Differential Leak-Down Compression Values: A Lesson Learned

[drdehave]

Background: The 912 ULS in my Sting is coming up on 1,500 hrs. At about 1,000 hrs, I had a valve (lapping) job--to ameliorate the leading effects from the 100LL run almost exclusively (with Decalin) through her for the first 250 hrs by the original owner. From 1,000 hrs to just recently, the differential compression tests (DCTs), engine warmed, were always quite tight at around 87/87, per cylinder (all DCTs discussed herein, were at 87 psi/in).

Then, in June, during my annual, with the warmed engine (not flown) the cylinders (in order) read 86/84/86/86; my mechanic advised checking it again in 50 hours, for any developing trend. But, the worrier I am, I only made it to 30 hrs, before requesting a followup DCT from him. This time, they read 85/83/84/86-–again #2 indicating lowest. One day later, I decided to have mechanic number two check her warm (not flown), and his DCT gave 83/79.5/84/84--again, #2 indicating lowest. Now, I am beginning the think there’s a trend on #2 that could warrant a valve-lapping down the road at some point. But, I opted instead for a third opinion from mechanic number three, just 4 days later; his results on the warm (just flown) engine were 87s across the board, with no perceptible leaking-down heard through the exhaust or rings (i.e., into oil tank). Whoopee!

What I Learned: This really opened my eyes to variability of DCT results on any given engine at any given point in time, and why some say it may be more informative to actually look into each cylinder with a scope to analyze conditions. I was also aghast at observing the different techniques of the three mechanics–-all with Rotax training and experience. In fact, in my view, only the last test (with the perfect 87s) was done exactly per the Rotax book (and videos). However, I cannot discount the possible 'beneficial effect' of flying the airplane, then relatively quickly doing the test (although the follow-up test above WAS done this way, too).

What I’m Going to Do: My new DCT tester, for $78.50, is in route to me from LEAF. I am going to finally start doing my own DCTs on a regular basis, as part of normal monitoring of my aging engine. And I am going to make a very concerted effort to hold all the variables that may affect results much more constant.

My Question: I assume, because I have not had any of the Rotax training courses–-or other related airplane-mechanic training--I will not be able to record these future DCTs in my engine logbook. If that is so, what level of training WILL be required, if I want to log them?

Well, that's my treatise and lesson in DCTs. Now, I’d like to hear some of your related results, comments and observations.–Rich

[drseti]

Rich, since you're flying an SLSA, what you can do as an owner/operator is determined not by Rotax, but rather by your airframe manufacturer. So, check your airframe maintenance manual. Does it allow you to remove and replace sparkplugs? If so, then you can do a compression check, since pulling and installing the plugs is the only part of the procedure that can be called preventive maintenance. Of course, you'll need a calibrated torque wrench for that.

As for taking the Rotax courses, they're great, but FAA cannot require them. All the FAR says is that you must be trained to perform a given operation, and follow the manufacturer's published procedures.

PS: very glad all your mechanics used a ref pressure of 87 PSIG (6 bars). All too many A&Ps use 80 PSIG (the correct number for Lycoming and Continental engines).

[drdehave]

Paul:

Yes, the Sting TL Ultralight Maintenance Manual allows me to do spark plugs--and I have been! Your answer is pretty much what I was thinking. But the third mechanic--the one who dialed the perfect 87s--was the one telling me, "You can't record your compression results in the log-book." Of course he was a long-time A&P (with major airlines) who became Rotax-certified recently, as an afterthought.

I will re-check my maintenance manual, just to be certain about the spark plugs being allowable. (Yes, page 4-16/17, Sting Sport MM, in writing.)

Rich

[CTLSi]

......

[Jim Stewart]

Flight Design defers to Rotax for all engine related issues and Service Bulletins. So Flight Design owners are expected to adhere to SB issues by Rotax without re-consulting with FD. Rotax offers a varied series of training for owners and A&Ps. The official factory Rotax site is Rotax-Owner.com

He flies a Sting Sport, not a Flight Design aircraft.

[drseti]

The official factory Rotax site is Rotax-Owner.com

Not entirely correct. Rotax-owner.com is run by an independent company, not owned or operated by Rotax, though they are factory authorized to provide training and service information. From their "about us" link:

Rotax-Owner.com is a Factory Authorized web based support system which allows our registered members a common access point for locating key Information - Education - Support for their Rotax Aircraft Engines.
<snip>
All iRTO programs are delivered by independent Rotax Technical Instructors (iRTI) of the highest quality and expertise.

The official Rotax website is http://www.flyrotax.com.

[CTLSi]

......

[roger lee]

Hi Dave,

First when using the differential test results can never be the same as the test pressure. There has to be a small leak or the test would be worthless. Air pressure will leak past the rings and you can hear it in the oil tank as a bubbling sound. If someone says the differential is equal to the test pressure tell them to buy a new set of gauges. No Rotax or air cooled aircraft engine is 100% air tight and leak proof.

Second any Rotax differential test result between 82-85 psi differential is perfectly normal (test pressure of 87 psi) and can change a pound or two on any given day so don't worry about a 1-2 psi change.
Even though Rotax states you need to pull cyl. at a loss of 25% if you ever see a Rotax even loose 10 psi over the other cylinders it's time to investigate. This just doesn't happen unless you have a real problem.

You can use any test pressure between 80-87 psi. If you use 80 test pressure then normal leak down is 76-78 psi. I'm still old school so I still use 87.

I did a test a couple years back testing the difference between doing a test with a hot Rotax engine verses a cold engine. I tested it on 10 planes. No what the difference was on a Rotax 912, nothing. The test came out the same. I don't worry about what the engine temp is on a Rotax any more. Loose tolerance Cont. & Lycoming's are totally different and can not be compared.

Technically if you did a differential leak down test then it would be for your own edification because if you put it in the logbook it would be worthless since you don't have that maint. rating and it would document you are working on your own plane. If you choose to do this keep it on a separate piece of paper.

Rotax this last year went from a 100 hr or annual differential test of 100 hrs. to 200 hr. test points because they haven't had any real issue over the last 22 years. That said if something starts to go wrong you usually know something isn't right and the test would then be warranted depending on what you thought was wrong.


p.s.
I still do a differential test at every 100 hr and annual on every one of my clients plane. It takes 15 minutes, it's easy and hopefully will show any trend if one ever did develop.


p.s.s
Sometimes lead can break off from the use of 100LL and get caught under a valve seat and cause a low differential test. This only requires cleaning of the valve seat and valve face.

[roger lee]

Hi Dave,

I see you said you have 1500 hrs. have you done the gearbox inspection?
If using 100LL it is due at 600, 1200 and 1800 hrs. If using auto fuel then 600 hrs. on a 1500 hr TBO engine and a 1000 hrs on a 2000 Hr TBO engine.

A perfect way to get lead off of valves and anything else inside the cylinders including piston tops, ring grooves, valve, ect... is a soda blaster. It is a sand blaster that uses soda. It works perfectly and cleans everything off without taking any metal off or ruining anything.

[drseti]

I'm still old school so I still use 87.

Me too, Roger. The only reason to use anything less than 87 PSIG is if compressor doesn't go high enough.

I still do a differential test at every 100 hr and annual

Me too, Roger!

[drdehave]

Thanks, Roger, I was hoping you'd chime in, because I so value you knowledge on such matters.

As to your points:

1. 87/87 Not Possible. Yes, my three guys know that; but they have different styles. The last test (mechanic three) with all 87s, did, in fact, have barely perceptible leakage that could be heard, but the needles were closer to 87 than 86, and this mechanic recorded them as 87s (on just-tested and calibrated gauges). When mechanic number one gets similar readings (i.e., closer to 87 than 86), he prefers putting them in the book as 86s.

2. Gearbox Service with My 1500 Hours. Yep, I'm good there! That baby's been done three times, already!

3. No "Authority" for Me Entering Compression-Test Results in My Logbooks. Yep, I see your point. They would have no "value," towards demonstrating good condition--or promoting a sale, or whatever--since they lack the "force" of a rating or certificate of training. I'll keep them to myself--until I have such a force behind me!

Thanks!--Rich (P.S. Dave is the ugly one.)

[roger lee]

Sorry for mixing up the name.

[drseti]

Dave is the ugly one.)

Rich, if Dave is the ugly one with the beautiful plane, does that make you the beautiful one with the ugly plane?

[3Dreaming]

Rich, since you're flying an SLSA, what you can do as an owner/operator is determined not by Rotax, but rather by your airframe manufacturer. So, check your airframe maintenance manual. Does it allow you to remove and replace sparkplugs? If so, then you can do a compression check, since pulling and installing the plugs is the only part of the procedure that can be called preventive maintenance. Of course, you'll need a calibrated torque wrench for that.

As for taking the Rotax courses, they're great, but FAA cannot require them. All the FAR says is that you must be trained to perform a given operation, and follow the manufacturer's published procedures.

PS: very glad all your mechanics used a ref pressure of 87 PSIG (6 bars). All too many A&Ps use 80 PSIG (the correct number for Lycoming and Continental engines).

The FAA in part 43 says what maintenance a pilot can perform on a SLSA.

[Jack Tyler]

Dave, are you familiar with Mike Busch and the seminars he does? Altho' he is (mostly) addressing the conventional air-cooled engines in Part 23 a/c, you might find the content of his programs to be helpful. They certainly are informative.

As just one example (given your engine hours), here's a youtube-based seminar on when to overhaul:
https://www.youtube.com/watch?v=Ts_irHU35yc
BTW he's a big advocate of using a borescope to evaluate engine health.