Unbelievable Compressions
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- Опубликовано: 15 авг 2023
- The FARs require a differential compression test to be performed at every annual inspection, but it's a really terrible test that is neither reliable (i.e., repeatable) nor a valid indiction of cylinder health. In this webinar, Mike Busch A&P/IA demonstrates its shortcomings, explains its pitfalls, and emphasizes whay poor compression alone should never justify cylinder removal unless backed up by more trustworthy supporting evidence such as a borescope inspection. Savvy Aviation offers Professional Maintenance Services to owners of General Aviation aircraft, such as: SavvyMx (Professional Maintenance Management), SavvyQA (Expert Consulting), SavvyPrebuy, SavvyAnalysis (Engine Data Analysis) and Breakdown Assistance. Savvy also publishes a monthly newsletter with lots of interesting information for the general aviation enthusiast; subscribe to it at www.savvyaviation.com or text the word "Savvy" to 33777. This webinar was hosted by the Experimental Aircraft Association (EAA).
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I just got your book "Engines", and have listened to 75% of your video catalog, so far. I now feel over confident in preparation for my upcoming O&P's for my Powerplant certificate. Paying $30 for your book is nothing, considering all of the free quality information you have given in your podcasts.
Mike, the TCM Service Bulletin you referred to pertaining to opposite direction rotation to check a cylinder with low compression is M84-15.
I love that comment about tearing down a compression tester to find "a tiny pair of dice". Brilliant. Love your content. I wish you had like-minded guest speakers (or yourself) with significant Rotax experience. Keep up the good content.
Thanks a lot for another great webinar! It was great to attend the „live version“ of the „Ask the A&P“ podcast in Oshkosh!
The VA-400 would be the borescope most would want to use and is available for $250 (I purchased mine years ago so don't have the updated camera). The VA-450 you mentioned is an $800 piece of kit that includes a dedicated display. A shop might consider the additional cost/dedicated display worth the expense, but I don't believe most owners would.
Excellent information as always. I am personally aware of a badly flunked compression (leak-down) test on almost all cylinders, where the compression tester was at fault. Grabbing another one and it indicated similar fails, and IT was faulty. The clue was when hand turning the prop, the compression seemed to REALLY hold. I have seen engines with low compression where hand turning the prop shows a fairly rapid release of cylinder pressure against the prop. A NEW correct compression tester (drove out to Spruce) gave believable answers.
I am aware of another plane that had TWO cylinders in the very low teens and almost no perceived resistance to turning the prop against the compression. Tester was fine. Boroscope wasn't terrible. The owner was in annual, so took the plane on a trip and saw no problematic temps on the engine monitor. Upon returning, testing cold, both low cylinders tested in the 70+ range and "felt normal" when hand turning the prop.
I LIKE Mr. Busch's assessment that compression tests aren't very useful. I care about flying an airplane in safe reliable working order with cylinders that won't fail in flight, but for me, compression tests are just something to get through in an honest way during annual.
Lots of relevant legal based info, killing the confusion. Great content asvusual, Mike!
Very informative, we never stop learning
Great great content, one of the best on youtube, i would like to also see some info on turboprops, i will be in charge of a king air now and would love to know all about the maintenance and how to fly it to the thread, your videos have helped me a lot with my 182 and just with knowledge in general so thank you so much.
Excellent!
None of the shops I've worked in have ever done hot or even warm compression tests
I agree, let the cylinders cool down.
Also the comment that it might cost $1,000 to change a cylinder is just ridiculous if the shop rate is $100 an hour it shouldn't take more than 2-3 hours to change a cylinder ($200-$300) in labor 🎉
Near and dear to my heart too. Brought many cylinders back with a quick simple in-place lapping. Doesn’t make a mess. 20’s to 70’s. Clean with swabs soaked in AeroKroil.
Pressure and temp conditions only impact calibrated pressure from a range of 42 to about 46. Engine will make full rated power at calibrated pressure.
Make sure ring gaps are correct
Could you do a short video on how to use the master orfice tool? Is it used on just cont. or lycomings too?? I have your engine book and the manifesto as well. Love them and refer to the engine quite often. Thank you.
You literally hook it up and input 80 psi. That is your “ failure” limit where you attempt to fix before replacement.
Just continental as they are lower compression, and people are failing them illegitimately
I agree with this, with one exception Mike. If you rely on manufacture SB to evaluate compression testing numbers for airworthiness, then you have to also rely on them for TBO, numbers. They all limit the motors to TBO of 2k Hrs run time. In other words, even a good Compression test on a motor over TBO is not recommended according to manufacturer. You can't pick Manufacture for specs on compression evaluation, and pick AC 43-13-1b for TBO , which says as long as compression is ok, your good. that's mixing apples and oranges and calling them the same. The manufacturer specs presume if you are following them on compression evaluation, you are also following them on TBO time.
The differential compression test has huge usefulness, in that you can hear where compression is being lost. A dynamic compression test can't do this. You will still need to do a leak down (differential pressure) test to diagnose the problem.
Diagnose WHAT problem? That was the ENTIRE point of this discussion! That the compression test results are useless!
Clearly you didn't bother to watch the video and listen to Mike's excellent dissertation on why they are a waste of time. But please, do go and explain to us how you are a bigger genius than the man that created Savvy Aviation and has decades of A&P experience.
This is the crap that ends up driving the average person out of aviation. Cold/improper compression checks=$$$$$
When you mentioned about "crocked piston", I'm thinking about the common piston slap on Continentals (commonly on the small ones like the O-200), would really like a video about this...
Retired auto tech here. Compression readings are used to find a problem only if you are diag.a problem.
How do "gapless" rings affect this dynamic of unreliable compression testing and anomalies associated with the compression test?
I missed this presentation at Oshkosh, I went to most of the others though. Since returning from Osh I did an owner assisted annual on my plane. All of the cylinders were a good 10PSI lower than they normally are with one right at 60. The IA signed off but is requiring a retest after flying. This is the first year that I've done an owner assisted annual and thought it was strange we weren't heating up the engine first. I'm going to fly it to the Mooney service center near by and have them retest it and give me a quote to fix the squawks that were found.
Those out of the back of a truck mechanics need to be watched closely. You get what you pay for.
@@TheReadBaron91 You couldn't be more right. The first year I had my plane I took the plane for an annual based on a friend's referral, he had the plane for 6 weeks and missed so many things. I'd stop in to see how it's going and found cracked spark plug insulators on plugs he had cleaned and "were ready to got back in".
I've been working on cars for 25 years so during this annual it was nice to be involved so I could inspect and lube things myself. I wish I could replace all the AN lines that need to be replaced myself but since I'm not an AP I can't.
Do you prefer Ly coming or continental engines?
Very Interesting Content.....Engines failing the current Differential compression test Requirements using verfied Calibrated test tools Certainty contain 1 or more Abnormal combustion Sealing or Valve train Componets.....That engine will be Operating in "Elevated Risk" conditions.....Why Don't these Aircraft engines have Removable cylinder Heads to make Upper Cylinder head Maintenance pratica & easy ?? ....
Probably because these engines were designed 70 years ago, when parts were expendable and cheaper
So would it be accurate to say that a bad cylinder will always fail a compression test but a good cylinder will sometimes fail a cylinder test
Good question! I think so, but just IMHO.
This is a Leakdown test? A compression test is with engine turning over,preferably while hot. Giving the actual pressure of cylinder.I was getting confused .
It's a leakdown test. Coming from the automotive maintenance world it was confusing to me too. A friend of mine is a fan of doing leakdown tests on cars where I only do it to find the source of the low compression. He did a leakdown on another friends Miata that had a loud rattle but not quite sounding like a spun rod bearing. His tests showed no issues and they tore the engine down anyway. Had they done a normal compression test they would have seen a couple cylinders lower than the others. There were two bent rods from a poorly tuned rolling anti-lag. Either way the engine had to come apart, but to me it shows the compression test would have more likely seen the issue. Maybe not though, not sure how much the compression ratio was changed by the bent rods.
In aviation a compression test is a leakdown test. I don't know why they call it that but that simply is what it's commonly called..
This is a differential leak down test, not a compression test, people are always saying compression. Your cylinder makes compression when it's running, the differential test gauges do not make compression, they fill the cylinder with 80 psi of supplied shop air, then the other gauge differentiates what the cylinder can hold out of that 80 psi based on leaks. Using 80 psi of cold shop air to seal a slightly leaking exhaust valve isn't a very good test when considering that valve is completely sealed when it has 400 psi of hot combustion gasses trying to push it through the valve cover. Differential leakage test. Not compression
Yes and no. Yes, this is what the auto world calls a leak down test vs what the auto world calls a compression test. However, it is a distinction without a difference. A rotating engine compression test is also measuring air leakage just as does the leak down test. It doesn’t really matter how the air in the cylinder gets compressed. A 6:1 CR engine is going to develop about 70 psi on a gauge at TDC. The engine doesn’t know the difference between 70 psi it created through rotation or 70 psi of shop air in the cylinder. Same difference. And even auto style compression tests don’t really simulate engine operation as the RPM the starter can produce is nothing compared to a running engine so the compression measured while cranking will be less, possibly much less, than the compression from a running engine.
@@LTVoyager Ok bud, just stop, just stop, do yourself a favor and go lay down.
@@521CID You are the one with the tired brain. Better take your own advice. And taking two weeks to respond? Wow, you are even slower than I imagined.
I'm a car/motorcycle guy that (unfortunately) got into aviation almost 30 years ago.
I agree, leakdown and differential test are better names, but it's an argument about semantics...it's industry jargon, and all the A/Ps and FAA aren't going to change now.
The Feds like certain words. We are all "certificated" mechanics and pilots, I guess "certified" lacks syllables.
Testing for those certificates involves an "oral examination", which sounds either a little perverted, or like something a dentist does. It involves talking- it is a test administered "VERBALLY".
It annoys me to say both certificated, and oral exam. I think there are more dumb words and phrases.
Yup, I prefer leakdown, but say compression.
Aviation people are still in disagreement about whether or not exhaust primary header tube length matters, too. I think of it anytime I install or remove any of the expensive, light, but "tractor like" exhausts. Exhaust and intake ports resemble something somewhere between proper engine ports and plumbing. But they do work OK for low rpm and approximately 1/2 hp per cubic inch. Fixed timing.. etc, etc.
Wahoo.
DOH!
Yes he is
As always an interesting video. But, with every respect intended, it may be time to stop saying "Aviation Maintenance Technician of the Year 2008" at the top of every webinar. Basically what you are saying is that one and a half DECADES ago, Mike knew his stuff. Better of just not saying it. When it's that long ago, it just comes across as strange to mention and I think detracts from the credential rather than adding to it.