Borescope Initiative: Savvy’s Latest Program

We’ve been actively tracking the work and customer offerings of Mike Busch’s Savvy Aviation (www.savvyaviation.com) as it, in our opinion, has endeavored to drag general aviation maintenance into the 21st Century. (Full disclosure: About 10 years ago, as part owners of a Cessna T210 we were a Savvy customer.)

Savvy started life by offering sophisticated maintenance management to piston aircraft owners. It expanded into providing a service that analyzed digital engine monitor data. As the volume of engine monitor data Savvy had received became truly massive, the company became increasingly able to diagnose engine issues and provide warnings to owners when the data showed that there may be a risk of engine damage or failure.

Exhaust Valve Damage

In 2021 Savvy introduced an outgrowth of its engine analysis program called Failing Exhaust Valve Analytics (FEVA) based on scanning engine monitor data from more than 3,000,000 piston-engine aircraft flights to detect evidence of a burned exhaust valve. Savvy would then warn the aircraft owner that FEVA had found evidence of a burned exhaust valve and recommend a borescope inspection.

The current version of FEVA is 2.1 as Savvy’s machine learning software has evolved with experience and review of more engine monitor data (its database now includes some 5,000,000 flights).

Engine Failures

In our opinion, FEVA has prevented engine failures by alerting owners to potential exhaust valve damage, leading those owners to borescope cylinders, detect burned exhaust valves and take appropriate action before things got truly ugly.

We think that this is a big deal because every month we review the 100 most recent NTSB accident reports of the type of aircraft we’re reviewing in the Used Aircraft Guide. We’ve been doing this for decades and we have observed that, depending on the type of airplane, 10 to 25 percent of the accidents arise out of engine power loss or stoppage (with big-bore engines being near the upper end of the range).

Of the engine power loss/stoppage accidents, about a quarter involved a cylinder issue or a burned exhaust valve. The cylinder issues were often directly due to improper cylinder installation. We’ll talk more about the risks of cylinder installations in the field and the probability that cylinders are unnecessarily replaced in a bit. For now, we’ll note that it is our opinion that Savvy’s warnings of potential burned exhaust valves have saved lives.

Borescope Imagery

With all of that as preamble, what grabbed our attention recently is that Savvy has just announced its newest addition to its engine analysis program—the Savvy Borescope Initiative. The goal is to add borescope imagery to its database to monitor cylinder condition and, with machine learning, hone its ability to identify and diagnose cylinder issues earlier and more accurately.

We’ll tell you about the new program and then bore into cylinder issues in general, exhaust valve issues in particular, the danger of pulling cylinders that are healthy and how the increased use of borescopes and a standardized system for collecting and analyzing their images can reduce the number of engine failures and safe lives.

With an announced introductory date of Jan. 1, 2024, Savvy “will expand its web-based cylinder condition software program with new facilities for uploading, archiving and viewing borescope images and associated descriptive metadata to identify each image. That will be coupled with a team of professional analysts experienced and trained in interpreting of borescope images and diagnosing cylinder problems early and accurately.”

Standardized Protocol

Rather than just collect borescope photos, Savvy is following what we believe is an excellent example from the medical radiology world—where there are standardized procedures for taking X-ray and MRI images and the order in which they are taken—and applying it the piston-engine aircraft world.

Because no one has established a standardized protocol for the order and fashion in which piston aircraft engine borescope images are taken and collected/uploaded, Savvy is grabbing the bull by the horns and creating one. We think standard protocols promote safety—witness that the standardization of the “six-pack” round-dial instrument panel in 1968 made it significantly easier for pilots to transition between airplanes.

Working with its in-house borescope guru, Dave Pasquale, who has produced teaching videos on borescope use, Savvy is creating a detailed “how to” video on using a borescope and setting up a standardized borescope imagery protocol. A written reference guide will be included. The video and guide are targeted at A&P technicians and owners who are maintenance involved.

As we were writing this article we spent some time talking with A&Ps about borescopes. Uniformly, they told us that current A&P training courses include either nothing on how to use a borescope effectively or very little. One tech who is working toward his A&P certificate told us that he feels fortunate in that the chief mechanic in his shop does give training in borescope use. He said that his boss has impressed upon him how important the borescope is as a diagnostic tool when it comes to cylinder condition and that the more he can learn, the better he will be able to provide what may be lifesaving information to his customers about engine health.

We agree.

Savvy’s borescope initiative will be available to all “Savvy Analysis, SavvyQA and SavvyMx” clients as part of their annual subscription. There’s more information on those programs on the next page.

Borescope Importance

Now we’ll take a step back and explain why using borescopes to monitor cylinder health is of significant importance to aircraft owners. It boils down to a combination of identifying exhaust valves that are at risk—and fixing the problem before the valve fails—and, of more importance, preventing the needless replacement of healthy engine cylinders.

We’ll state right here that on the risk spectrum for engine power loss events a burned valve falls in the uncommon to rare range. That’s good news. However, when it does happen, the event can be catastrophic, so having and using a tool to prevent one is a no-brainer.

The greater risk, in our opinion, is an engine failure caused by improper cylinder installation in the field. It is more common than exhaust valve failure, from our observation and research, putting it in the common to not unusual class in the engine-failure risk spectrum. Put bluntly, removing a cylinder for any reason is an event that elevates the risk of a subsequent engine failure.

We are of the opinion that A&Ps may not have enough respect for the risk generated by a field replacement of a cylinder. We feel that cylinders should only be pulled as a last resort when there is a cylinder problem—there must be hard evidence that there is no other alternative.

From that conclusion, we’re going to look at diagnosing a cylinder problem. We note that most times a problem usually presents itself during a compression check. In our opinion, it cannot be overemphasized that low compression, by itself, is never a suitable reason to remove a cylinder.

Compression Checks

Long required for annuals and 100-hour inspections, the differential compression test to assess cylinder health has been recognized as “spectacularly unreliable” by more and more aviation maintenance techs. Multiple tests of the same cylinder over the course of an hour can and will produce dramatically different results.

The problem is that techs have historically been conditioned to yank a cylinder whenever a compression check generates a number below a certain, often arbitrary, level, often 60/80. That is simply wrong, and, in our opinion, dangerous.

For example, Lycoming’s Service Instruction No. 1191A only says that removal and overhaul of a cylinder should be considered if the pressure reading is below 60 psi. At best, a low compression reading is only a first step in deciding whether a cylinder is sick—and it’s likely that if it’s sick, the problem can be remedied by repairs that do not require removing the cylinder.

At this point we’ll note that Continental is concerned about pulling cylinders needlessly and says so in its Manual M-0, and lowers its compression concern threshold to the low 40s. Further it mandates a borescope inspection whenever a compression test is performed. It further says that if compression is below its threshold and the borescope exam doesn’t reveal an obvious cause for low compression the aircraft should be flown for 45 minutes and another compression test performed on the hot engine.

Air Leakage

There are two reasons for low compression readings: air leakage past the compression rings and air leakage past the exhaust valve.

Leakage past the rings is usually caused by the piston being slightly cocked in the cylinder because it is moved into position by the connecting rod or the gaps on the piston rings have lined up. To check, Continental calls for the piston to be brought back up to top dead center (TDC) by rotating the prop in the other direction.

Conclusion—air leakage past the rings is usually because of the compression test procedure, not an indication of an engine problem.

Leakage past the exhaust valve during a compression check can be an indication of a serious problem or no big deal. If it’s due to a burned exhaust valve that has warped or has significant metal erosion, it’s a problem that needs immediate attention. If the problem is caught early it may be possible to fix it by lapping the valve in place, without pulling the cylinder. It’s usually a good idea to also replace the rotator cap or rotocoil because failure of the valve to rotate during engine operation is often the cause for a burned valve.

Other air leaks past the exhaust valve can be due to a worn guide or a dirty valve seat. They are usually not an issue because of the force applied to close the valve during engine operation—800 to 1000 psi.

Continental and Lycoming recommend staking a valve to help the valve seal during compression test. That involves removing the rocker cover and tapping the valve rocker with a mallet while the cylinder is pressurized. Depending on the reason for air leakage, it may fix the problem.

The best way to determine the cause of air leakage past the valve is the borescope exam. It will determine the extent of the damage to the exhaust valve and how we’ll the valve is seating. Bottom line, cylinder removal is only appropriate if the valve is so badly burned that it’s not a good candidate for lapping.

Savvy recommends borescope inspections of all cylinders every 100 hours (50 hours for turbocharged engines). It is Savvy’s position, and we agree with it, that doing a borescope exam at the recommended intervals is frequent enough to detect “a burning exhaust valve, failing rotocoil, detonation signatures or other developing cylinder issues before they reach the point of causing an inflight power loss and often early enough that cylinder removal can be avoided.”

Which Borescope?

The great news is that there are an amazing number of high-quality borescopes on the market for under $300. As far as we’re concerned, that means that no shop should be without one—that goes for maintenance-involved owners as well.

We are not going to make a recommendation as to a particular model, because the landscape is changing so rapidly. However, based on our conversations with Mike Busch and other maintenance techs we do have a recommendation as to the type of borescope.

Choose a rigid probe with an articulating tip. Next best is a semi-rigid unit. We think the unit should be at least 8.5 mm in diameter or greater. Stay away from “flexible” borescopes—ones less than 8.5 mm in diameter—as they tend to flop around when you’re trying to look at something.

Buy directly from the manufacturer so that you get the most recent version of the unit—not always the case if you buy from someplace like Amazon.

Conclusion

We are going to be following Savvy’s borescope initiative very closely for several reasons; top among them is that there is a good chance that we’ll finally have a standardized protocol for taking and posting/storing borescope cylinder images. Further, Savvy’s data collection is likely to produce solid information on identifying and evaluating the nature of damage to exhaust valves and the risk they pose to engine operation.

We also like anything that will help owners and techs develop a healthy level of hesitancy to pull a cylinder unless there is hard evidence that there is no other alternative. We want the number of engine failures due to errors in field cylinder installations to drop because the number of field cylinder installations drops.