Now that previous columns have littered the runway with theory and attempted to explain manifold pressure, propeller RPM, and mixture, I’d like to present some suggestions for in-flight techniques to extract better performance from these engines. Some of it will be a little repetitive from prior columns in an attempt to make this column more or less “sink-alone.”
First, a Little Housekeeping
Much to my amazement, these columns have been very well received. I’d like to thank you for reading them, and my special thanks to those who have taken the time and trouble to comment in the public message threads, or via email.
Public Responses, Please…
Speaking of which, please note that it is to everyone’s advantage if you leave any questions or comments as public replies, rather than emailing me privately. There are several compelling reasons for this. Most important, everyone learns from a public exchange, and often others will have wanted to post the same comment or question, but didn’t want to appear foolish, or ignorant. Folks, I firmly believe the only dumb question is the unasked one!
Also, a public response and exchange helps keep me straight! More than once, a reader has corrected something I’ve said, or added to it considerably. There are a lot of people out there who know a whole bunch more than I do, some of whom should be writing a column themselves! If I err, or put something badly, and they are spurred into responding, we all benefit.
This feedback is incredibly valuable to me, and to readers who might be misled by my ravings. Finally, these columns and my other activities seem to produce a huge volume of email, and I find myself getting into some fairly deep one-on-one discussions with a number of very nice people, when those discussion would be of much greater benefit in public. I love the email I get, but on the technical issues or issues of broad interest, it would be far more efficient to carry on with them in public, please.
I’m also gratified at the interest in some of the old books I’ve been able to get reprinted, and at the neat responses from those who now have them. So far, there has not been a single complaint or request for a refund, and I’m now into the third reprinting. If any of you should happen to have other old publications that should be made available, let’s talk.
Carbureted Engines, and Fixed-Pitch Props
The single most-asked question I’ve had from these engine management columns has been some wistful variant of, “But what about my 182, which has a carbureted engine?” Or, “What about my 172, with its fixed-pitch prop?”
Folks, I’m really sorry, there’s just not that much that can be done, beyond a few simple tips that have been commonly used for years, like leaning on the ground (see below).
Here’s another tip for carbureted engines only. When operating at full throttle, it is often helpful to back off on the throttle until you see just the tiniest drop in MP (if you have a MP gauge, otherwise look for the slightest drop in RPM), and leave it there instead of fully open. That cocks the throttle plate just enough to set up a slightly turbulent flow, and that helps mix the fuel and air for better combustion. A touch of carburetor heat may help, too.
But I’ve flown a couple of 182s that are simply hopeless, and I’m tempted just to leave the mixture full rich and forget it. Pity, the Skylane is a marvelous airplane, but that TCM O-470 engine has the worst mixture distribution of any powerplant on the planet.
If you’re fortunate to have an all-cylinder engine monitor on a carbureted engine, see if you can operate lean enough to get all cylinders lean-of-peak (LOP) without the engine jumping off its mounts from vibration. I haven’t seen one that would yet, but there may well be some engines that will do it. If so, some of this column may be helpful.
While the knowledge of these things may be helpful in a general way for everyone who flies, it is only the high-performance, fuel-injected engines that allow some of the more sophisticated techniques. Even on those, it almost always takes GAMIjectors to make these tricks work well enough to use them at all. This is the primary reason you have not seen some of this information in the past – it simply hasn’t been very useful, and there’s been no need for it.
Once again, this column will refer entirely to normally-aspirated engines, unless otherwise noted. Yes, yes, I know, there are two or three turbo owners out there who want me to do a column for them, and I’ll get to it, one day, I promise (but I’m not saying when!)
Gentlemen (and Ladies), Start Your Engines
I ran this section by both my editor Mike Busch and GAMI’s George Braly for their comments, and I’d like to quote George’s response here:
“For the record, I think you are a damn fool for doing an article on “starting”! So there! Even GOD doesn’t know how to start some of these engines!”
Now, that was said in George’s usual high good humor, but it’s an excellent comment on the problems in trying to discuss this subject. Behind the scenes, there was a “lively discussion” on this before this column went public. Both Mike and George see a LOT more different engines than I do, and if anything is obvious, it is that there are as many starting techniques and ideas as there are pilots! I have shamelessly incorporated some of their tricks and techniques, too. If they work for you, I’ll take the credit – if they don’t, blame Mike and George.
Starting often seems to be an art form, but there is science involved, too. We know that if we provide air and fuel in the proper proportions and a spark, the engine must run, in-flight. This is also true of the initial engine start, but there are some complicating factors. Let’s take a look at some of them, in no particular order.
For the real techno-freaks, a really important factor is “valve overlap,” (where both the intake and exhaust valves are open at the same time). The pressure (suction) in the manifold is lower than ambient, and lower than that in the exhaust system during starting and low-power operations. Because of this, there is some tendency for the “stuff” in the combustion chamber to get sucked back into the intake manifold. Even some of the exhaust may be subject to this reverse flow during valve overlap.
Another problem is poor vaporization and atomization during the very low cranking RPM. There is a whole host of engine problems that can lead to hard starting, but for our purposes here, we assume a properly adjusted engine.
Cold engines will generally start better with a throttle nearly closed (less air) and relatively more prime than warm engines. To tell the truth, most cold engines start right up without a problem no matter what you do, so I won’t waste much time here discussing them.
Since these “cold starts” require the richer mixture, most aircraft engines are set up for this case, making cold starts the easiest of all. That’s all well and good, but it provides too much fuel for a warm start, which leads to the common problem of “flooding,” which simply means a mixture too rich to burn. Many pilots, when faced with this situation (no start) will think the engine isn’t getting enough fuel, and they will make it worse by priming more, turning the boost pump on, or even pumping the throttle (a bad idea at any time). For this reason, we must preset considerably more throttle with a warm engine to provide more air, and use a lot less prime (perhaps even none) for a proper mix.
If the hot engine is flooded, it is often beneficial to just let it sit for 30 seconds or so. This allows some of the liquid fuel to evaporate into vapor. The chances are that will start the engine instantly on the next try when that vapor gets sucked into the cylinder, and if the fuel pump is pumping fuel and not vapor, a good start will result.
Through experience (or the smell of fuel!), you may recognize a flooded condition. Many pilots will correctly recognize that more air is needed, but they will then make the mistake of quickly shoving the throttle all the way in. This may even work, but quite often, it will cause the mixture to go so quickly from “too rich to burn” to “too lean to burn” that the engine hasn’t a chance to start. So push that throttle in slowly, taking perhaps five seconds for full travel. Chances are that slow opening of the throttle will at some point create just the right mixture conditions, and the engine will fire. That’s probably a good place to leave the throttle for a second or so until the engine starts, but be ready to pull it back quickly to keep from getting a huge surge of RPM.
I absolutely hate hearing an engine start, then instantly run at some very high RPM for many seconds. Some pilots will do that, then bring it back down, but others will compound the error and sit there getting the ATIS, and doing an after-start checklist, with what sounds like 1,800 RPM on a cold engine. Many experts estimate that 90% of all engine wear takes place in that initial cold starting process, when done right. Running at high RPM on bearing surfaces marginally lubricated by cold oil is a terrible thing for your engine, not to mention that the prop blast and noise will make enemies on any ramp.
If you need to prime the engine for starting, use the primer. That’s what it’s designed for. (Well, Duh!) Some pilots will pump the throttle, either before cranking the engine, or while doing so. This is a bad idea, even if it does seem to work sometimes.
Engines with the usual Marvel-Schebler carburetor (as found on Cessna 182s) have an accelerator pump just like your car engine does. Well, let me qualify that, because I haven’t the faintest idea if modern car engines have such devices, and virtually no interest, either! I lost track of what’s what in car engines in about 1964, I think. Dreadful, dirty things, I never did really like working on them. Now airplane engines, that’s another story! Works of art, mostly. But, I digress. Back to pumping the throttle.
Any fuel produced by pumping the throttle is squirted into the carburetor throat, at a point too far from the cylinders to do much good at cranking RPM. Instead of helping the start, the squirt of raw fuel just dribbles down the low point in the intake system and puddles there, creating a major fire hazard in the event of a backfire.
It may also dribble out a drain tube onto the ground, making a puddle of fuel. This becomes an extreme fire risk if any torching occurs. I once very nearly lost a Lambert Monocoupe (an old wood and fabric airplane) from this very thing. Luckily, I saw the smoke, leaned out the door, saw the puddle burning merrily away with flames licking at the belly, and was able to leap out (nearly took the seatbelt with me!) and push the airplane back away from the fire.
Fuel-injected engines don’t have an accelerator pump, so pumping the throttle on one of them does nothing at all but demonstrate ignorance (not stupidity, which is an entirely different phenomenon).
Many problems are mitigated with fuel injection, but others are added. Fuel injection usually puts all fuel right into the intake port (a few, like the superb Wright R-3350, put the fuel directly into the cylinder). It is possible for fuel to run down the “log runner” intakes on flat engines and create some of the above hazards and problems, but it’s far less likely.
Fuel injection is great stuff. But its major problem is the dreaded hot start. If you shut down, then re-start within about 15 minutes, or if you wait an hour, it’ll probably start right up and run just fine. But it is that period of time somewhere in between (30 to 45 minutes after shutdown is generally the worst) that can drive pilots to drink. Nothing seems to work. The problem is that the hot engine “cooks” all the fuel in the engine compartment. It cooks it in the main line coming into the engine-driven fuel pump, the lines after that, and even the tiny stainless lines going from the distribution valve to each cylinder. The result is a system just full of bubbles, and these engines don’t run well on bubbles.
The only effective technique for this is to make sure the mixture control is pulled all the way to idle cutoff (ICO), and then run the boost pump for 30 seconds or so. (On “high” if you have a two-speed pump.) Yes, that’s a long 30 seconds, but it needs to be done.
Note added 06/06/02: Later data strongly suggests the boost pump be run on the highest setting for AT LEAST 60 full seconds, by the clock, and 90 seconds is better.
The electric boost pump is usually located somewhere outside the engine compartment, and thus has no problems with bubbly fuel. What you’re doing here is to circulate cool fuel into and through the engine-driven fuel pump, up to the fuel control unit, and then back through the vapor return lines to the fuel tank. The idea is that when you do crank the engine, the engine-driven pump will really pump liquid fuel, and not starve on bubbles. A normal start should result. While cranking, you might need a quick shot on the boost pump to help the cold fuel get beyond the fuel control unit, and blow out the lines to the fuel distributor (“spider”), and the tiny lines to each cylinder. But make sure it’s really quick, otherwise you risk flooding the engine.
Another hot-start trick is to park downwind (i.e., tail into the wind), and open anything you can on the top of the engine. On Bonanzas, this would be the cowling, on Cessnas, the oil filler cover. The wind will be scooped up into the bottom of the engine by the cowl flaps, and the opening on top allows the hot air out. This helps keep those bubbles from forming in the first place.
Support for this is found in the TCM manuals:
“…During subsequent [after the thirty minute heat soak] starting attempts, the fuel pump will initially be pumping some combination of fuel and fuel vapor. At the same time, the injection nozzle lines will be filled with varying amounts of fuel and vapor. UNTIL THE ENTIRE FUEL SYSTEM BECOMES FILLED WITH LIQUID FUEL, DIFFICULT STARTING AND UNSTABLE ENGINE OPERATION CAN NORMALLY BE EXPECTED.” Page 8-5 TCM Form X30565, August 1990 FAA approved M & O Manual, Model IO-550A, B, C, G.
Some suggest a full-rich mixture, with the boost pump on for a few seconds. That may solve the immediate problem of bubbles in the lines, but it can also pump so much raw fuel through an open intake valve that you’re almost certain to flood the engine, and you might even create “hydrostatic lock” that could damage a cylinder (maybe). Some say that much fuel may also wash the oil off the cylinder walls, which may not be a good thing, and with a non-turning engine, fuel will accumulate, leaving just that much more raw fuel to get rid of. Who knows, I sure don’t, but I’m pretty sure that will create a fire hazard you don’t want. If you absolutely must use this technique, do it while cranking the engine, so at least you have a chance of a start if you happen to get everything “right” during the process.
In any event, for any start with a significant amount of throttle, as soon as the engine fires and the RPM starts rising, move the throttle to the idle position, quickly and smoothly, then increase throttle so as to “meet” the RPM at about 1,000.
Alternator On or Off?
In some circles, there is much ado about whether to start the engine with the G/A (Generator/Alternator) on or off. If you turn it on with the battery before the start, you won’t forget it, but some say that’s too much drain on the battery. Okay, maybe there is a small drain of an amp or two to excite the field, but if your battery is that critical, you probably ought not to fly with it anyway! A similar comment applies to the rotating beacon, which I leave on at all times, so the master switch controls it. (In my opinion, rotating beacons should be on circuit breakers, not switches, so they always turn and flash with the master on.)
A point against leaving the G/A off until after the start, if I may? Watch the ammeter when you do turn it on, and you’ll see an instant huge surge as the G/A instantly tries to recharge the battery after the massive drain of starting. That instant shock is seldom good for shafts and gears, although in theory they’re designed for it. If the G/A switch is on to start with, the system assumes that load much more gently as the starter is released and the engine RPM comes up.
Leaning on the Ground
Finally, you may find it beneficial to lean your engine after start, and for all ground operations. In theory, a properly set up engine will run at “taxi power” without fouling plugs, but the reality is that most general aviation piston engines are set up so rich (for easy cold-starting) they do often foul plugs. You can test this on your own engine by setting minimum idle RPM, then leaning until the engine quits. Watch the tachometer very closely for a small rise just before the engine quits. The more rise you see, the richer your idle mixture setting. On the big radials, this rise should be almost imperceptible, or “barely detectable,” but some of the flat engines call for as much as 100 RPM rise. Again, this is mostly for optimum starting, not running. It’s perfectly safe and often desirable to correct this with manual leaning, once the engine is running.
The downside of leaning on the ground is the very distinct possibility of attempting a takeoff that way, so if you lean on the ground, lean it brutally! You can’t hurt the engine by leaning at “taxi power,” but you sure can cause some heavy damage if you take off with the mixture partially leaned! If you attempt a takeoff while “brutally leaned,” the engine will simply wheeze and die when you try to apply throttle. If you enrich at any time, for any reason, either go right to full rich and leave it there for takeoff, or re-lean it “brutally” once again.
You, there! Get off the brakes!
There seems to be a large group of pilots (including some high-time pros) who set some slightly-high power for taxi, then hold it back by dragging brakes and using differential brakes for steering. That heats up brakes very quickly, destroying some of their effectiveness, wearing them out, and even creating a potential fire hazard. On most of the big airplanes, this is deadly.
Try to control taxi speed with power, without using brakes at all. If you must use some higher power for some reason, allow the speed to build up to “slightly too fast,” then brake it down to “too slow,” and get off the brakes again. That gives the brakes a lot of time to cool in between applications. Brakes like to be applied or released, not “dragged” or “ridden.”
What has that got to do with engine management? Nothing. It’s just one of my pet peeves, and I couldn’t resist sticking it in here!
In the Runup Area
Watch where your tail is pointing!
Yup, you’ve got it, another one of my pet peeves. Consider the wind. Some say this is for cooling, but I’ve seen good data that suggests a downwind runup provides better cooling for a Bonanza! In my opinion, it is far more important to consider where your runup will blow your dust cloud and debris.
If you have properly leaned for ground operation, you might be too lean to get runup RPM, in which case go full rich for runup. When the runup is done, you can leave the mixture full-rich for takeoff, or – if you anticipate a significant delay – re-lean it “brutally.” Remember, we don’t want to risk taking off with the mixture leaned. I don’t care what kind of checklist you use, or how many times “Mixture Rich” is on it, you WILL forget it. This is a perfect example of using a “fail-safe” procedure that will protect you from yourself.
The usual 1,700 RPM for running up most TCM engines (or 2,000 RPM for most Lycomings) is NOT critical. I’ve seen pilots diddle and dawdle trying to get exactly 1,700 but all this does is heat the engine up for no good purpose. Plus or minus a couple hundred RPM won’t hurt a thing, so push it up to “about 1,700” or “about 2,000” and get on with it.
During the mag check, you must see some drop on each mag, but not more than the POH specifies. One exception: If you do a mag check while leaned, the drop will usually be much greater than when the mixture is full rich, perhaps even out of “book limits.” This is expected for runup with a super-lean mixture, and it’s no problem. But, if you’re concerned about that, just do the runup at full-rich mixture! After you runup lean a couple of times, you’ll know what to expect for “normal.” If you have one of the all-cylinder engine monitors, watch the display on that, rather than (or in addition to) the tachometer. That monitor will tell you more about your engine than the simple mag drop.
It is also becoming very clear that the mag check at low power (anything less than cruise power) is not very useful for catching problems; it’s nothing more than a quick check to catch major problems like severe plug fouling, a “hot mag,” or a dead plug, or cylinder. This was well-known in the big old radials, where mag checks are almost always performed at about 30″ MP, and up around 2,300 RPM (varies with model).
I would suggest that everyone should get into the habit of checking the mags once each flight, perhaps near the end of the flight, using the engine monitor, at cruise power. Select each mag, and let the engine run there for 30 seconds, watching for EGT/CHT changes and dropouts. This is an excellent check of the entire ignition system, and very often detects problems long before they show up on the usual runup. The engine should run smoothly on each mag during this check, even when LOP.
Cleared for Takeoff
It is my firm belief that all takeoffs in ALL recips should be done at full power, as specified in the POH. I cannot think of a single exception.
(Pop-quiz question: At full takeoff power, would you expect higher CHT on a cold day, or a hot day? See answer below.)
If the book says full throttle and 2,700 RPM (or 44″ and 2,800, or whatever) then that’s what you should use. Get the governor setting tweaked so the prop delivers the book RPM on takeoff, as measured by an accurate tachometer. I won’t settle for more than 50 RPM either way, and it can be held to even tighter limits, with a little effort.
Similarly, get the fuel flow tweaked so that you get ALL the fuel flow the book allows. This is important for cooling. If it’s a hair over, that’s fine, but don’t settle for less than “book” fuel flow.
Some are confused by the fact that some engines require the use of a boost pump for takeoff, and some do not. The big radials have carburetors that are set up to have auxiliary boost pumps on for takeoff, and most of them require them to be switched on to cover the possible failure of the engine-driven pump. On the other hand, many general aviation flat engines are set up to not use the auxiliary boost pump for takeoff, while others have a two-speed pump that should be set to “LOW” for takeoff. Simple solution, just “Do what the book says!”
On flat engines, “LOW” boost may be helpful on hot days, but if you use it, you may need to manually lean the mixture to prevent power loss. “HIGH” boost is generally prohibited on flat engines, because it will deliver enough fuel to flood the engine out at high power. If the engine-driven fuel pump does fail, then it’s time to use “HIGH” boost.
On a climb with hot fuel in the tanks, and a hot day, you may need “LOW” boost to keep bubbles from forming, and making the engine surge and sputter. Go ahead and turn it on, but you may also need to lean just a bit more to keep the fuel flowing at the proper rate.
Generally speaking, flat engines with a single-speed auxiliary boost pump with only an “ON” and “OFF” setting are really in the “HIGH” position when on, and for this reason, “ON” should not be used unless clearly needed for a fuel pump failure.
On normally-aspirated engines (carbureted or fuel-injected) when operated at high altitudes, it is necessary to lean for takeoff, because the enrichment function will cause a “too-rich” mixture. The TCM IO-550 is sometimes an exception to this, as some models have an “altitude compensating” device. In my experience they don’t work very well, and I have mine tweaked to not do very much, leaving mixture control to me.
Just how to do that leaning for takeoff will produce heated debate in any airport lounge. But it’s not that hard! Look at the TCM power curves in “Mixture Magic” and notice that the BHP curve is pretty flat in the high-power range. That tells you that the mixture setting isn’t critical – we just need to get it in the ballpark. All we need to do is get rid of some of the “extra enrichment” and we’ll be somewhere on the flat part of that curve, and that’s about as good as you can do. Personally, I just go full forward with everything, then on the roll, I grab the mixture control and make a gross movement “too far,” feeling for the power loss, then I shove it back in to the point where it “feels good,” and let it go at that. We do not need to be super-precise here! Isn’t that simple? The momentary “too lean” mixture won’t hurt a thing, especially at the high elevation when the engine is putting out a lot less than full rated power.
To pound on the poor dead horse one more time, you are not “saving” an engine by using less than full power for takeoff, you are probably hurting it! All high-performance aircraft engines have some means to greatly enrich the mixture at takeoff power. When you attempt a takeoff at partial power, you often defeat this, and both EGT and CHT will be higher, often much higher before the gear comes up. Full power will also get you higher, faster, sooner, and this is good from a general safety standpoint. Also, CHT rises continuously throughout the takeoff in all engines, because at low speed, there is insufficient airflow for cooling. The quicker you can get to an airspeed that does provide good cooling, the sooner that CHT will stop rising, stabilize, and even start down again. The operators of one large warbird persist in taking off with reduced power, and incur the double whammy of slow airspeed acceleration on the runway and a less-than-optimum mixture, leaving them looking at redline CHTs by the time the gear is coming up. They “just don’t get it.” If they’d just go ahead and use the bloomin’ power the manufacturer specified, CHT would probably not even be an issue!
Partial power takeoffs in a multiengine aircraft are really dumb, because performance is greatly reduced, and the power setting greatly complicates the engine failure procedures. You may think you’re good enough to add power on the good engine while simultaneously handling the failure of the bad one, but trust me, you’re not. You’ll blow it every time. Again, you do not hurt an engine by operating it at full takeoff power as specified by the factory! This will generally keep the engine cooler.
The only case I know of where reduced power (thrust) is safe and useful is in jet aircraft, where the CHT problems do not apply. The fuel control units are so good that proper mixtures are maintained at all settings, and the aircraft have such an excess of thrust that the engine failure case is near-trivial. In fact, when briefing a “reduced thrust” takeoff, I make it very clear that if we do lose an engine, we will normally NOT advance the thrust on the others at all. If that doesn’t seem safe, we go to the next higher available thrust setting.
Oh yes, that takeoff CHT question above? CHT will go higher on a cold day. This is because the normally-aspirated engine produces more power with cold air (denser air, more fuel flow), and even more importantly, the engine is producing that slightly higher power at a leaner mixture (more air, same fuel). More power and a leaner mixture equates to higher CHT. This normally overrides the small effect of colder cooling air.
Climbing to Altitude
My pet peeve in climbs is using too low an airspeed. One famous training organization insists on using 95 knots to 1,000 feet AGL in a Bonanza, most of the twin operators push for a climb at or near the blue line, and virtually all the transport operators use a V2+10 climb with all engines operating. I think it’s stupid to climb that slowly in any airplane (including jets, just so I offend everyone here!) once actual obstacles are cleared.
95 knots in a Bonanza runs the engine temperatures up. Yes, they may remain within limits, but why get any hotter than necessary? Additionally, the nose will be so high you simply cannot see where you’re going – you’re blind to traffic that may very well be in your path. Finally, I firmly believe that if a total engine failure occurs at 95 knots in a Bonanza below a few hundred feet, most pilots will stall before they can get the nose down enough to maintain flying speed. If they just happen to succeed at that, they will probably end up so slow and descending so steeply that there isn’t enough energy left to flare. A crash, and an ugly one, is all but inevitable. At such low speeds, with the usual nose-high attitude, a recovery from an engine failure is very nearly an acrobatic maneuver, and not one pilot in a thousand has practiced it with any realism. It’s a bad deal.
For this reason, I prefer to see a very early shift to a higher climb speed once real obstacles are cleared. In the absence of real obstacles, I set up a gradual climb right from liftoff, pulling the gear up as soon as I am well clear of the ground (Oh, boy, I’m gonna get mail on that one! Hmm, might be a good subject for a column?) In the Bonanza, I accelerate to about 120 knots, reaching that speed by the time I’m at 100 feet AGL, or 200 AGL. This gives much better cooling, much better visibility, and makes the engine-out case far more manageable (single or twin). Yes, yes, I know, I’ll be at a slightly lower altitude when the engine quits, but not as low as you might expect. Some are very fond of quoting the sharp rise in drag with higher speed, but an often-missed factor is the improvement in prop efficiency that also takes place. The real result is that the actual climb rate on a Bonanza will suffer very little. The angle of climb (gradient) drops, of course, and it is possible for this to become a terrain clearance issue.
I’d rather have just a little extra speed here, and I accept the slightly lower altitude.
Power reductions after takeoff have many considerations and variations, so it’s a little hard to generalize. In all cases, use full takeoff power until the “flight situation” has “stabilized.” This is a very subjective point, since some pilots won’t be fully stabilized until the airplane is in the hangar and they’ve had a transfusion of some type of spirits. In general, though, the gear should probably be up, the airspeed stabilized, some altitude beneath the wings, and the workload of takeoff should have abated somewhat. (On a low-IFR departure, perhaps you’ve turned to your initial departure heading and broken out on top of the low stratus deck.) That’s the time to set whatever lesser power the limitations section of the POH calls for. Some of the larger engines have a “METO” (Maximum Except TakeOff) power, which must be set within one, two, or even five minutes of takeoff. Some go further, with an even lower setting for “Climb.”
Many of our flat engines have no such limits, and are rated for full takeoff power “essentially forever.” Some have RPM limits, and those should be observed. With normally-aspirated flat engines, the increasing altitude we normally see automatically reduces the power within a couple of minutes, so it is almost silly to retard the throttle, then keep adding it back within minutes.
There is a fast-growing major problem, however, and that is noise. A primary reason airports are closing by the dozen is noise at takeoff power, or more correctly, noise at takeoff RPM. It is irritating, sometimes even to people who love airplanes! In Seattle, where I live, there is a constant procession of floatplanes overhead at 1,500′ to 3,000′, and many of these pilots and operators don’t have this picture, because they’re boring holes in the sky on sightseeing and training flights at 2,500 RPM or more, engines just screaming. I can only imagine the effect on people who don’t like airplanes at all, or those who feel endangered by them.
For this reason, I suggest pulling off a couple hundred RPM as soon as possible after liftoff, no matter what the book says. The difference in noise from my IO-550 and its three-bladed McCauley prop is dramatic, at the cost of just 15 HP. The Germans are so anti-noise they made Beech/TCM deliver a “special” engine on Bonanzas to be operated there, and the only change is a limit of 2,500 RPM (instead of 2,700) on the same engine. Since Beech had to do certification testing for this, we know what the HP is. The IO-550 is also quite happy at 2,500 RPM for a lot of reasons, so it’s a good trade-off, I think. I usually do that as soon as the gear is up.
At the risk of being accused of beating that poor dead horse again, please, please do NOT reduce the MP to 25″ after takeoff or for climb, as has so long been the accepted practice on the flat engines! EGT and CHT will go up, not down. You are not doing your engine any favors, and you may be hurting it.
Unless there is some limitation in the POH, climb at WOT (Wide Open Throttle), 100 or 200 RPM below the max (for noise), full-rich mixture and cowl flaps open (cooling), or cowl flaps as called for in the manual. You can’t go wrong doing this. Monitor your CHTs, preferably on a modern all-cylinder monitor, and if any CHT goes above 380°F to 400°F, do something about it. There are several things you can do. The most effective one is to lower the nose and increase airspeed. Open the cowl flaps if they’re not already open. Enrich the mixture, if you’re ROP (or lean it more if LOP). If you’re already full rich, this might be a good time to turn on the boost pump to “LOW” (if available) or “HIGH” to increase the fuel flow. With the boost running, you may get too much fuel, and the cure for this is to lean a little. All of these will reduce climb performance, but unless you’re about to hit something, it always makes sense to take the “hit” on climb rate in order to keep the engine nice and cool.
No matter what your POH says, limitation or otherwise, I suggest you consider 400°F an absolute redline CHT on any cylinder with a bayonet probe feeding an all-cylinder monitor (spark plug thermocouple types may show higher). There is mounting evidence that factory limits on CHT are much too high. In any event, try to keep the time above such temperatures at a minimum.
Leaning for Climb?
The question is not whether to lean for climb, but when to start the leaning. Many POHs will state pretty strongly that no leaning at any power setting be done below some fixed altitude, often 5,000 feet, and then in “cruise” only. This is patently absurd, but some people carry that to ridiculous lengths, even saying that it applies on the ground, for taxi operations!
I suspect stuff like this comes from a quick meeting with a non-flying lawyer present, who insists that the POH must be written for the lowest common denominator among pilots, the utter moron. Various suggestions get kicked around, and finally someone observes, “Well, you won’t hurt the engine, or reduce safety with full rich below 5,000 feet, and you won’t hurt the engine, or reduce safety, by leaning above 5,000 feet.”
“Aha! That’s easy, we’ll just put in one line saying ‘Lean only in cruising flight above 5,000 feet.'” Simple, effective, undeniably safe – who can argue? The whole mixture discussion is cut to one simple, clear line, over which the manufacturer will never get sued.
Even GAMI is conservative about this subject, as misuse of the mixture control can indeed ruin an expensive engine, and I suppose they could be sued. Be forewarned, my defense in court will be, “You did what? You took advice from some nut on the Internet? Stupid!”
Can I reduce “leaning in the climb” to one simple sentence? Sure, it’s easy! Like the book says, “Lean only in cruising flight above 5,000 feet.”
But even this can get you in trouble! Take the case where you’re climbing out, ROP or even full rich. Hot day, hot fuel, and the engine-driven fuel pump is hot enough to heat the fuel even more. Bubbles begin to form, and these move through the lines, to the combustion chambers. The fuel flow sensor won’t detect the bubbles, so you show the same old fuel flow, but with all the air bubbles, it’s actually a very lean mixture. This can drive one or more CHTs up very quickly. If you have a cylinder that runs hot already (as most #2 jugs do on Bonanzas), this may be just enough to drive that jug into detonation.
On the other hand, if you are running LOP, this will drive the CHTs down, and cause a power loss. Nice to have the settable warnings on an engine monitor, as the JPI has.
To operate lean-of-peak (LOP) during climbs, or in cruise below 5,000 feet, you need at least all of the following:
- A thorough knowledge of just what is happening when you lean. How thorough? Well if anything in “Mixture Magic” is a complete mystery to you then don’t do it!
- A good, accurate all-cylinder monitor showing at least EGT and CHT in a digital display (preferably with one-degree resolution),
- A fuel-injected engine with GAMIjectors installed,
- A higher climb speed than you may be used to.
If you lean aggressively during climbs at the lower altitudes without all those factors present, you’re playing with fire. It can probably be done, but I’m not sure you can learn how, safely.
When I’m ready to go LOP, I’ll have 120 knots, and I’ll start out with WOT (Wide Open Throttle), 2,500 RPM, and the mixture in full rich. I’ll have a “stable situation” with a low workload. (I don’t fool with things like this during an instrument departure, for example.)
(If you are irritated by TLAs (Three-Letter Acronyms) I’m afraid you’ll have to deal with it. The “WOT” and “LOP” terms are spreading rapidly, and the GAMI folks are even tossing around “WOTLOPSOP” for “Wide Open Throttle Lean of Peak Standard Operating Procedure.” But, I digress.)
I’ll set my engine monitor to show the hottest CHT. Now if you’re going to lean at this point, you can’t pussyfoot around and do it slowly! If you do, the EGT will soar, and the CHT will follow fairly quickly, as you move the mixture control. You must grab it, take a deep breath (the first time), and firmly do the BMP (Big Mixture Pull) moving the fuel flow fairly quickly to LOP in about one or two seconds. In my engine, pulling it right back to 16 GPH will be very close, giving me about 1,450°F EGT, and about 370°F CHT. How can you tell how far to pull it that first time in yours? A major immediate indication will be a noticeable loss of power. The next indication will be the trend of that CHT. If it rises, you need to lean more. If it falls quickly, you leaned too much. If that sounds backwards, remember you’re now on the lean side of the mixture curve, where “leaner is cooler, richer is hotter.” This concept may take considerable “retraining” because you’ve probably spent your whole flying life thinking just the opposite. Even the FAA exams teach that “leaner is hotter” but that’s true only on the rich side of peak.
Okay, let’s assume you’ve successfully transitioned to LOP. The idea now is to set the mixture so that the CHTs ride somewhere between 350°F and 380°F. I think the upper limit is important (see “Mixture Magic”). The lower limit is somewhat arbitrary, but in my opinion, you lose too much power if you lean it below that. By using a lower number at first, you’ll have a chance to look carefully at all your CHTs, and get a feel for how the engine likes this mode of operation. Personally, I shoot for about 380°F in the climb.
CHT reacts very slowly, however. Most people find it easier to use the EGT for changes, as it reacts instantly. Then see how that affects the CHT. With just a couple tries, you should be able to determine an EGT that will give you pretty close to the target CHT; thereafter, you can just set the EGT. I use 1,450°F on the lean side for climb, yours may vary.
It is very important to control your airspeed! Pick a climb airspeed and stick to it! If you let the IAS drop off, your CHTs will certainly rise.
Some airplanes will go all the way to cruise altitude with no further adjustment. With some, the CHT may rise, or it may fall. You may need to make one or two adjustments during the climb, remembering that on the lean side, “leaner is cooler.” Tweak the EGT by 20°F, then later see what that did to the CHT.
Once you reach cruise altitude, very little adjustment is needed. Pick the RPM you want (I use 2,500 at altitude, 2,100 when very low) and let the airspeed build. As it builds, you’ll see the CHTs drop from the increased cooling. If you like, “toss another log on the fire,” and enrich the mixture to bring the CHT (and the power) back up to the level you want, but always less than about 380°F.
As far as the engine is concerned, we could skip this “cruise” section entirely. By just leaving everything alone (or just tweaking the CHT) when reaching cruise altitude, you’re all set in the “LOP go fast” mode. But there are other considerations. You may not want the “go fast” mode – you may need more range, for example.
In my opinion, it is utterly stupid to set some arbitrary percentage of rated power for cruise. That’s getting the cart before the horse. What you need to know is the airspeed you need for the purpose of this flight, then set the power to produce that airspeed.
This is not a new concept. Charles Lindbergh used it in 1927 in the “Spirit of St. Louis,” and later taught the technique to military pilots in WWII. Airlines and military transports used it as a normal procedure in prop aircraft for decades, and modern jet transports do it, too. In cruise flight in jets, we haven’t the faintest idea of how much “power” or “thrust” we’re pulling, all we’re concerned with is the SPEED we want, with the only real engine limit being – would you believe – EGT?
Try this yourself, with your airplane at gross. Make a table with four columns, and label them “Fuel Flow,” “IAS” (Indicated Airspeed), “TAS” (True Airspeed) and “MPG” (Miles Per Gallon”). Next time you’re flying pretty close to max gross for your airplane and the air is smooth, set up “LOP Go Fast” mode as above, and note the fuel flow and IAS. (Do the calcs for the “TAS” and “MPH” later.)
(Yes, you need a good fuel flow indicator for this.)
Next, reduce the fuel flow a few tenths of a gallon, and let things stabilize for a minute or so. Note fuel flow and IAS again. Repeat this at intervals of a few tenths of a gallon. At some point, the engine will become slightly rough, but don’t panic, just note the fuel flow. Reduce the RPM by 200 or 300 (which will reduce the fuel flow even more,) then set the fuel flow back to the setting you noted, and resume gathering your data. At some point, the engine may get rough again, so run the RPM down another couple hundred.
What you want is a table that shows LOP fuel flow and the corresponding indicated airspeeds. Because horsepower varies almost linearly with fuel flow when operating LOP, it doesn’t really matter what the RPM is for this exercise – the results are “close enough.”
When time permits, run the numbers to convert IAS to TAS for each line, then simply divide the TAS by the fuel flow for each line, and you’ll have “MPG.”
If you really want to do it right, do this at low altitude, again at some intermediate altitude, and at the highest normal altitude you use. For me, that’s 1,500 feet, 9,000 feet and 19,000 feet. Plot this data out on graph paper, with fuel flow across the bottom, and TAS up the side, or better yet, let Excel’s superb graphing feature do it for you.
You’ll probably find that by decreasing your IAS 10 or 15 knots, you can get a pretty fair improvement in MPG, and this is worth knowing. Your best mileage will be at a very slow speed, but very few of us have the patience to do that. You must also take wind into account – you’ll need more IAS/TAS in a headwind, and less in a tailwind.
(I apologize for not having the data and a nice neat color chart showing all this. I had one, but my computer had other ideas about it, and lunched it.)
How do we use all this? For the normal flight, I just set “LOP Go Fast” and forget it. But when I see that fuel might be critical, and I want to make it nonstop, I’ll reduce the power (and thus IAS and TAS), and see how that looks for a time. If I know I’ve got a headwind, forget it and set “LOP Go Fast,” since it’s not worth the effort. With a little knowledge of the data, you can even crank it into your flight planning software, and play some “what if” games.
Descents and the OWT of “Shock Cooling”
I know of no real data, old or new, that supports any of the theories about “shock cooling” being particularly harmful. I think it’s a load of poppycock, invented to give pilots something to sound knowledgeable about, when talking to the less experienced, and it gives CFIs yet another procedure with which to hammer on trainees.
On the contrary, there seems to me to be considerable evidence that “shock cooling” is not particularly harmful. Airplanes flown regularly (and hard) seem to be the ones that regularly go to TBO and beyond. Some of these are flown in the harshest conditions found on earth, such as the arctic, the jungles and the desert. Above all, training aircraft are subjected to the very operations that cause the most severe shock cooling on virtually every flight, with constant simulated engine failures, aborted takeoffs, long power-off glides and sudden applications of full power. Aircraft that tow gliders routinely go to TBO, and they are doing constant full-power climbs at low airspeeds, followed by steep, power off descents, often at high speed. How about acrobatic aircraft, which go from wide open to power off, in all attitudes, at all airspeeds, show after show, sometimes multiple shows per day? Bob Hoover goes from a full-power setting into instant feather at very high speeds (probably beyond redline), flies for several minutes that way doing his wonderful act, then fires them up and within seconds, goes to full power again. He reports that he routinely goes to TBO!
Even if there is something to the various “shock cooling” theories, I don’t think it’s the major factor many make it out to be. On the other hand, if we are to worry so much about shock cooling, what about “shock heating”? Every takeoff involves going from near-idle power to full power within a few seconds.
For a normal descent from any altitude, I simply start down without changing any of the engine controls. The increased speed will cause the CHTs to drop very slowly. If you wish, you could enrich (from LOP), or lean (from ROP) to modulate this, but it hardly seems worth the effort, to me.
At some point during the descent, this will produce an IAS that is “too fast,” either because of turbulence, or the yellow arc on your airspeed indicator. Leaving the mixture alone, bring the RPM back a couple hundred, and watch, maybe tweaking the mixture as needed to keep the temps up, but probably not. When the RPM is back “too low” or the prop hits the high-pitch stops and won’t hold the RPM down, and you’re going too fast, that’s the time to start back with the MP. That first reduction from WOT will move it out of the “power enrichment” range, so you may notice a sharp drop in EGT and CHT (if LOP), or a rise (if ROP.) You could control this with mixture, but I find it’s usually easier just to pull five to ten inches off the MP and be done with it. That gets the engine down into a low power range where nothing will hurt it.
Some pilots spend a lot of money installing speed brakes, because they want the ability to get down faster, without pulling much power off. If I’m cruising at high altitude, and ATC wants me down, or I need to dump it through a layer of potential icing conditions, I’ll put the gear down (with the 156 IAS Vlo on my Bonanza, the gear is a wonderful speed brake), pull off five or ten inches of MP, and head on down at 2,000 FPM or better. Does my engine get cold? No, because I’ll enrich to peak EGT, then another 50 to ROP, and that will keep my CHTs nice and toasty. They may slowly drop a bit, but I’m not concerned about that. In other words, at lower power settings, I’m running at the “best power” point on the mixture curve for temperature control. This gives a surprising amount of temperature control, too!
In the Pattern
I’m a firm believer in doing as little as possible in the pattern, basically gear and flaps only. Everything else should be done long before you arrive in the pattern, maybe 10 miles out, or more, and any and all checklists (written or mental) completed. The traffic pattern deserves your full, undivided attention outside the cockpit.
The classic pre-landing procedure method calls for the prop at a very high RPM setting, and the mixture full rich for a potential go-around. But that high RPM is totally unacceptable around most of today’s airports (see previous comments on noise.) I strongly suggest you keep the RPM at some very low setting (1,800 to 2,100 on mine, please). When you are on very short final, with the speed decreasing, and the power coming back, that’s a good time to push the prop control up, because the prop has probably dropped out of governing range, anyway. Even better, leave it alone.
A somewhat more controversial technique is to leave the mixture leaned where it was in cruise or descent throughout the approach and landing, and all the way to the hangar. I like to do this, because it avoids dumping a whole bunch of cold fuel into the engine when it’s not needed, and it leaves me pretty well set for the taxi to the hangar. But it does have its risks. The one thing that must be done with this technique is to train, train, train for the go-around with MIXTURE, PROP and THROTTLE. This is not a bad training idea no matter what technique is used.
I use the GUMP check myself (Gas, Undercarriage, Mixture, Prop), and when I get to the last two I say (usually aloud) “Mixture is leaned, prop is 1,800, a go-around will require MIXTURE, PROP and THROTTLE.”
Remember, if you get in trouble doing any of this, you’ll have to tell the judge that you were following the advice of some nut on the Internet.
Use Your Head
The techniques in this column are not for unthinking pilots who prefer the easiest way of doing everything, and who can’t be bothered with advanced techniques. If you’re not willing to study and learn WHY these procedures do what they do, then don’t use them! If you cannot use them regularly, in a careful, considering manner, then don’t use them!
On the other hand, if you are prepared to study and understand how that incredible piece of machinery under your cowling really works, to use judgement about when these procedures might be appropriate, and if you’re willing to put up with the abuse of those who prefer dogma over logic, this column is for you.
Be careful up there!