Engine Operation Superstitions: Frustrating Persistence

It's again time to combat engine operating superstition with facts.


During a recent conversation with a pilot I had just met, he mentioned that one of the things he did as a volunteer pilot for Civil Air Patrol was to tow gliders in the squadron’s Cessna 182. He expressed frustration with the very time-consuming and complex power reduction procedure imposed on the pilots to prevent cylinder cracking due to the scourge of shock cooling. It turned out the procedure took so long that, even if the glider did several minutes of soaring on its flight, it was well on the ground before the tow plane. As a long-time tow pilot, this struck me as ludicrous.

The anti-shock cooling procedure the pilot described would have made Freud get out his notebook to redefine anal retentive. It required a small reduction in manifold pressure and then flying around for a period of time before making the next, miniscule reduction and time delay, while descending slowly and burning a boatload of fuel. And it was all to avoid one of aviation’s seemingly invulnerable mythical beasts—shock cooling. If shock cooling did actually exist and cause cylinder cracking, it would probably be cheaper for that CAP squadron to buy a brace of cylinders and keep them on hand for replacement than pay for the fuel they were going through to avoid a phantasm.

I used to be astonished at how aviation myths, particularly when it came to engine operation, have such incredible staying power. Now, when I hear one spouted, I just shake my head in admiration of the power of ignorance and belief over data. With some folks, the laws of physics, aerodynamics, metallurgy and thermodynamics are trumped by unwavering faith in their particular superstitions.

Nevertheless, when aviation superstitions get in the way of safe, efficient engine operation, they need to be exposed for the nonsense they are, particularly when they are adversely affecting others—such as the CAP operation that can only get off a few glider flights a day because someone in a position of power is imposing ridiculousness on others and teaching it as fact to young men and women who are learning to fly.

The widely-respected Daniel Patrick Moynihan put it eloquently: “Everyone is entitled to his own opinion, but not his own facts.” It’s again time to combat aviation superstition with facts. Let’s look at four lingering myths: shock cooling causes cylinder cracking; lean of peak (LOP) operation will damage the engine; over-square (the number of inches of manifold pressure being higher than the first two digits of the engine RPM) operation will cause the engine to “lug,” damaging it; and, the power must be reduced after takeoff for climb if the airplane has a constant speed prop.

Shock Cooling

There is absolutely no hard evidencethat making a large power reduction will cause cracking of the cylinders of a piston aircraft engine. Such claims have been made for years—often backed up statements along the lines of, “Yeah, my mechanic says that he makes good money from pilots who yank the power back at the end of cruise and crack their cylinders;” or, “I used to work in an overhaul shop and I saw lots of cracked cylinders come in due to shock cooling;” and other odes to the scientific method.

Nearly 20 years ago, Kas Thomas wrote what was probably the definitive piece on the shock cooling myth, which was reprinted in AVweb. He started out by noting that Bob Hoover regularly shut down and feathered the engines on his Aero Commander Shrike during airshows—going from max power to none—and never cracked a cylinder. That’s consistent with what skydiving and glider tow operators report—their engines hit TBO without much in the way of cylinder problems, even though they descend rapidly at very low power settings. Flight schools, with their repeated touch and goes, don’t go through cylinders at a disproportionate rate. (What appears to really matter is operating an engine regularly—sitting is hard on engines.)

Thomas went on to examine the small role that the cylinder fins play on engine cooling—only about 12 percent of the heat generated by combustion departs from the engine via the cooling fins. The big fraction, 44 percent, goes out the tailpipe. Eight percent, almost as much as is handled by the cooling fins, is dissipated through the oil.

Going through the published test data, Thomas showed that cutting engine power by half only reduces CHT by 10 percent or so. That kind of CHT drop isn’t capable of trashing cylinders—and isn’t anywhere close to the CHT change that occurs in the opposite direction on takeoff—shock heating, so to speak. And there’s never been any data to indicate that the massive shock heating during takeoff harms the cylinders.

In his article, Thomas also pointed out that flying through rain reduces CHTs by nearly as much as a 50 percent power reduction. There’s no history of airplanes regularly flown through rain, such as the night freight folks, having to constantly replace cylinders.

In fact, the real shock cooling comes at the end of the flight when you pull the mixture to idle cutoff and the CHTs drop at more than 100 degrees per minute right away—yet every engine goes through that sort of shock cooling and manages to survive it.

In the time since Kas Thomas’s article, graphic engine monitors have become common in general aviation—and the data they provide further support his conclusion. Many are set to alarm if the CHTs show a drop at a rate of more than 60 degrees per minute. Pilots are discovering that it’s nearly impossible to hit that rate without slamming the throttle shut and diving—which isn’t comfortable for anyone in the airplane. Mike Busch, A & P and principal of Savvy Aircraft Maintenance Management, told us in a conversation at the AOPA Fly In at Colorado Springs that he’s been tracking how fast CHTs will drop with various power reductions in his Cessna T310R and has found it unusual to have them drop at a rate of even 30 degrees per minute with aggressive power reductions when ATC gives a slam dunk approach.

I think it’s time to put the shock cooling myth to bed, so that pilots can worry about things that really are a risk to them in aviation—such as runway loss of control in crosswinds, and spend time practicing crosswind landings. Where we really spend money tearing up aircraft engines is when we lose control on landing, run off the runway and hit something or collapse the landing gear.

Lean of Peak

Despite all the work George Braly and his team at General Aviation Manufaturing, Inc. (GAMI) have done in the last two decades, and publishing their data, it’s just plain weird that there are those who insist that LOP operation is some sort of evil force designed to desroy engines. AVweb’s Paul Bertoreilli recently wrote an in-depthblog about the LOP-will-damage-your-engine Old Wives’ Tale continuing to hang around. The engine superstitious insist that LOP causes burned valves because the engine is being operated too lean, yet they never seem to explain how LOP operations can generate the heat necessary to cause a burned valve. While they shake their garlic necklaces, the continuing success of LOP ops for engine longevity due to cooler temperatures and money saved as a result of better longevity and lower fuel burns is speaking for itself.

It’s pretty basic, as you lean the mixture, EGTs and CHTs increase with the CHTs peaking shortly before EGTs. Continuing to lean the mixture causes CHTs and EGTs to drop and continue dropping (that’s why the term “peak” is used). LOP means cool.

Some years ago, AVweb’s John Deakin published a series of engine operating columns that are still considered the best guidelines around on overall engine and LOP operations.

With an increase in the understanding as to how the combustion process works, let’s hope the phrase “too lean” can be relegated to the trash heap of time, to be replaced by the more accurate term, “wrong lean.”

Oversquare Operation

There is nothing magic about the fact the numbers on the manifold pressure gauge are sort of the same as the first two digits of the numbers on the tachometer on a horizontally-opposed, normally-aspirated piston engine. That means you are not going to hurt anything by setting the prop rpm at 2200 rpm and the manifold pressure at 24 inches. In fact, it’s probably better for the engine than turning the prop faster to get the same percentage of power because the slower rpm power setting means less wear and less fly-over noise.

For some reason you’ll hear pilots claim that oversquare operation “lugs” the engine as if it’s analogous to a manual-transmission automobile running slowly in a high gear. It’s not.

Good grief, running at low rpm and high manifold pressure is the technique Lindbergh taught the Army so it could radically extend the range of the P-38. The performance data was clear—and it didn’t “lug” the engines.

If the POH gives an acceptable rpm/manifold pressure combination, the only reason not to use it is if it generates a vibration in your particular airframe that you don’t care for.

Power Reduction After Takeoff

I recall going through a checkout in a Diamond DA-40 Star in which the owner of the FBO was my instructor. Despite the POH pointing out that the 180-HP Lycoming engine was rated for continuous running at full power, he absolutely insisted that it was necessary to make a power reduction to an arbitrary setting for the post-takeoff climb. When I inquired about why he wanted us to ruin the rate of climb, the response was, “I’ve taken dozens of engines to TBO, I know how to run these engines.” Hoo boy.

If you look at the POH for the airplane, it gives you a choice: You can do a maximum performance climb or a “normal” climb. You burn about the same amount of fuel in each one: The “normal” climb has a lower burn rate, but the reduced rate of climb makes up for it. It takes about the same amount of avgas to lift an airplane to altitude at a given airspeed no matter if full power or “climb” power is used.

Radial engines often had time limitations on the use of maximum or takeoff power. It is necessary to make a series of power reductions after takeoff because of the limitations of the engine. The larger, early horizontally-opposed engines that came out after World War II also had time limits on full-power operation. As I recall, the first Bonanzas called for a power reduction within one minute of applying full power. In any event, check your airplane’s POH for the straight scoop. There are models of some big-bore Continenals and Lycomings that still have time limitations on full power operation. If the engine doesn’t have a time limitation on full-power operation, there’s no need for you to create one, you aren’t helping the engine live longer.

At full power, the engine runs cooler than at reduced power because it gets more fuel for cooling. Full power is a good thing from the standpoint of being nice to the engine. From the perspective of being nice to yourself, getting as much altitude as you can as soon as you can is also a good thing, so why increase the amount of time you are going to be fumbling around down low? You want to get up to where that tailwind is whistling along, plus, you want altitude so that you have a decent radius of action should that engine decide to take the day off.

Besides, the engine only makes rated power at sea level on a standard day, anyway. If it is hot, no matter at what altitude the airport resides, you aren’t getting full power, so making a reduction from that is foolish, unless you prefer to subject your pax to a sauna for as long as you can. Leave everything against the wall and climb with as much power as you can get if the manufacturer allows it.

Stomp ‘em Out

It’s time for these engine superstitions to go away. It’s time to give yourself the freedom to make a reasonable power reduction to descend and get the airplane on the ground before you need a birthday cake because you’re a year older; to save fuel and prolong your engine life by operating at the fuel mixture setting that is right for your engine—including LOP—according to data, not blind belief; you aren’t going to hurt your engine by running at an oversquare power setting if it’s in the POH, and it may save wear and tear on both the engine and airframe; and, your engine will run cooler and you’ll climb better if it’s approved for continuous full power operation and you take advantage of that as you climb out.

Rick Durden is a CFII, holds an ATP with type ratings in the Douglas DC-3 and Cessna Citation and is the author of the newly-released Volume 2, The Thinking Pilot’s Flight Manual or, How to Survive Flying Little Airplanes and Have a Ball Doing It.