Rough Engine: Hardware or Software?

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Fred’s Cirrus SR22 was giving him trouble. The engine would regularly “hiccup” during cruise flight, and this was really spooking Fred. Worse than that, it was spooking Fred’s wife to the point that she was no longer willing to fly in the airplane with him. 

This was now a crisis for Fred. He took his plane to the nearby Cirrus Authorized Service Center, and described the hiccuping engine issue he was having. The shop manager said they’d investigate.

Fred’s airplane was only a couple of years old, and its “Platinum” Continental IO-550-N engine came with a five-year warranty that covered both parts and labor for the first two years and parts only for the remaining three years. Fred was in the parts-only portion of the warranty.

Since the engine was still under warranty, the Service Center called Continental product support, described the problem as Fred had described it to them, and asked for the factory to help resolve the problem under warranty. The Continental tech rep thought it might be a problem with the engine-driven fuel pump, so he arranged to have a replacement pump shipped  to the shop under warranty. The shop installed the new pump, then performed the required fuel injection system setup adjustments in accordance with Continental’s service instructions. They notified Fred that his airplane was ready for pickup, and presented him with an invoice for six hours of labor (but not for the pump, which was covered under Continental’s warranty).

Fred flew the airplane home. The engine hiccuped twice during the flight home, and hiccuped several more times during the next flight. He brought the plane back to the Service Center and told them the problem had not been resolved.

The Service Center called Continental product support again. This time, the factory sent a replacement fuel control unit under warranty. The shop installed it, repeated the fuel injection system setup, and presented Fred with another labor-only invoice.

Fred flew his airplane home. The engine still hiccupped. Fred took the plane back to the Service Center for a third time. He was not happy.

The Service Center called Continental again, and the factory shipped them a new flow divider manifold valve under warranty. The shop installed it, repeated the setup, and presented Fred with yet another labor-only invoice.

If you’re guessing that Fred’s engine still hiccuped, you guessed right. The Service Center was now out of ideas. Fred’s wife still refused to fly with him until the problem was resolved. Fred was feeling desperate.

At the suggestion of a fellow Cirrus owner, Fred contacted Savvy. At our suggestion, he enrolled his plane in SavvyAnalysis Pro ($129/year), uploaded his engine monitor data (including several hiccuppy flights) to the SavvyAnalysis platform, and created an analysis ticket describing his tale of woe.

One of Savvy’s data analysts examined the data from the hiccuppy flights. At first, the analyst couldn’t find anything wrong. There was no evidence of a fuel system problem or an ignition system problem. The analyst looked harder and did find something that seemed odd. He posted a ticket reply to Fred:

“Fred, although I’m not seeing any evidence of problems with your engine’s fuel or ignition systems, I can’t help but notice that you seem to be operating the engine lean-of-peak (LOP) at 2700 RPM. The IO-550-N doesn’t like to operate LOP at such high RPM because there’s not enough time for the combustion events to complete before the exhaust valve starts to open. We recommend that you pull back the power lever to reduce RPM to 2500 or less before you go LOP.”

Fred replied that he was surprised by the analyst’s recommendation because he’d been taught that the engine should be operated at wide-open throttle (WOT) when the mixture is set LOP. Why was the analyst recommending something different?

The analyst explained that the Cirrus SR22 has an unusual engine control setup where a single power lever controls both the throttle position and the prop governor setpoint. With the power lever full-forward, the engine is running at WOT and 2700 RPM. When the power lever is pulled back, the first thing that happens is that RPM is reduced to 2500 RPM. Pulling the lever back further starts to close the throttle butterfly as well as continuing to reduce RPM. The optimal power lever location for LOP cruise is the point where RPM has been reduced to 2500 RPM but the throttle butterfly is still wide-open (WOT). That’s the point that is commonly referred to as “the bump” in Cirrus circles.

Fred said he understood, and would try pulling the power lever back to “the bump” before leaning the engine for LOP cruise. He did so on several flights, and reported that the engine was no longer hiccupping and, more importantly, his wife was flying with him again.

We love happy endings like this!

Fred’s experience with his Service Center was not unusual. Mechanics tend to focus on the hardware. When the hardware is under warranty, they tend to ask the factory for assistance. When asked for assistance, factories tend to send replacement parts (which is pretty much all they can do in these circumstances). 

Neither the Service Center nor the factory considered the possibility that Fred’s problem might be a software problem rather than a hardware problem. Neither ever asked Fred to describe how he was managing the powerplant in flight. 

The engine monitor data told the tale. The engine was talking but nobody was listening…until Fred finally asked Savvy to take a look at it. Once he did, the diagnosis became obvious.

For just $129/year ($199 for twins), SavvyAnalysis Pro clients get unlimited expert analysis of their engine monitor data. Sometimes this analysis will pinpoint specific problems with the engine—a misfiring spark plug, a dirty fuel nozzle, and induction leak—while other times it will uncover an issue with how the pilot is operating the engine. SavvyAnalysis Pro clients also receive regular Report Cards that show how their airplane stacks up against hundreds or thousands of other similar aircraft we follow with respect to various parameters affecting performance, efficiency, and longevity. They receive Trend Analysis Reports that identify statistically significant trends such as decreasing oil pressure or increasing CHTs. They also receive regular exhaust valve failure risk reports generated by Savvy’s proprietary FEVA 2.0 machine-learning algorithm. And there’s even more to come from the mathematical wizards at Savvy’s skunkworks.

Wouldn’t you feel more confident about your engine’s health and your powerplant management skills with Savvy’s analysts and algorithms looking over your shoulder? Learn more about SavvyAnalysis.

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