“UNBELIEVABLE COMPRESSION – HOW RELIABLE AND VALID IS THE ALMIGHTY COMPRESSION TEST?”

This is the title of Mike Busch’s article in the July 2023 issue of AOPA Pilot Magazine. He talks about the differential compression tests which we all do as part of our annual inspections. While in the US compression tests are mandatory in Canada they are not. Most of the engine manufacturers include differential tests as part of their annual inspection check lists.

Mike says that compression tests can be very unreliable as you can get wildly different results if you test a cylinder multiple times. A cylinder can test low and, re-tested after flying for 30-45 minutes, have excellent compression. A healthy cylinder can have poor compression readings while a sick cylinder may have good compression readings.

Compression tests on a particular cylinder may vary greatly from annual to annual. He cites a case where a cylinder had wildly different readings over a period of 12 years – all done by the same shop. Is it possible that the cylinder was sick then healed itself, got sick and then healed itself again? Probably not a realistic scenario!

Basically, Busch advocates performing the compression test and then putting the resulting numbers in the proper context before taking any drastic measures such as pulling cylinders.

I have included a link to his article on AOPA’s website: https://aopa.org/newsand-media/all-news/2023/july/pilot/savvy-maintenance-unbelievablecompression.

His column, Savvy Maintenance, is published monthly in the AOPA Pilot magazine and on the AOPA website where is available to view without an AOPA membership. I read it regularly and he provides some very good advice.

This is where I have some personal experience with differential compression test results on my Cessna 195’s Jacobs engine. Over the years some cylinders have had low compression test results initially but after running the engine and getting cylinder temperatures up, the subsequent test readings were well within the acceptable range. I have seen readings improve by up to 12 PSI. Another option is to “stake” the valves to remove carbon buildup on the valve seats.

Now this is where it gets interesting. While my oil consumption on the 195 has always been a little higher than what some of my fellow 195 owners are experiencing, early last year I noticed a gradual but obvious increase in oil consumption. On local flights oil consumption seemed to vary quite a bit and I was never really sure what it was using. I always checked the oil after shut-down and then before start-up and never seemed to get a consistent reading.

In July last year on a trip to Saskatoon, it appeared that we used over two gallons of oil in 2 hours. Again I wasn’t sure if the aircraft was sitting the same as in the hangar at home but we cut our travel plans short and flew home. I still felt OK to fly the aircraft around locally as the engine ran smoothly and showed no abnormal conditions. There was no obvious oil dripping down the belly or firewall. The plugs looked clean and were not all sooted up.

I asked a few 195 owners and maintenance people about the excessive consumption and was assured that the engine was not going to crater on us. Basically I was told that if the oil consumption reached 1 gallon per hour I should consider some maintenance. So with this positive information in September we flew to the 195 Convention in Joliet, Illinois. I checked the oil at every stop and added as required. Over the course of 21 hours of flying we burned 9 gallons of oil. It almost became a case of “check the gas and fill the oil” just like when we all bought our first ‘beater’ cars. I knew something was wrong but not sure what.

We did the annual in October 2022 after the Joliet trip and everything looked good, compression differential readings were good. The engine was running as smooth as always.

Late December I contacted the shop which performed the overhaul (@460 hours ago) and the owner was quite taken aback at the oil consumption. He asked me to confirm it was quarts I was talking about – “No Pete, I’m talking gallons”. After a brief silence he said he was very sure my #4 and 5 cylinders were the culprits and within a week two new cylinders were on their way.

I also contacted another engine shop and was told the exhaust valve guides were probably worn. The explanation was plausible and I was told how to confirm that this was the problem. I didn’t get a chance to confirm this.

So after all this, a smooth running engine, no oil leaks and good compression tests we finally pulled the “offending” cylinders last week. As we pulled the #5 cylinder several pieces fell in to the drip tray. When we removed the piston we found out what those pieces were! The Jake piston has four sets of rings on each piston – two compression, one oil control ring and an oil scraper ring. The top compression ring was in one piece however the second ring was destroyed and fell apart in our hands. The top oil control ring is also missing pieces and plugged. The bottom ring appears to be Ok. Luckily there is no indication that any of the broken pieces passed the top compression or bottom control ring and entered the crankcase. That would be catastrophic and expensive! The cylinder walls are clean and show no signs of scratching or gouging. I think I got lucky! I knew something had to be wrong but certainly didn’t expect this. It says something about the robustness and reliability of the Jacobs engines.

So, referring back to the Mike Busch’s article on the validity of compression differential testing be careful of interpreting the results and make sure you do further investigation before making rash decisions. See my photos of the piston – it’s quite impressive.

Mike Busch Profile:

Mike Busch is arguably the best-known A&P/IA in general aviation. He writes the monthly “Savvy Maintenance” column in AOPA PILOT and hosts free monthly EAA-sponsored maintenance webinars. Mike is a mathematician by training, having received his Bachelor of Arts degree in mathematics from Dartmouth College. After Dartmouth, he did graduate work in mathematics at Princeton University and in business administration at Columbia University. While at Dartmouth, Mike did pioneering work in computer software development, and ultimately retired from a long, successful career as a software entrepreneur. Mike then co-founded AVweb in 1995 and served as its editor-in-chief and investigative journalist until its sale to Belvoir Publications in 2002. Through his work as a type club tech rep for Cessna Pilots Association, American Bonanza Society, and Cirrus Owners and Pilots Association, and as CEO of Savvy Aviation, Inc., Mike has helped thousands of aircraft owners resolve thorny maintenance problems that have stumped their local A&Ps. Founded in 2008, Mike’s company Savvy Aviation, Inc. provides a broad palette of maintenance-related services to thousands of owners of piston GA airplanes. Those services include maintenance management and consulting, engine monitor data analysis, a nationwide prebuy management program, and 24/7 breakdown assistance that’s essentially “AAA for GA.”