Yes, 2 of the 4 the turbos did not meet spec and I had to get new ones. One cam shaft didn’t meet spec. She made spec in climb, MP, RPM, everything else. Not only that, she kept cabin pressurised well, which is the first sign of turbo problems. She climbed about 1200ft/min at SL, which is good but not extraordinary for type. They regularly do 1500ft/min. But it’s California and it’s hot, so… However, above 15K feet, the climb rate dropped and at FL175 it was well below 500ft/min. I don’t recall exact numbers right now, but I’d say down to 200-300ft/min. Normally this type should go up to FL250 without much trouble, and even higher (the non-pressurised one is certified to FL300).
However, any pressurised aircraft should be taken to altitude and test flown. It’s very common that they lose steam up there when they have a few hours on the turbos. This being turbo normalised it can be either a turbo problem, a waste gate problem or an induction/airbox problem – or a mix of all the above. There is no TBO for turbos, but if you get 1000hrs out of them, you’ve done well. These were not overhauled. Doesn’t make it unairworthy, though, and no mechanic could fail it for that. If the turbos are not blowing oil, seized or missing bits, they’re airworthy.
That accident report describes a problem (camshaft corrosion/spalling/wear) that I would guess at least 50% of the fleet suffers from. There is no requirement to cut open oil filters and it is very common to not do it (my experience says much less than 50% do it but I have no hard data).
Great experiment done by AAIB and the 10% loss of power is an interesting finding. However, they say it is very easy to detect: your RPM at 100% power during takeoff roll. This only works for a fixed pitch prop and you should look at the RPM at every takeoff and abort should it be lower than what the POH says. One thing they say is not correct:
There is no routine maintenance carried out on the engine that attempts to measure any cam wear, other than an examination of the removed oil filters
There is a SB from Lycoming, upgraded to an AD in EASA land that mandates valve testing every 400h (the “valve wobble test”). This SB also contains instructions how to measure valve travel. I have the fixture to carry out this measurement and it is dead simple.
This 4x fatal accident has some interesting engine data on worn out cam lobes affecting the power. The effect is smaller than one might expect.
I wonder if they ever traced the “maintenance” company and asked them if they ever opened up the oil filter.
I didn’t see that (FL175) … no this cannot happen by worn out camshafts, you are right. The engine would not reach max rpm/map either if it couldn’t breathe. Of course the turbochargers are responsible for reaching max power and rpm… so, yes, it might be them …
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… 
Yes, Bosco, worn out cam lobes would qualify for a reduction in power. How can you have power when you can’t breathe!
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.)
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.
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”.
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
