AOPA PILOT / October 2021

460 or 500 degrees F, but it’s good mental image to use to ensure maximum engine and cylinder longevity. These numbers should be adjusted for some special situations. They should be adjusted downward by 20 degrees F or so if you’re flying in unusually cold outside air temperatures (below ISA), and also if your airplane has an extraordinarily efficient cooling system (e.g., Cirrus, Columbia, Diamond). Also, the not-to-exceed values should be increased by about 20 degrees F when breaking in new or newly honed cylinders because CHTs naturally run hotter than usual for the first few hours until break-in is complete. Also, while we don’t want CHTs to be too hot, we don’t want them to be too cool, either. If CHTs get cool enough, there may be lead scavenging issues that cause excessive deposit buildup in the combustion chamber and particularly on exhaust valve stems, resulting in valve sticking issues (especially in Lycomings). For this reason, I suggest your mental CHT green arc extend downward to about 350 degrees F for Lycomings or 330 degrees F for Continentals, with a yellow arc below. That’ll keep you in the sweet spot. Keep in mind there’s nothing magic about any of these numbers. Nothing terrible will happen if a CHT runs in the “mental yellow arc” or if there a brief exceedances above the “mental redline.” I’m offering them only as suggestions, not commandments or operating limitations. Cures for hot cylinders You ought to be able to keep your CHTs in the sweet spot if you’re paying attention. But what if you can’t? In that case, you need to diagnose why your cylinders are running too hot. There are three common reasons for high CHTs, and your mission (if you accept it) is to figure out which one is the culprit. One common reason is that the too- hot cylinder is running too lean. If it’s a fuel-injected engine, perhaps there’s a partially clogged fuel nozzle; if the engine is carbureted, maybe there’s an induction system leak. This issue is super easy to diagnose. Simply do a test flight in which you first cruise with a rich-of-peak (ROP) mixture and then switch to a lean-of-peak (LOP) mixture. If a cylinder is running too lean, it’ll have higher-than-normal CHT during ROP operation and lower- then-normal CHT during LOP operation. Frequently such a cylinder will “switch rank” from being the hottest cylinder when ROP to being the coolest cylinder when LOP. If you see this, you can be sure the cylinder in question has a mixture issue. A second common reason for a hot- running cylinder is that there isn’t sufficient cooling air passing over the cylinder’s cooling fins. That’s generally due to some issue with the engine’s rigid 2109p_R&W.indd 91 2109p_R&W.indd 91 8/18/21 1:42 PM 8/18/21 1:42 PM AOPA PILOT / October 2021 92 RUDDER & W RENCH It’s almost impossible to achieve accurate ignition timing with an old-fashioned “flowerpot” timing indicator. Always use a modern digital inclinometer. cooling baffles or flexible baffle seals. If there’s a cooling air problem, the cylinder will run hot both ROP and LOP since the cooling airflow isn’t affected by the mixture setting. If you suspect this might be the issue, you may want to look at my column “It’s Baffling” in the December 2020 issue of AOPA Pilot for some tips. A third common reason for too-high CHT—particularly when all cylinders are running hot—is advanced ignition timing. Your magneto (or electronic ignition) is supposed to be set to a specific number of degrees before top dead center (BTDC) that is marked on the engine data plate— typically between 20 degrees and 28 degrees BTDC. It is essential that the timing be set very accurately—the tolerance is plus 0 degrees and minus 1 degree—and it should be set using a digital inclinometer accurate to 0.1 degrees. If the timing of one or both mags is even slightly advanced (i.e., too many degrees BTDC), it will have a profoundly adverse effect on CHT. The Lycoming IO-360-series offers an interesting case study. When the IO-360 was originally certified, Lycoming specified that the magneto timing be set to 25 degrees BTDC—a relatively aggressive value. But the engines had so many issues with excessive CHTs that Lycoming subsequently issued a service bulletin authorizing the engine timing to be retarded to 20 degrees BTDC. Making this timing change had an insignificant effect on horsepower but a dramatic improvement in CHT. Since the timing change was authorized by noncompulsory service bulletin rather than compulsory airworthiness directive, we now have a mixture of IO-360s flying, some timed to 25 degrees BTDC and some to 20 degrees BTDC. When I generate a CHT histogram of the thousands of IO-360s in our database, I see two distinct peaks in the data. Curiously, the Lycoming service bulletin applied only to the IO-360 family, not the O-360s, O/IO-320s, or O/IO-540s, all of which remain specified at 25 degrees BTDC. I’ve counseled numerous operators of RVs and other experimental aircraft using these engines to try retarding their ignition timing by a couple of degrees, and they’ve been very pleased with the results. I can’t really offer the same advice to operators of certified aircraft, but at least backing the timing off to 24 degrees BTDC (the bottom of the tolerance band) will provide some improvement. mike.busch@savvyaviation.com savvyaviation.com Ask the A&Ps / Every month, experts Mike Busch, Paul New, and Colleen Sterling answer your toughest aviation maintenance questions on our Ask the A&Ps podcast. Submit your questions to podcasts@aopa.org. aopa.org/news-and-media/ podcasts/podcasts/ask-the-a-and-ps Switch today to the AOPA World Mastercard® , and stop paying too much on your aviation purchases. Visit AOPA.org/creditcard to learn more. *Certain points and purchases restrictions apply, see full Rewards Terms and Conditions for full details at AOPA.org/creditcard. 1. Rewards points can be redeemed for Cash Back or other items pro- vided through AOPA Pilot Rewards. A Cash Back redemption is applied as a statement credit. The statement credit will reduce your balance but you are still required to make at least your minimum payment. A minimum of 2,500 points is needed to redeem for Cash Back. Values for non-cash back redemption items such as merchandise, gift cards, and travel may vary. Cash Back1 Every Purchase 1% A CASH BACK 1 REDEMPTION IS APPLIED AS A STATEMENT CREDIT. QUARTERLY BONUS POINT CAP OF 2,500 POINTS FINALLY! A CARD FOR PILOTS. 2109p_R&W.indd 92 2109p_R&W.indd 92 8/18/21 1:43 PM 8/18/21 1:43 PM