Pelican’s Perch #46:
“But My Mechanic Says …”

Ever notice that whenever a piston aircraft engine has low compression or some other cylinder problem, chances are the mechanic will blame it on


Pelican's PerchLargely as a result of these columns, I am privileged to get a lot of email about engines and the basic principles, and I monitor a number of forums and mailing lists where pilots and owners hang out online. GAMI gets even more mail and phone calls of course, with over 8,000 sets of GAMIjectors “out there” and more going out the door every day. George Braly is the founding genius at GAMI, and we share some information in both directions – when I get a question I can’t handle, I “ask George.”

I feel the need to insert my usual disclaimer here: I am in no way financially connected with GAMI, or Tornado Alley Turbo, or any other such entity. The folks there are trusted and valued longtime friends, and we have long shared data and ideas. I have their products in my airplane because I believe in them, and I paid for them.

I think it’s fair to say that I get to see a pretty representative sampling of the common squawks and problems (and sometimes the uncommon ones!) in the field.

One constantly recurring theme we hear from owners and pilots is, “But my mechanic says…”

There usually follows some statement that seems downright silly, given what we now know and use every day in the fast-growing WOTLOPSOP community (“Wide Open Throttle, Lean of Peak, Standard Operating Procedure”). Even very good mechanics and highly experienced pilots often buy into what can only be called “Old Wives’ Tales” (OWTs). When asked to produce some data to back up their statements, the answers will range from a dismissive “Everyone knows that,” to “I saw it somewhere, don’t remember where, too busy to look it up,” to “That’s what I learned in school.”

What Do They Teach in A&P School?

Students in Aviation Maintenance ProgramThis business of running lean of peak (LOP) is but one example. Mike Busch – my distinguished, wise, handsome, erudite and world-famous editor – finally got serious about getting his A&P certificate. He’s now passed the three FAA knowledge tests (AMT General, AMT Airframe, and AMT Powerplant) with near-perfect scores, and is scheduled to take his oral and practical exam later this month.

Mike sent me a few of the FAA knowledge test questions he found silliest, plus a couple others for comment. He also made a most interesting connection between some of the common OWTs, and this particular test. (I think Mike knew that what he sent would light a fire under me to do a column on it. He’s sneaky that way.)

In Mike’s words:

I’ve long noticed – and I’m sure you have, too – that whenever an aircraft owner brings his plane into the shop with a top-end problem of any sort (and particularly a valve-related problem), the mechanic always blames the problem on “running too lean.” Burned valve? Obviously leaning too aggressively. Stuck valve? You must’ve been running too lean. High oil consumption? Low compression? No choke? Your fault, you leaned too much. (So far, I’ve yet to hear a mechanic blame fouled plugs on leaning too much, but I’m sure it’s happened. )

Where the heck do they get this? Well, let’s explore that a bit. Most OWTs have some basis that got them started, and this one is no exception. As it happens, some of them come straight from the FAA, and the questions on the various “knowledge tests” – those computerized tests some still call “writtens.”

A Quick LOP-sided review

GEM before GAMIjectors I’ve beaten the subject to death previously, but to review briefly, very few “flat” engines will run LOP at all, the mixture varies too much between cylinders. This has long given us only an “available spectrum” of mixtures somewhere between full rich, and somewhere around peak EGT. We know that the CHTs are at their hottest when the mixture is set to roughly 50 ROP EGT, and if any more than about 65% of rated power is set, those CHTs will become “too hot” at 400 F and above, even if the engine manufacturers do set the usual redline at 460 or so.

A lot of practical experience with these engines over the years has forcefully driven the point home (through top overhauls) to most people that anything above 65% is “pushing the engine,” and when you’re pulling 65% or more, it’s best to run a bit richer, for cooling. This is all true and correct, when applied to an engine that can operate only in “the rich half” of the mixture spectrum. This was hard-won knowledge, gained over 50 years of experience in the industry, and the knowledge was passed on to the younger mechanics and pilots. It became an absolute, with anything else simply dismissed, or, as my teen son might say, “dissed.”

A very few engines (as on the Piper Malibu) were carefully tuned to be able to run LOP, but even on these, pilots and mechanics were so nervous at this “new” mixture setting, they’d often richen the mixture “just a bit, for safety,” thus putting the engines right back in the hottest part of the mixture curve! It’s no wonder those engines had so many problems! If only someone had known (a few did) to simply lean the engine a bit, instead of enrich it, things would have been a lot better, and MUCH cooler.

We now know that if the fuel and air are properly balanced between all cylinders, all these engines can run much cooler and cleaner when LOP, and of course, they can also still run the same old sorry, dirty ROP we’ve always used. It’s still clear that one should generally avoid only the 50 ROP area at high power. But 50 years of experience is hard to overcome, and the idea that “Leaner is always hotter” sticks tight. It is, until it ain’t, then it’s cooler!

Nothing New

Radial engineThe really old hands with radial experience are no problem at all, one demonstration of LOP in the airplane, watching the control movement, and they instantly flashback and say, “Why, that’s exactly what we used to do with the big radials!”

The ONLY difference is terminology, and instrumentation. The radials use an accurate power measurement device (“Torque,” or “BMEP”) to lean to peak power, and then they lean further by some increment of power, depending on what is desired. Modern engines do not have any way to accurately display power, but we use engine monitors that show EGT and CHT, and we’ve long known the relationships. The end results are exactly the same.

Mike also pointed out to me that the current A&P knowledge tests not only don’t address any of these modern issues, but are mostly based on the ancient radial engines now operated only by flying museums, and a few wonderfully anachronistic folks in Alaska who still run them. As Mike wrote:

Another fascinating aspect of the AMT Powerplant exam is that the overwhelming emphasis of the recip-oriented questions is radial engines, pressure carburetors, and Hamilton Standard hydramatic propellers. Out of the 1000+ questions in the question bank, I think there are only a handful about opposed engines, perhaps two or three about continuous-flow injection, only one about modern Hartzell compact-hub props, and none at all about McCauleys. It would be too kind to say that the test is anachronistic in the extreme. (OTOH, if you ever have a question about adjusting valve clearances in an R2800 or trimming the fuel control on a DC-8 engine, I’m your man! )

I think we need to get Mike interested in the Confederate Air Force, where that knowledge can be put to good use!

Q&A for A&Ps, FAA-Style

He also sent me a few sample questions from the FAA’s current “AMT Powerplant” knowledge test, and I’d like to kick a few of them around here.

8072. Which fuel/air mixture will result in the highest engine temperature (all other factors remaining constant)?

A-A mixture leaner than a rich best-power mixture of .085.

B-A mixture richer than a full-rich mixture of .087.

C-A mixture leaner than a manual lean mixture of .060.

FAA-approved answer: C.

Mike Busch’s comment: If memory serves, stoichiometric is around 15:1 or .067, so answer C (“leaner than .060” or about 17:1) would be VERY lean-of-peak and leaner than most engines can run smoothly. I’d imagine that the closest-to-correct answer is probably A.

Mike is absolutely correct on this one, of course. That mixture setting would be very cool, on the lean side. However, engines equipped with well-tuned GAMIjectors will run quite smoothly that lean, and leaner, perhaps out to about 18:1.

But think of the generations of A&Ps who have studied their hearts out, and learned this? Study guides probably have it, and while I haven’t checked, I’ll bet the “official FAA-approved manuals” leading to the A&P probably have this, too.

(Say, am I the only dinosaur who still has difficulty typing “A&P?” Throughout my youth, it was “A&E,” for “Aircraft and Engine.” WHY did the FAA change it? Now we’re moving on to “AMT,” I guess, but I don’t think I’ll live long enough to adapt to that. Similarly, when I was 23, I got my “ATR,” now it’s an “ATP.”)

Mike is also quite correct that most of the questions are from the radial engine era, and may not apply directly to flat engines, even if the combustion characteristics and metallurgy are more alike than different. If nothing else, the language is very different, as I shall point out.

“Rich best-power mixture,” and “Lean best-power mixture” are straight out of the old radial manuals, and to my knowledge, are never used with the flat engines. They certainly could be, of course, and perhaps somewhere they are, but radial music plays in my head when I see those terms.

We need to stop here, and get something straight. We throw a ton of numbers around in these discussions, tests, and textbooks, so let me take a stab at some clear definitions.

“BSFC” is for “Brake Specific Fuel Consumption.” Pay no attention to the techno-speak, it’s simply the pounds of fuel used to make one horsepower (HP) for one hour. On our big-bore flat engines with 8.5:1 compression ratios, you’ll see figures like 0.40 when “pretty lean.” The lower the number, the more efficient the engine is at making HP. If you’re using 100 HP, with a mixture set fairly lean, you’ll burn about 40 pounds per hour, about 6.8 GPH. All kinds of things affect BSFC, including fuel, spark timing, compression, piston size/shape, mixture, manifold pressure, RPM, the list goes on forever.

“Fuel/Air Ratio,” so common in the old manuals, is simply the ratio of the weight of fuel to air. At a fairly lean mixture setting, you’ll see a number of around 0.062, or sixty two thousandths of a pound of fuel, to one pound of air. In more normal numbers, that would be 6.3 pounds of gasoline (roughly one gallon) to 100 pounds of air. Yes, 100 pounds of air is a lot of air! That’s 1,300 cubic feet, or about 9,700 gallons of air. Picture a room full of air – 13′ long, 10′ wide, and 10′ high – and a gallon of fuel, and you’ll have a mental picture of this fuel/air ratio. We can further simplify those dreadful decimal numbers to whole numbers, and call “0.062” by its inverse, 16:1.

(In other words, 0.062 pounds of fuel to 1 pound of air is exactly the same mixture as 16 pounds of air to one pound of fuel. Engineers seem to always use those pesky decimal numbers, instead of nice, whole round numbers I can relate to!)

Don’t confuse these two very different measurements. BSFC is pounds per horsepower, the other is the fuel/air mix, by weight.

The charts for the old engines tend to use fuel/air ratio, the modern charts for the flat engines tend to use temperatures rich and lean of peak EGT. The results are the same.

Here’s another question from the FAA’s current AMT Powerplant knowledge test:

8094. Which of the following would most likely cause a reciprocating engine to backfire through the induction system at low RPM operation?

A-Idle mixture too rich.

B-Clogged derichment valve.

C-Lean mixture.

FAA-approved answer: C.

AMT study guide explanation: A lean fuel-air mixture burns slower than either a rich or a chemically-correct mixture.

Well, sorta. The truth is that a mixture just slightly richer (maybe 25-50F ROP) than a chemically-correct mixture burns the fastest, while the flame front slows down if the flame front is EITHER richer or leaner from that point. That’s precisely why we use an excessively rich mixture at very high power, to SLOW combustion, and put the peak pressure far enough after TDC to prevent detonation and keep CHTs down to reasonable levels. The AMT study guide continues:

There is a possibility that a lean mixture will still be burning as it is pushed out through the exhaust valve.

This is true in theory, but it requires a mixture that is so lean that it is not a useful mixture setting, even when normally operating lean of peak.

During the time of valve overlap, when both the intake and the exhaust valves are open, the burning exhaust gases can ignite the fresh fuel-air charge being taken into the cylinder through the intake valve. This can cause a backfire through the induction system.

Mike Busch’s comment: At low RPM, it’s difficult enough to keep combustion going past TDC. By the time the intake valve opens more than 360 degrees of crankshaft rotation after ignition (and that’s a loooooong time at low RPM), there’s not a snowball’s chance in hell that combustion would still be going on.

Mike gets that one right, too, but the truth is hiding in there somewhere, because many old radials WILL backfire, and it’s ALWAYS because the mixture is too lean. There is one key point that the FAA misses, perhaps in an attempt to simplify.

Many of the old radials have large exhaust collectors, leading to one or two big outlets. Like almost all internal combustion engines, there is “valve overlap,” where BOTH the intake and exhaust valves are open at the same time, “overlapping” the end of the exhaust stroke, and the beginning of the intake stroke.

In most of the big radials, they are both partly open for a full 45 degrees of crank rotation! Long ago, engineers found this helps the bad stuff leave the combustion chamber, and the good stuff to enter. One pushes, the other pulls, and the valves snap closed and chop ’em off at the best point, so almost all the bad stuff is gone, and very little of the good stuff sneaks out early. The engineers speak of “volumetric efficiency,” but I stick to “bad stuff” and “good stuff.”

During the time both valves are open, there is a direct channel open between the intake manifold and the exhaust manifold. Any “fire” in the exhaust can light off the mix in the intake. During normal operation, the pressures and high flows prevent “backflow,” and the fire can’t get to the intake. Good thing, too!

However, at very low RPM, just barely above cranking RPM (about 50 RPM), and below normal idle RPM (about 600 on most), the pressure in the intake manifold is very, very low (i.e., high suction), with the pistons trying to pull the “good stuff” in against the nearly-closed throttle plate. This might show up as 15″ to 20″ of MP, with an ambient pressure of 30″. What happens? Anything in the exhaust manifold gets sucked back into the combustion chamber, and back into the intake manifold during “overlap.”

Sometimes, during the start, raw fuel, or a partial charge of fuel and air, can get through the combustion chamber without being lit off. A spark plug might be oily, or perhaps that shot of mixture wasn’t quite combustible as it went through the combustion chamber, either too “wet” (rich), or too “dry” (lean). But once in the exhaust manifold, it will get lit off by the fire coming from other cylinders. If there’s enough raw fuel, it can cause a “torching,” up to 20 feet of slow-burning flame that burns until the fuel is gone. That’s generally spectacular, but usually harmless. It may scorch paint a bit. Of course, if the engine is all greasy and oily, more serious external fires can develop, a good reason to wipe the old birds down when done. One the other hand, if there is a combustible mixture lurking in the exhaust manifold, and enough of it, it can burn much more quickly, and make a big bang, an “Afterfire.” Very hard on exhaust manifolds.

Finally, if any fire in the exhaust manifold gets sucked back into the intake through ANY cylinder when both valves are open, you’ve now got a “fire in the hole,” so to speak.

How to prevent this? The engine manufacturers were very clever. During a normal start, at 50 RPM or so, with normal priming, the mixture in the intake manifold is MUCH too rich to burn. If you could stick a lit match in there, it would be snuffed out. No problem when fire blows backwards into that chamber, it simply can’t burn, and whatever fire there is is snuffed out for lack of air.

Once the exhaust valve closes, the piston is able to suck in a charge, it is compressed (and heated), and mixed with the air that came in from the exhaust manifold, becomes combustible, and lights off.

All this can make starting a big radial pretty tricky. To get the backfire, you must first have “fire” in the exhaust, probably from a mixture that was momentarily too rich. You must then create a “too lean” mixture in the intakes, probably with too much throttle, or too little prime. You’ll see this happen when the engine coughs, tries to start, then “BANG,” when the pilot either let go of the primer, or had too much throttle, just as the engine started.

Mercifully, this is one characteristic that did NOT carry over to flat engines, where you really have to work at it to get a “bang.”

8107. One cause of afterfiring in an aircraft engine is:

A-sticking intake valves.

B-an excessively lean mixture.

C-an excessively rich mixture.

FAA-approved answer: C

AMT study guide explanation: Afterfiring, or torching, is the burning of the fuel-air mixture in the exhaust manifold after the mixture has passed through the exhaust valve. After-firing is usually caused by operation with an excessively rich mixture, such as would be caused by over priming, improper use of the mixture control when starting, or by poor ignition.

This one is correct, I think. Afterfire happens when the charge makes it all the way through the cycle without being lit off at all DURING THE START. It gets squirted out into the exhaust, where there just might be enough heat/fire to light it off. During start, there will be more than enough air (and oxygen) in the exhaust pipes to mix with it, and make it flammable, or even explosive.

8678. Why must a float-type carburetor supply a rich mixture during idle?

A-Engine operation at idle results in higher than normal volumetric efficiency.

B-Because at idling speeds the engine may not have enough airflow around the cylinder to provide proper cooling.

C-Because of reduced mechanical efficiency during idle.

FAA-approved answer: B

I can’t believe even the FAA could go this far wrong! Tell me it isn’t so! See the previous question and answer for a better look at this. Mike offers an additional reason:

Mike Busch’s comment: Which is presumably why most pilots taxi around at full-rich and foul the crap out of their spark plugs. The actual answer to this question is “because a very rich mixture is required for cold-starting, and aircraft carburetors don’t have a choke to provide such a rich mixture, so the idle mixture has to be set extremely rich … which is why as soon as the engine starts to warm up, you need to come back on the mixture control.” Of course, that answer isn’t one of the choices offered.

8773. Carburetor icing is most severe at

A-air temperatures between 30 and 40 degrees F.

B-high altitudes.

C-low engine temperatures.

FAA-approved answer: A

Mike Busch’s comment: Oh really? At OATs in the 30-40F range, the air seldom contains enough moisture to create severe carb icing. (Maybe the air in OKC is frequently supersaturated at those temperatures, but not here in California.) The risk of carb icing is greatest on warm moist summer days. The jerk who wrote this question was probably a non-pilot.

I couldn’t have said it better myself! Can you spell “Carbureted Cessna 182, in Florida?”

Remember, to pass the A&P exam, you have to memorize this and other wrong answers, or risk failing!

8808. In addition to causing accelerated wear, dust or sand ingested by a reciprocating engine may also cause

A-silicon fouling of spark plugs

B-sludge formation

C-acid formation

FAA-approved answer: A

AMT study guide explanation: Sand that gets into an engine acts as an abrasive and causes accelerated wear of the cylinder walls. Silica in the sand also forms a silicon glaze on the nose core insulators of the spark plugs. This form of contamination is an insulator at low temperature, but becomes a conductor when it is heated.

Mike Busch’s comment: Has ANYONE ever heard of silicon fouling of plugs? I’ve never heard of it in 35 years of flying and 14 years of swinging wrenches. The Champion Spark Plug manual certainly doesn’t mention it, although it covers both oil fouling and lead fouling in detail. On the other hand, “sludge” is caused by particulate contamination that comes out of suspension from the oil, so my vote for the correct answer is “B” … but the FAA didn’t consult me.

This one’s a total mystery to me, too. I’m willing to learn, so can any reader come up with anything on this? Silicon is an element of sand, so if it shows up in the oil analysis, it’s an indicator that you’ve got a dirty filter (bypassing it), have been operating in dusty conditions too much with the carb heat on (bypasses the filter on most engines) – or your mechanic used too much DC-4 silicon grease when he installed your oil filter. Any dirt and sand that comes in the intake gets pulverized, a bit of it blows by the rings into the case, that mixes with the oil, and the oil becomes a bit abrasive.

8829. Which of the following defects would likely cause a hot spot on a reciprocating engine cylinder?

A-Too much cooling fin area broken off.

B-A cracked cylinder baffle.

C-Cowling air seal leakage.

FAA-approved answer: A

Mike Busch’s comment: In my experience, uneven cylinder cooling (hot spots) are most often caused by defective or misaligned baffle seals (answer C) and, less frequently, missing or cracked baffles (answer B). Broken cooling fins are down at the noise level as a cause.

Let’s see here, the FAA makes up three answers, and says the “wrongest” is right. Mike is right again. (Durn, that boy is good, he ought to have an A&P certificate!) I suppose if you broke off enough fins, A could be correct, but baffling is so very important, it’s far more likely to be either B or C.

8982. If a flanged propeller shaft has dowel pins

A-install the propeller so that the blades are positioned for hand propping.

B-the propeller can be installed in only one position.

C-check carefully for front cone bottoming against the pins.

FAA-approved answer: B

Mike Busch’s comment: Modern opposed engines use flanged crankshafts with holes for dowel pins that are an interference fit into the propeller hub and mate with registration holes drilled in the crankshaft mounting flange. There are two identical pins spaced 180-degrees apart, which permits the propeller to be installed in either of two orientations. If the prop is three-bladed, one of these orientations results in the vertical blade pointing down when the engine stops, and the other orientation results in the vertical blade pointing up. Only one of these two orientations is correct, and which is correct depends on the particular installation, so you have to check the service manual. Installing the prop in the opposite orientation often results in serious vibration problems. Answer B is just plain wrong (as are A and C).

If this were an internet forum, I’d be ROF,L (rolling on floor, laughing)! Hand-propping? A current FAA test, talking about hand-propping? But I do seem to have a very dim memory of wooden props on the very old birds being installed so that it was easy (or hard) to hand-prop them, for that was the only way to get them started.

Mike is too young, but this old phart can remember the days when you could holler at almost anyone on the ramp, “Hey, gimme a prop, willya?” The old litany would ensue, “Off and closed,” a few flips, and then “Make it hot,” or (more formally), “Brakes, and contact!” We used to have contests to see who could make a Champ (N1943E, why do I remember that?) prop spin the most turns, or who could prop a Bonanza, or a T-6.

Then there was a period when that first call across the ramp might be, “Hey, do you know how to hand prop an airplane?”

Now? Well, I guess it’s safer now, but damn, it ain’t near as much fun.

But, I digress ….

But My Mechanic Says …

Aircraft mechanic at workIn Mike’s case, the test is a pain, but relatively harmless. But what of the person who doesn’t have the vast background he does? Mike can learn the bogus answers, regurgitate them (an appropriate simile), and then (as he says) “dump them into the bit bucket where they belong.” But what of the person learning this junk from the FAA manuals, at the FAA-approved schools, who takes the tests, gets his A&P, and goes out to practice. (Oh, and what of the turbine questions, where Mike is not an expert? Are there as many bad answers there?)

Now, with all this, you bring your favorite flying flivver in, your mechanic finds a cracked cylinder, and next think you know, “Yep, I tole ya, you’re running too lean!”

Or, the other scenario we’re seeing/hearing a lot. Read the old ads in Trade-A-Plane, you’ll find a HUGE percentage of the big-bore flat six engines mentioned there were “topped” or overhauled at 500, or 600 hours. This is almost the rule, rather than the exception. That was LONG before anyone was running LOP, because they couldn’t.

Today, someone runs the same old engine to 500 or 600 hours, decides to put in GAMIjectors, runs LOP a few times, and suddenly finds his compression dropping. Our mechanic nods wisely and, “Yep, I told ya running LOP would do you in, it ain’t natcheral!”

ASA Fast Track Test Guides for Aviation MaintenanceIsn’t it time someone updated some of these tests? Shouldn’t the tests cover modern engines to a greater extent, and the radials a bit less, more in proportion to the engine count? From my personal viewpoint, loving radials, I hope they don’t do away with the radials entirely, the lack of qualified mechanics is one factor killing the old airplane, but at least the questions and answers on radials ought to identify them, and be right.

Couldn’t we make all FAA knowledge tests so that instead of three ridiculous and sometimes equally wrong answers, we could have two that are wrong logically, and one that is RIGHT, 100% right, instead of one that is “least wrong”?

Be careful up there!