Great Story, cool pictures – and very interesting!
I’m intrigued by your statements about the airplane’s poor performance due to run out engines. That should not be possible. If your engines do not make their rated power, then something is terribly wrong and the airplane miles south of being airworthy. Do your engines produce the MP and RPM they should? The combination of MP and RPM is the power output and it should match POH. In case it doesn’t, then the problem is not the engines but something else in the airframe.
Even engines with 10/80 compression thousands of hours past TBO generally make their rated power. To me it sounds more like induction leaks or a broken turbocharger. Should be visible in low MP values.
Well, 10/80 is a little exaggerated, but many test have been made by TCM and Lycoming showing that 25/80 or 30/80 compression did not lead to loss of power. The pressures inside a cylinder are 100 times higher when the engine is actually running than in the (ore or less stupid) compression tests, that really don’t say much about the engine’s condition without additional borescope inspection etc.
I agree that the poor performance should have nothing to do with the engines over TBO!
It remains a continuing mystery as to why Conti engines have such low compressions when Lyco ones don’t.
My IO540-C4D5D measured at around 77/80 on the last test, and that was without even having been warmed up, while the A&P tells me he regularly sees 50/80 on Conti 550 engines.
It has to be a different level of QA on the valves and/or the piston rings, because there isn’t anything else…
I would agree it doesn’t lose much power (because the gas can’t escape through the leaks fast enough to make a difference) but it might point to a growing problem somewhere. The solution AFAIK is to do regular borescope inspections.
No, 50 is really on the low side of any IO-550-N, but 65 can been very often. But even 50 are NOT a problem if the borescope doesn’t reveal any problems. It’s not that such an engines doesn’t lose “much” power – it should not lose ANY power.
A WARM engine is a completely different engine altogether. And what most people are not aware of that the compression test is only a snapshot. And many times the compression will have a completely different levek if you measure again after flying for an hour.
Also: measuring compression with non-calibrated tools without taking ambient air pressure ito account is still done, but more or less useless.
Today IO-550s in the SR22 are only opened by the real specialists if
a) the borescope reveals a problem
b) the engine has excessive oil comsumption and
c) oil analysis shows there’s something wrong
The whole concept of static compression is flawed, representing 1940s thinking and diagnostics. The piston rings which provide the combustion chamber seal (and thus compression) have a certain tension, providing a seal of a certain strength. This is the static compression, measured at 80psi.
In real life the pressure is up to 1000psi with the engine running. The major difference is that the pistons are shaped in a way that the internal pressure pushes out the piston rings. This does not happen during the static compression test. If you look very carefully at your piston you can see that there is space for the gases to get behind the ring and pushe it out. This is what is called “dynamic compression”.
Static compression is irrelevant to the engine’s performance. Only dynamic compression counts. Good static compression is a positive sign but bad static compression isn’t necessarily a bad sign. One should check where the air escapes (valves or piston rings) and then look for signs of burnt/worn out valves or broken piston rings. Typically a real problem means 0/80 compression.
Still, something about this Aerostar must be wrong. I would certainly not have flown it as there is no such thing as “tired engines”.
And I might add this from Mike Busch’s article about the topic:
COMPRESSION TEST RESULTS
The bible for compression testing of TCM cylinders is TCM Service Bulletin SB03-3. In it, TCM throws the old 60/80 standard right out the window, and establishes a variable go/ no-go compression limit based on what your mechanic’s compression test gauges read when hooked up to a calibrated standard known as a “master orifice tool.” For most compression testers, the no-go compression limit is somewhere in the low 40s. Thus, according to TCM, a cylinder that measures 50/80 is every bit as airworthy as one that measures 75/80.
SB03-3 goes even further, saying that even if a cylinder measures below the no-go limit (say 35/80 , bold by me), it should not be removed from the engine unless a borescope inspection of the cylinder reveals some obvious reason for the low compression (like a burned exhaust valve or excessive barrel wear). If the low-compression cylinder looks okay under the borescope, TCM says the engine should be returned to service, flown for a while, and then the compression retested. Only if the cylinder flunks the compression test a second time should it be pulled for repair.
I’m intrigued by your statements about the airplane’s poor performance due to run out engines. That should not be possible.
Achim, I am not so sure. A frequent case is older engines that simply have very worn cam lobes. As far as I understand, this will drastically decrease power output (due to imperfect valve movement geometry), yet all else will be quite normal (MAP, RPM, CHT, oil usage, etc.)
Yes, Bosco, worn out cam lobes would qualify for a reduction in power. How can you have power when you can’t breathe!
I doubt worn cam lobes would reduce the service ceiling from FL270 to FL175. The valve travel on our engines is very long with plenty of room for degradation over time. To be more precise: a given combination of MP/RPM and FF will give the POH power output. I can hardly imagine a situation where this would not be true.
Most likely the turbochargers don’t work correctly and/or there is an induction leak. The overhaul will fix that for sure but it would have been interesting to diagnose the real problem. Equally interesting who considered this airplane to be airworthy… 
