The Data Doesn’t Lie

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“I just finished the annual condition inspection on my RV-10,” Bill reported on his SavvyAnalysis Pro ticket. “On the test flight, I was shocked to see that all the left bank CHTs (cylinders 2, 4 & 6) were around 30℉ higher than I have even seen before.”

Savvy’s chief analyst Joe Godfrey grabbed this one. “What work did you do on the engine during the annual?”

“The only thing I changed was that I rerouted the ignition harness wires on the right bank of cylinders so that my SureFly electronic ignition system (EIS) now fires the bottom spark plugs all cylinders and the Bendix magneto fires all the top spark plugs,” Bill replied. “Nothing was changed on the left side ignition harness, but it’s the left cylinders that are running way too hot.”

“Did you adjust the ignition timing?” asked Joe.

“Well, I checked the ignition timing, but I didn’t really change it,” said Bill. “The ignition timing spec for my engine is 25° before top dead center (BTDC). The Bendix mag checked to be 24.9° BTDC and the SureFly was 24.5° (which is about 0.5° retarded). I also removed the Tempest fine-wire spark plugs and installed new Tempest massive-electrode plugs.”

“On the test flight, max fuel flow was about the same as it has always been,” Bill continued. “I checked the cooling baffles and baffle seals carefully and I don’t see any problem with them. I also checked the fuel nozzles and they are all flowing about the same. Your advice would be greatly appreciated.”

Easy Diagnosis

Joe pulled up a chart of the engine monitor data that Bill had uploaded after the post-maintenance test flight, and the problem was immediately obvious. “Bill, looking at the two takeoffs from today’s test flight, EGTs are relatively low when CHTs are abnormally high,” Joe reported. “Here’s what your EGTs and CHTs looked like during your second takeoff.”

“This combination of high CHTs and low EGTs almost always indicates that the ignition timing is too advanced—in other words, greater than 25° BTDC,” Joe explained. “I know you said you checked the ignition timing during the inspection, but I suggest you check it again. It’s not hard to get it wrong, and that goes double with electronic ignition systems like the SureFly. Don’t ask me how I know this.” 

(Joe owns and flies a Bellanca Super Viking on which he does most of the maintenance himself.)

The Data Doesn’t Lie

A few days later, Bill reported back in…

“Joe, you nailed it. I finally realized that in my attempt to slightly retard the SureFly’s ignition timing to be a little less than 25° BTDC, I actually had it advanced to a little more than 25° BTDC. The SureFly is supposed to be mechanically timed to 0° BTDC, and then the DIP switches on the EIS are set to electronically advance the timing by 25°. Well, I had the DIP switches set correctly, but the mechanical timing was actually about 0.7° BTDC so the actual ignition timing was about 25.7° BTDC, which is clearly too aggressive.”

“I have now reset the timing on both the Bendix mag and the SureFly EIS to a much more conservative 24.3° BTDC. I mechanically timed the SureFly to 0.7° ATDC, which combined with the DIP switch setting of 25° yields an actual ignition timing of 24.3° BTDC.”

“That works for me,” said Joe. “We advise setting the ignition timing to what’s on the engine data plate (25° BTDC in your case) with a tolerance of plus 0° and minus 1°. If you’re now timed to 24.3° BTDC, you’re within that tolerance and on the conservative side. This should help your CHT problem quite a bit.”

“It also occurred to me that I may have been finding TDC incorrectly using a piston stop plug and a digital inclinometer,” Bill said. “This time I followed the directions that came with the timing kit very closely, and I am confident I did it accurately this time.”

“The proof is in the pudding,” Bill concluded. “If you pull up my latest flight, you’ll see that all the CHTs peaked below 400℉ this time. It’s all good. Thanks so much for steering me in the right direction.”

“Thanks for the update,” said Joe, “and kudos for your attention to detail. Also for your excellent judgment in making that post-annual test flight and correcting this problem before launching with passengers. I’ll mark this case closed, but holler if you need any further help.”

Bill’s airplane is one of hundreds of Van’s RV experimental amateur-built
(E-AB) airplanes enrolled in our SavvyAnalysis service. We love working with their hands-on maintenance-involved owners, and they seem to love what we offer them. 

A subscription to SavvyAnalysis provides regular Report Cards and Trend Reports to keep you continually apprised of the condition of your engine’s critical systems—ignition, fuel, oil, electrical, valves, etc. It gives you unlimited access to our team of expert analysts (like Joe) who offer precision diagnosis of any problems that you may have—such as the unexpected maintenance-induced failure (MIF) that Bill encountered on his post-annual test flight. 

In fact, we strongly urge our SavvyAnalysis clients to fly a recommended flight test profile and submit their data for expert analysis BEFORE each annual inspection, so they know what needs to be worked on during the annual—and equally important, what doesn’t need to be messed with. There’s no need to be cleaning fuel nozzles or fiddling with ignition timing if the data indicates that those systems are working fine.

Finally, SavvyAnalysis includes FEVA 2.1 reports that utilize advanced machine-learning technology to analyze your engine monitor data algorithmically to alert you to possible incipient exhaust valve failures before they can compromise safety of flight.

You bought a plane to fly it, not stress over maintenance.

At Savvy Aviation, we believe you shouldn’t have to navigate the complexities of aircraft maintenance alone. And you definitely shouldn’t be surprised when your shop’s invoice arrives.

Savvy Aviation isn’t a maintenance shop – we empower you with the knowledge and expert consultation you need to be in control of your own maintenance events – so your shop takes directives (not gives them). Whatever your maintenance needs, Savvy has a perfect plan for you: