture
management destined to tell me things I already know. Well, maybe.
It's entirely possible that some information will seem repetitious,
but, then again, redundancy in aviation isn't such a bad thing
either. judging from some reader questions, such a revisit to the
topic is warranted.
Mixture Control is written from the pilot's perspective, by a professional test-pilot, and is designed to answer the why, when, where and how of mixture control and proper leaning procedure for various fuel delivery systems. It is an encyclopedia of fuel management. To the layman, it translates into verse as "How to avoid burning a hole in your wallet". Inspite of the proliferation of digital instrumentation, the Basic EGT is still an outstanding instrument for monitoring Fuel Flow vs Percent Power. Most of the material in this article is based on this basic level of instrumentation and control. Units of measurement remain in the original Imperial units (oF, for example) because over 60% of aircraft in use are more than 50 years old. |
Air-Fuel Ratios/Distribution
In general terms, mixture is defined as the ratio of air to fuel by weight (or more accurately, mass). Most engines will burn air-fuel ratios of 8:1 to 18: 1. Eight-to-one being very rich and eighteen-to-one being very lean.
The "chemically correct" (otherwise known as stoichiometric) mixture is about 15:1. This is the mixture ratio where you would expect to find peak EGT in a perfect burn, but atomization inefficiencies put the peak EGT ratio probably closer to 13 or 14 to 1.
You might say, "So what's this gobbledygook about ratios? I don't have a direct air-fuel ratio indicator in my plane." That's true, but it will help in understanding the different leaning procedures on different fuel metering/distributing systems.
The most common fuel distribution systems found in general aviation are: carburetor, pressure carb, Continental fuel injection, and Bendix fuel injection. A brief explanation of the operating principles of each system will be important in understanding the flight management portion of the pilot's duties.
Carburetor
The carburetor is a fairly simple device that meters fuel according to the pressure difference between the downstream side of the fuel jet and the net pressure of the fuel in the float bowl (affected by net bowl vent and bowl fuel head pressure).
This is accomplished using Bernoulli's Principle (venturi in the carb throat) and is affected most by the volume of air flow through the carburetor. The system is, therefore, not very good at compensating for changes in air density (weight) caused by any factor, most noticeably, altitude.
The fuel jet (main jet as it's sometimes called) is calibrated to give the correct rich mixture for the particular engine at full power, sea level, on a standard day. A variable valve in series between the float bowl and the calibrated jet most usually accomplishes mixture control. (Although some accomplish mixture control functions by varying the bowl vent pressure, either way, the result is the same).
Pressure Carburetor
Don't let it fool you. The name implies similarity to the aforementioned carburetor but the similarity ends there (as anyone who has had to pay for one will tell you).
The pressure carburetor is a fairly complex unit that controls fuel/air ratios by sensing pressure differences in a venturi and ram air pressure in what's called a bullet (for what it does to your pocket book if you have to replace the bellows inside).
The ram-air pressure is highly dependent on air density (weight) and is therefore much better at altitude compensation than the carburetor. Mixture is controlled by manual adjustment of an internal air control valve that varies fuel discharge pressure. This is essentially a single-point fuel injection system [similar to the Throttle-Body Injection (TBI) system used in GM Astro Vans, etc]
Continental Fuel Injection
in the naturally aspirated engines (except the 10 & G10-550) this system is purely mechanical. It determines fuel/air ratios solely by reference to pump RPM, throttle, and mixture valve position. It, therefore, has no way of sensing density altitude whatsoever.
Once the pilot sets up the engine for a particular air density (altitude), minor throttle and RPM adjustments will not require a mixture adjustment. Properly adjusted and rigged, this system provides correct rich mixture for the particular engine at full power and climb at sea level only.
Any operation at altitude requires the pilot to manually control the mixture to the optimum setting for the particular MAP and RPM. This is important; any significant change in air density (generally altitude) requires a corresponding mixture adjustment by the pilot.
Turbocharged versions of the Continental Fuel Injection System use an aneroid to sense upper deck pressure and adjust fuel pressure, hence fuel now, accordingly, and therefore don't require the constant adjustment with air density changes. The GIO and I0-550 use a similar aneroid but sense ambient air (hence altitude) instead of upper deck pressure.
Bendix Fuel Injection
These systems are very similar, in operation and design, to the pressure carburetor (including cost, ouch). Enhancements have been made, however, that make them more accurate and easier to operate.
This injection system, like the pressure carburetor, is a fairly complex unit, which controls fuel-air ratios by sensing pressure differences in a venturi and ram air pressure in the bullet.
Most of these systems, however, do not have the AMC (automatic mixture control) bellows in the bullet. This makes the unit somewhat sensitive to air density (weight) changes, providing some altitude compensation. However, manual mixture control is still required by the pilot at altitude for optimum performance.
A few of the Bendix Servos do have the AMC bellows in the bullet (some turbocharged engines) and compensate almost totally for the air density changes encountered from sea level to as high as 30,000 feet and sometimes higher.
Operation Block to Block
So how do these differences in systems affect the way you operate the engine, in particular the mixture? Let's go over four basic engine operation phases and how to operate each system in each phase.
The four operational phases covered will include; taxi & run-up, takeoff & climb, cruise, and descent & landing. First, though, let's look at the basic mixture requirements of each phase from an operational standpoint.
Taxi & Run-up
The main consideration on taxi and run-up mixture control is smooth operation and the prevention of spark plug fouling. A properly leaned engine can easily mean the difference between a good mag check and a bad one. Fouled plugs or an overly rich mixture will decrease power and cause bad mag checks. Ground leaning will help keep the plugs clean.
Takeoff and Climb
NOTE: All properly adjusted, supercharged and turbocharged engines are to be at full rich for takeoff. There are two major concerns in adjusting and monitoring the mixture in takeoff (full power operations) and climb mode. Power and Exhaust Gas Temperature (EGT) or Turbine Inlet Temperature (TIT).
Best Power mixture comes at about 125oF. Most general aviation engines have the capability to carry away the heat generated at Best Power mixtures up to about 70-75 percent power. Above this power setting, very few, if any, of the engines and engine installations we operate can use this mixture setting because of the high levels of heat energy being passed through the engine.
This is especially true for takeoff and climb. The higher RPM and manifold pressures increase the heat passing through the engine in a given time frame (i.e. more combustion and exhaust scavenge events per second), putting more heat into the engine.
There are two ways to decrease this heat and cool things down. Reduce power (fewer combustion-exhaust scavenge events) or cool the charge in the combustion and exhaust events. The idea of takeoff and climb is power, therefore, the first option is not so desirable.
By default, the other option becomes the method of choice-cooling the combustion-exhaust event. This is done nowadays by enriching the mixture. Some of the "tried and true" will remember ADI systems (Anti Detonant Injection-usually a mixture of water and alcohol) in some of the old round engines and V-12s. ADI does the same thing as enriching the mixture.
At high or emergency power settings, ADI fluid was injected into the intake air. Some models of the R-2800 (e.g. F4-U and P-47D) could pull about 80 in. Hg MAP with the ADI system engaged and only about 55-60 in. Hg with it off. It got them an extra 300 to 400 horsepower when it was needed most.
Contrary to popular belief, actual EGT or TIT temperatures are important to monitor, especially at takeoff and climb. Many an engine has annealed the rings because combustion temperatures got too hot. Rings are annealed (lose their hardness) by prolonged and elevated temperatures.
As temperatures in an iron alloy (used in piston rings) increase, the time to anneal the alloy decreases. Therefore, the less time spent at elevated temperatures, the better. In practical engine operating terms, keep CHT's below 400oF and TIT's below 1425oF for this phase.
EGT temps will vary according to probe placement, but a good rule of thumb in climb settings is to set the mixture 200oF rich of the peak mixture temperature you would get at the 70 percent power setting. This will give good power but adequate cooling for climb.
Those of you running intercooler systems on your turbocharged engine take note: If your fuel system does not fully compensate for changes in air density, the fuel system must be set up again after installation of the intercooler.
Fuel systems that do not sense air density from the intercooler will run the mixture too lean in takeoff end climb (in the neighborhood of 1500oF or higher) and will anneal the rings in 50 to 100 hours (oil consumption starts to go up and compressions start to drop).
These systems must be set to give a full fuel flow at an equivalent full power MAP (generally 3-4 in. Hg below redline). This compensates for the higher air density brought by the intercooler. I have brought many a plane back to the shop to pull all six cylinders for this very reason.
Cruise
Best Power is used mainly in cruise when you want speed and is shown in power charts under the "Best Power" curve. There is also a "Best Economy" curve that is a leaner mixture used with the same power setting and gives slightly less power than the "Best Power" setting (by experience, somewhere around 2-3 percent lower fuel flow or 3 to 5 knot airspeed loss).
The name is as implied. It gives the best economy for the chosen power setting but also gives higher EGTs. "Best Economy" mixture settings generally are not given for power settings above 65 percent because of the potential for preignition and detonation.
Safe EGT levels vary directly with the engine power setting. At low power settings (65 percent and less) the mixture may be adjusted to give best economy because of the engine's ability to carry away the heat energy' at these lower settings.
At higher power settings, the mixture is adjusted to allow for extra fuel to help cool the engine. This is because of the engine's inability to carry away the heat energy developed at lean mixtures with high power settings (70 percent and higher).
This is also the reason most operating handbooks specify seemingly overly rich mixtures for climb and high cruise power settings, especially on the larger engines.
Descent and Landing
Descent is similar to climb in that the mixture must be watched closely in systems which don't compensate well for changes in air density (carburetors and naturally aspirated Continental Fuel Injection systems). The main problem here is the reverse of climb: mixture becoming too lean as air density increases with a decrease in altitude.
In descents to the pattern altitude from 10,000 to 12,000 feet, if the mixture is not increased, opening of the throttle at level-off can be accompanied by spits and sputters and possibly an engine that quits from fuel starvation.
Good rule of thumb here: set the mixture to maintain the same EGT/TIT as cruise minus 50oF to the rich side in high power descents (65 percent or higher) and smooth engine running in low power descents (50-60 percent). If you don't have an EGT gauge, enrich the mixture to keep the engine smooth plus a little extra, as you enter the pattern or level off.
Most normal descents are accomplished at low power (50-60 percent) where running close to peak EGT (25oF) isn't a problem because of the lower temps. A smooth engine is a happy engine and temps are kept up to help stave off the "shock" cooling gremlin.
Density compensating systems will do a much better job of mixture control in descent but still need monitoring and an occasional adjustment. For landing, the mixture should be set for a position that will allow for full power operation for the particular system. This is to provide sufficient fuel for an immediate response if a go-around or evasive maneuver is needed.
Flight and Mixture Control
In [the next section], we'll go through these four flight phases for each particular fuel system. Keep in mind the time delay (5-10 seconds) for an EGT or TIT system reading to stabilize wherever accurate mixture adjustments are required. Also note that it is important to have your EGT or TIT gauge calibrated regularly.
Have it checked at each annual if possible, and sooner if you fly more than 150 hours between inspections. This is especially important for turbocharged engines and naturally aspirated engines that fly regularly at altitudes below 5,000 feet MSL.
Or those of you who have Lycoming engines, it wouldn't be a bad idea to get a copy of S.I. 1094D, fuel mixture leaning procedure, and look over the procedures for your particular engine. You'll find that this Instruction will vary substantially from the POH or Airplane Flight Manual for larger, high-horsepower engines.
2. Mixture Control by Type
[In Part 1] we gave a general mixture operational overview. In parttwo of this effort, we will detail the power settings and leaning procedures for each fuel system type in various flight modes. Note that this was written by a very experienced test pilot of GA aircraft, but mixture management is still an area of some debate.
Carburetors
Note: The following advice is for naturally aspirated engines only. All turbocharged and turbo-normalized, carbureted engines must be at full rich for takeoff.
Taxi and Runup: Leaning during taxi is sometimes required to prevent plug fouling and/or to provide smooth operation, especially at highaltitude airports. At altitude (3000 feet and higher), leaning is definitely required to get an accurate mag check.
Rich mixtures can give mag drops of 200 RPM and of only 75 to 100 RPM when leaned. To find the best mixture for mag checks, lean until the engine gets rough then enrichen back to highest RPM. Readjust the throttle to the mag-check RPM and do your check.
Some carburetor systems provide some challenges to this simple procedure in cold weather. The most common example that comes to mind is the 0-470 in the C180 and C182. This is a great engine/airframe combination but can get cantankerous in very cold weather.
The carburetor sits down away from the engine where it can't pick up much heat. Therefore, it will ice up a little easier and in very cold weather (low teens and subzero) has some problem getting the fuel to atomize once it is distributed into the induction airflow.
This can cause lean mixtures of sufficient magnitude to give really rotten mag drops; bad enough to fool some into thinking that the mag has just gone south with the geese. The best way to compensate for this malady is to pull the carb heat on, lean the mixture as previously described, adjust throttle to the mag check RPM and try again, all with carb heat "on."
The mag drop will be a little higher than normal (125-150 instead of 75-100), but will be smooth if everything with the mags is okay. This is to be expected because of the hotter induction air (causing lower engine power) with carb heat in the "on" position. (Carb heat in for TO.)
Takeoff and Climb: Leaning for takeoff (non-turbo) is a very important pre-takeoff item at high-altitude airports (higher than 3,000 feet). Unfortunately this is a practice still not exercised by some. Proper leaning at takeoff will shorten takeoff rolls and increase climb rates.
At the high-altitude airports in the Rockies, for example, proper leaning can mean the difference between a successful takeoff or ending up in the trees at the end of the runway. Proper high-altitude leaning can shorten takeoff rolls by 500 feet and increase climb rates by 200-300 feet per minute.
Leaning should be accomplished just before the takeoff roll. At full or near full throttle, lean to peak RPM then enrichen only slightly (barely enough to see an RPM drop, no more than 25).
This will be best power for takeoff, plus just a little extra fuel for cooling (cooling isn't quite as much of a problem because of the lower temperatures produced at the lower power found at higher altitudes). The same procedure should be used for fixed-pitch or constant-speed props. If an EGT is used, it should be set for about 150oF rich of peak. At sea level, of course, go full rich.
Climb is not much different. The idea here is to keep the mixture at best power plus about 50oF Mixture adjustment (re-leaning) is required about every 2,000 feet of altitude change for maximum performance and smooth operation above 5,000 feet density altitude.
As altitude increases, and engine power decreases to 65 percent or lower, the mixture can be leaned much closer to peak EGT (within 50oF). This will help maintain power at the lower settings where best power mixture gets closer to peak EGT.
Cruise: This is the easy part. Set cruise power and lean to 50-75oF rich of peak for settings of 65 percent or greater and 25-50oF rich of peak for settings below 65 percent. This will give you the best mixture setting for longevity of your engine and good performance for cruise.
As always, if the engine gets rough before you reach peak EGT, enrichen the mixture until the engine gets smooth again and then enrichen a little more. This is also the procedure to follow if you don't have an EGT gauge. This will put you slightly on the rich side of peak EGT.
If your engine is turbocharged (TR182, for example) run 75oF rich of peak TIT at 65 percent power or greater, and 50oF rich of peak below 65 percent. Anything above 75 percent power, should be at least 200oF rich of peak or full rich, and no hotter than 1500 F, whichever comes first.
Running at peak EGT anywhere above 55 percent power is not recommended because of uneven fuel distribution and this high temperature operating zone has the smallest margin for mixture errors.
Again, some engines can be very difficult in subzero (Fahrenheit) weather. You may have to fly your 0-470 with the carb heat on just to get the mixture to atomize and the engine to run smooth. This is where a Carb Air Temp gauge comes in real handy.
Descent and Landing: Since the carburetor is rotten at air density sensing, the mixture will need to be enriched every 2,000 feet or so during the descent. If you have an EGT gauge, you can enrichen the EGT 50oF lower than cruise and maintain this EGT all the way to the pattern.
Once you level Off, enrichen the mixture to the approximate position (by your own experience) that you would need if you were taking off at the same airport. From here on out, the mixture can stay where it is until shutdown.
Pressure Carburetors
Taxi and runup isn't much different than normal carbureted engines. Lean for smooth running to prevent plug fouling and follow the same runup procedures. At mag-check RPM, lean the mixture until the engine gets rough or loses RPM, whichever comes first, then enrichen back to highest RPM. Readjust the throttle to the mag-check RPM and do your check.
Takeoff and Climb: For all high-power operations (75 percent power or greater) below 5,000 feet density altitude, the mixture control should be full rich. All takeoffs (except density altitudes of about 8,000 feet or higher) should also be full rich.
The pressure carb is pretty good at sensing actual air density and adjusting mixture accordingly for fulland high-power operations. (Some installations even specify no leaning in any phase of flight; the pressure carb should do it all.
There are not many of these installations left flying. If you do have one, follow the book. These mixture controls are not made for leaning but are mainly a fuel "on" valve).
For climbing at higher density altitudes where power is limited to below 75 percent, the mixture can be leaned. For long cylinder/piston/ring life, keep EGTs at 1425oF or lower, preferably around 1350oF and do not exceed 400oF cylinder head temp.
Cruise: Setup cruise mixture to no less than 50oF rich of peak at power settings of 65-70 percent and 75 to 100oF rich at 70-75 percent power. If you don't have an EGT, run book settings plus one-half to one gallon per hour more to extend cylinder life. In all cases hold CHTs no more than 400oF.
At power settings of 50-65 percent some manufacturers will allow you to run at peak EGT. This may be okay for 50-55 percent, but it will prove easier on your engine (and pocket book) to run, at minimum, 50oF rich of peak at 60-65 percent. For supercharged or turbocharged engines add 25oF to the above EGT/TIT figures at minimum.
Descent: For descent, subtract 50oF from the cruise EGT-TIT by enriching the mixture. The AMC bellows should keep the air-fuel ratio pretty stable throughout the entire descent.
Some slight adjustment may be required occasionally, but a healthy pressure carb will compensate for air density changes, hence altitude, marvelously. Before applying power at level off, or once in the pattern, enrich the mixture to the setting needed for full power at that density altitude.
Some leaning may be required for taxi at high density altitude airports. Lean as you would for taxi before takeoff unless extra cooling before shutdown is required.
Continental Fuel Injection
Taxi and runup will be almost exactly the same as carbureted engines due to this system's lack of air density sensing capability. Lean for smooth running during taxi and lean the same as the carburetors for runupset mag-check RPM, lean until the engine looses RPM, then enrichen back to highest RPM. Readjust the throttle to the mag check RPM and do your check.
During this procedure the engine should lose RPM before it gets rough. If it doesn't this is usually an indication that the system needs a little attention (dirty nozzles or injector line obstructions). This is true of injected engines in any flight phase.
If it's really noticeable (like a low frequency rumble or panel vibration that comes and goes), it would be a good idea to get it checked out before you go any further. Obstructions in a line or nozzle can destroy a cylinder in less than an hour and may even cause complete piston failure in the takeoff or climb phase.
Takeoff and Climb: Naturally aspirated engines, as stated in the description, have no air density sensing mechanism (exceptions are the 10 and GIO-550, addressed later). For takeoffs from sea level to 3,000 feet density altitude, use full rich mixture. For takeoff at density altitudes of 4,000 feet or higher, adhere to the altitude fuel flow settings on the face of the fuel flow gauge. If its not marked, consult the POH.
Be sure to use density altitude. Using only pressure altitude will give inaccurate air-fuel ratios on all but standard temperature days. If for some reason your flow gauge or POH doesn't give these figures (some old installations don't) lean as follows: Just before takeoff, at full or near full throttle, lean to peak RPM then enrichen 2-1/2 to 3 gallons per hour (if your gauge is calibrated only in psi, enrichen two psi).
If you have an EGT, enrich 150oF rich of peak. If your en- gine reaches red-line (constant-speed prop) before you get to full throttle, pull the throttle back to 100 RPM below red-line and follow the above procedure. This will give you best power for takeoff and climb for the first one or two thousand feet.
For best climb performance, the mixture will need to be monitored and adjusted every 1,000 feet. If you have an EGT gauge, keep the EGT at the same temperature it was on take-off. This will give you good perfor- mance and an economical climb up to about 10,000 feet. At this point re- check peak and adjust for about 50oF rich of peak for 10,000 feet and higher.
The I0- and GIO-550 series engines have a bellows similar to the aneroid bellows on the turbocharged fuel pumps that compensate for changes in air pressure. If set up properly, these 550 systems do a pretty good job of mixture control and are left at the full-rich position for takeoff and climb until well above 12-13,000 feet.
EGTs on these engines generally run in the low 1400s for takeoff and climb and, therefore, don't need leaning even on fairly hot days. The little extra fuel pumped in this case will be needed to help cool the en- gine anyway.
The turbocharged Continental fuel system is flown like all other turbo- charged-supercharged systems, with one exception. Set full rich for take- off, but for climb, the engine may be leaned to the climb settings on the fuel flow gauge face.
For top-end longevity, however, keep climb TITs at or under 1400 degrees F This may give you a fuel flow setting of one to one and a half gallons per hour higher than book but will pay off in the long run.
Cruise: Cruise is not much differ- ent for any naturally aspirated, fuel injected engine or pressure carburetor equipped engine but it is worth repeating here. Set up cruise mixture to no less than 50oF rich of peak at power settings of 65-70 percent and 75 to 100oF rich at 70-75 percent power.
If you don't have an EGT, run book settings plus one-half to one gallon per hour more to extend cylinder life. In all cases, hold CHTs to no more than 400oF. At power settings of 50-65 percent some manufacturers will allow you to run at peak EGT. This may be OK for 50-55 percent, but it will prove easier on your engine Land pocKet book) to run 50oF rich of peak at 60-65 percent.
For turbocharged engines add 25oF to the above EGT-TIT figures. The 10-550 series is again an exception here. The book allows the engine to be run lean of peak at low power settings. The fuel system is designed for the very accurate fuel distribution required for this type of operation, but must be maintained well to keep the engine healthy.
Remember, with the standard Continental fuel injection system on naturally aspirated engines, any altitude change will require a corresponding mixture adjustment. The Continental engine in the early Piper Malibu (TSIO-520-BE) is an exception to all of the above. This engine is run either full rich or lean of peak, no in between. Run it by the POH.
Descent and Landing: For descent, enrichen the mixture about 50 F and maintain the same EGT while descending. Remember, the metering assembly is leaning the mixture when you pull back the throttle to maintain the same manifold pressure, so you will need a corresponding mixture adjustment to keep the EGT the same.
Upon pattern entry, the mixture should be enriched to the approximate position for a full power setting at that air density (pressure altitude and temperature), in case full power is needed.
Bendix Fuel Injection
Taxi and Runup: A number of engines with this fuel system like to foul plugs while taxiing. At the lower RPMs, a fuel valve attached to the throttle valve system sets the fuel-air ratio.
The fuel servo does not sense air density or flow until the engine gets to about 1,700-2,000 RPM. This necessitates manual leaning until approximately 2,000 RPM at which point the throttle valve rotates fully open and allows the servo to regulate the fuel-air ratio.
Leaning the mixture to peak RPM during taxi and runup will keep the plugs clean and give accurate mag checks at runup.
Takeoff and Climb: This system compensates pretty well for changes in air density, especially the models with the AMC (Automatic Mixture Control) (almost exactly the same as the pressure carb).
Most Bendix systems on naturally aspirated engines, however, have no AMC, [just a simple, pitot-like sensor] to sense ram air pressure in the induction. This still gives an accurate enough reference for determining air density and does a pretty good job of controlling air-fuel ratios. Still, some minor mixture adjustments must still be made for altitude operations (4,000 feet and higher).
Just before [entering] the runway, or just before the takeoff roll, run up to full or near full power and lean to peak EGT or until the RPM drops slightly. Enrichen the mixture 200oF (if you have an EGT) or one and one-half GPH for four-cylinder engines and two to three GPH for sixcylinder engines. This setting will give best power and cooling combination for takeoff and climb.
During climb, the servo will compensate fairly well for air density changes. A minor mixture adjustment every 2,000 to 3,000 feet will be needed to keep the EGT at the same temperature until cruise altitude is reached.
Again, cruise is not much different for any naturally aspirated, fuel injected or pressure carb equipped engine. Set up cruise mixture as described previously in this article.
When changing altitudes during cruise with this system, no mixture adjustments should need to be made unless the change is more than about 2,000 feet.
Descent and Landing: This is also the same as other injected engines except mixture adjustments during descent are not as frequent as with the Continental system. Enrichen the mixture 50 F just before descent and adjust to keep it the same until level off.
Throttle adjustments for descents will need to be made only about every 2,000 feet. If you don't have an EGT installed, enrichen one gallon per hour for four cylinder engines or one and one half GPH for six cylinders and maintain the same MAP and fuel flow for descent.
Once in the pattern, adjust the mixture to the approximate position for a full power setting at that air density (pressure altitude and temperature) in case full power is needed. Lean for taxi if at high altitude.
Lean of Peak Operation
Most pilot operational handbooks do not address LOP operation. Lycoming ' particularly, does not like it-although they authorize operating at peak EGT in many engines at a limited power level.
Continental is less against it, since they designed the 10-550BE used in the original Malibus to operate this way, and have comments on LOP operation in some of their fuel injected engine handbooks.
Most carbureted engines will not operate very well LOP if at all due to less than optimum fuel flows to the individual cylinders.
That said, LOP is a viable option for fuel injected engines-even turbos with the proper engine monito~ing instrumentation (engine monitors) and proper pilot technique. When done properly, LOP can extend engine life with cooler operating temps compared to running at high power rich of peak. But again, proper operational training is key.
One source of such training is offered by Advanced Pilot Seminars, www.advancedpilot.com. Ph 888359-4264 (this is the phone number for GAMI, in Ada, Oklahoma who hosts the seminars).
POH Discrepancies
A note about the differences between this article and what you will see in most POHs. The fuel flows given in the above advice, as most of you have probably noticed, are a bit richer (higher) than the POH numbers.
The manufacturers, in an effort to give us better performing aircraft, have usually used numbers that allow for the most performance and/ or the greatest range for a given flight profile (see illustration below). However, these numbers often do not lend themselves to long engine life.
There is unfortunately a battle between the marketing department and the engineering department in a given aircraft manufacturer, and the marketing people usually prevail. Don't believe for a minute that the numbers listed by the competition don't significantly influence what ends up in the POH.
Some of the numbers may well be actually obtained by a test pilot in a perfect airplane with a perfect engine. Then all the other parameters are extrapolated mathematically.
If you look at the engine maker's fuel flow, and other important engine operational numbers (as opposed to the airplane manufacturer) in their performance charts you are more likely to see fuel flow numbers significantly higher for a given percent of power. Excess fuel in aircraft engines plays a significant role in cooling the cylinders at high power settings.
There is a high engine longevity price for marginally higher cruise speeds and climb rates. Remembering who pays that price will help you get the life you want from your engine, if you fly with care.