A common maintenance issue the older aircraft
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Hours are no indicator of condition. A factory ovehauled engine built to 'as-new' limits can have a third run set of crankcases - 6000hrs and conrods and crank which could have been in action since the ark first floated.
Because the camshaft and valve gear is poorly designed it will almost certainly be new. The rest? Could be almost any age. What this tells you is that hours run are almost irrelevent. Condtion is everything.
Cylinders? Good ones can last for 4000 hrs if they are well made and treated well. Alternatively they can crack within a couple of hundred hours.
I sold an aircraft with good oil pressure, good compressions and spotless oil - within 200 hours it had no compression and burnt 2 lt an hour. When opened up it was tired, very tired. But if you give an aircooled engine immediate high power and run it hard with little oil surprising enough it wears - quickly.....
Blanket 'rules' on time or hours are not going to represent the actual condition of the engine - careful maintenance and inspection will.
Because the camshaft and valve gear is poorly designed it will almost certainly be new. The rest? Could be almost any age. What this tells you is that hours run are almost irrelevent. Condtion is everything.
Cylinders? Good ones can last for 4000 hrs if they are well made and treated well. Alternatively they can crack within a couple of hundred hours.
I sold an aircraft with good oil pressure, good compressions and spotless oil - within 200 hours it had no compression and burnt 2 lt an hour. When opened up it was tired, very tired. But if you give an aircooled engine immediate high power and run it hard with little oil surprising enough it wears - quickly.....
Blanket 'rules' on time or hours are not going to represent the actual condition of the engine - careful maintenance and inspection will.
Yes, of course, I meant compression tests. You pressurise the cylinder from an airline and measure the rate of leakage (in essence).
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The original question related to what to do with an engine that is close to exceeding an arbituary calender life, but is only halfway through the recommended time between overhauls.
As long as the engine is performing well and showing no signs of premature deterioration in performance, why would you needlessly pull it apart?
If there are no obvious problems, the only way to ascertain whether or not the performance is impaired sufficiently to warrant a re-build/replacement of the engine is to monitor performance trends over a period of time. Trend monitoring can include factors such as oil consumption, cylinder compressions and elemental analysis of engine oil samples.
Oil analysis is certainly an excellent tool, which is used extensively in the industry that I work in, however, it requires quite a few analyses over a period of operating hours to establish meaningful trends and enable accurate interpretations of the results.
Basically, the decision to renew or not should be based on your knowledge of the engine performance trends/maintenance history, knowledge of the engine operating history and your personal risk acceptance. If the engine is performing well and you are comfortable with the operating history, then my opionion is that there is no reason to replace it just because it has reached a certain age.
A new engine that is poorly operated or badly maintained is quite capable of breaking down just like an older one, but if you feel more comfortable flying behind a new or newly overhauld engine, then you should replace it.
As long as the engine is performing well and showing no signs of premature deterioration in performance, why would you needlessly pull it apart?
If there are no obvious problems, the only way to ascertain whether or not the performance is impaired sufficiently to warrant a re-build/replacement of the engine is to monitor performance trends over a period of time. Trend monitoring can include factors such as oil consumption, cylinder compressions and elemental analysis of engine oil samples.
Oil analysis is certainly an excellent tool, which is used extensively in the industry that I work in, however, it requires quite a few analyses over a period of operating hours to establish meaningful trends and enable accurate interpretations of the results.
Basically, the decision to renew or not should be based on your knowledge of the engine performance trends/maintenance history, knowledge of the engine operating history and your personal risk acceptance. If the engine is performing well and you are comfortable with the operating history, then my opionion is that there is no reason to replace it just because it has reached a certain age.
A new engine that is poorly operated or badly maintained is quite capable of breaking down just like an older one, but if you feel more comfortable flying behind a new or newly overhauld engine, then you should replace it.
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Engines can have great compression and a leaky o-ring pushing oil out the exhaust.
You mean "oil control ring".... allowing oil to be blown out of the exhaust
Compression alone is not a sufficient indicator. Engines can have great compression and a leaky o-ring pushing oil out the exhaust.
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OTOH checking valve lift is quite easy and quick. Just need to take off the rocker covers.
OTOH all that will show is that the amount of the camshaft which ended up in the oil filter was under 25%
It won't show a camshaft which has only just started to badly break up.
OTOH all that will show is that the amount of the camshaft which ended up in the oil filter was under 25%
It won't show a camshaft which has only just started to badly break up.
Apparently engine operation doesn't strongly reflect Lycoming cam condition either - the engine runs fine but loses a little power over time, presumably reflecting reduced valve lift with unchanged timing. I know of one aircraft that's been through two cams in 3000 hrs and 39 years - both instances uncovered through measuring lift at annual inspection, neither being very noticeable in operation. The oil filter apparently catches the debris and therefore (or so I'm told) oil analysis doesn't reflect cam condition well either.
I'd be interested to hear if more hazardous cam failures have occurred, warranting periodic replacement versus condition based maintenance after annual inspection.
I'd be interested to hear if more hazardous cam failures have occurred, warranting periodic replacement versus condition based maintenance after annual inspection.
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There was a 4x fatal PA28 crash coming out of Sandown on the Isle of Wight, a few years ago. The AAIB report reported, from memory, a 25% loss of valve lift and a 10% loss of power.
The average renter would probably not notice a 10% loss of power especially as it shows up as only a ~3% loss of RPM on the runup, which could easily be masked by wind passing through the prop.
It should be apparent to a regular flier who flies by the numbers.
The average renter would probably not notice a 10% loss of power especially as it shows up as only a ~3% loss of RPM on the runup, which could easily be masked by wind passing through the prop.
It should be apparent to a regular flier who flies by the numbers.
Thanks for that info. A 10% dyno-measured peak power loss due to a flat cam would be a funny thing - if the engine is driving a constant speed prop you'd see it in significantly reduced climb rate. With a fixed pitch prop, maybe not so much because the power loss due to a flat cam lobe would be proportionately less at lower rpm.... But OTOH with the fixed pitch prop you'd have less climb rate to lose in the first place!
Whether an accident could result from a 10% power loss would depend on the application, I think. For my 40 year old Lycoming powered aircraft (climbing at around 1100 fpm normally), I don't think so. I'm therefore interested in the potential for catastrophic failure between annual inspections of valve lift - I haven't so far heard of them.
Whether an accident could result from a 10% power loss would depend on the application, I think. For my 40 year old Lycoming powered aircraft (climbing at around 1100 fpm normally), I don't think so. I'm therefore interested in the potential for catastrophic failure between annual inspections of valve lift - I haven't so far heard of them.
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Probably one reason is to put a cap on liability.
The warranty is just 12 months but any half decent lawyer will make a liability stick way past that point.
However if you run an engine to 13 years and it fails, kills you, etc, they are (probably) not liable.
From the engine maker's POV, it is a necessary position to adopt because only about 2% (according to Lyco) of the GA engine population is replaced each year. So there is a huge number of very old engines out there. If you could not cap the liability somehow, you would never be able to buy product liability insurance.
Manufacturers also use the 12 year "life" in specific cynical ways. One example is the recent Lyco SB569A 12 year crank life limit. Within the 12 years, if you overhaul the engine at Lyco, they will replace the crank FOC, otherwise they won't "because an engine must be overhauled at 12 years, Sir".
The warranty is just 12 months but any half decent lawyer will make a liability stick way past that point.
However if you run an engine to 13 years and it fails, kills you, etc, they are (probably) not liable.
From the engine maker's POV, it is a necessary position to adopt because only about 2% (according to Lyco) of the GA engine population is replaced each year. So there is a huge number of very old engines out there. If you could not cap the liability somehow, you would never be able to buy product liability insurance.
Manufacturers also use the 12 year "life" in specific cynical ways. One example is the recent Lyco SB569A 12 year crank life limit. Within the 12 years, if you overhaul the engine at Lyco, they will replace the crank FOC, otherwise they won't "because an engine must be overhauled at 12 years, Sir".
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why do engine manufactures set a calendar life limit such as 12 years?
Probably one reason is to put a cap on liability.
The warranty is just 12 months but any half decent lawyer will make a liability stick way past that point.
However if you run an engine to 13 years and it fails, kills you, etc, they are (probably) not liable.
From the engine maker's POV, it is a necessary position to adopt because only about 2% (according to Lyco) of the GA engine population is replaced each year. So there is a huge number of very old engines out there. If you could not cap the liability somehow, you would never be able to buy product liability insurance.
Manufacturers also use the 12 year "life" in specific cynical ways. One example is the recent Lyco SB569A 12 year crank life limit. Within the 12 years, if you overhaul the engine at Lyco, they will replace the crank FOC, otherwise they won't "because an engine must be overhauled at 12 years, Sir".
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I’m sorry if this is a blatantly naive question but if there is nothing in an engine that is deteriorating as a result of age – an engines condition being dependent on wear and how it is operated - why do engine manufactures set a calendar life limit such as 12 years?
Seals, diaphragms and gaskets deteriorate over time. Even new parts that are well packed and shielded from light and heat in a storage environment have a finite calendar life.
Also, seldom-used engines can suffer from accelerated wear on start-up due to loss of oil film and/or corrosion between components. There's a Service Instruction or Bulletin about this subject but I don't have the specific details to hand at the moment.
Google will probably throw up the answer.
Also, seldom-used engines can suffer from accelerated wear on start-up due to loss of oil film and/or corrosion between components. There's a Service Instruction or Bulletin about this subject but I don't have the specific details to hand at the moment.
Google will probably throw up the answer.
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A Google search revealed Lycoming Service Instruction No. 1009AU
http://www.lycoming.textron.com/support/publications/service-instructions/pdfs/SI1009AU.pdf
which states:
“Engine deterioration in the form of corrosion (rust) and the drying out and hardening of composition materials such as gaskets, seals, flexible hoses and fuel pump diaphragms can occur if an engine is out of service for an extended period of time. Due to the loss of a protective oil film after an extended period of inactivity, abnormal wear on soft metal bearing surfaces can occur during engine start. Therefore, all engines that do not accumulate the hourly period of time between overhauls specified in this publication are recommended to be overhauled in the twelfth year.”
http://www.lycoming.textron.com/support/publications/service-instructions/pdfs/SI1009AU.pdf
which states:
“Engine deterioration in the form of corrosion (rust) and the drying out and hardening of composition materials such as gaskets, seals, flexible hoses and fuel pump diaphragms can occur if an engine is out of service for an extended period of time. Due to the loss of a protective oil film after an extended period of inactivity, abnormal wear on soft metal bearing surfaces can occur during engine start. Therefore, all engines that do not accumulate the hourly period of time between overhauls specified in this publication are recommended to be overhauled in the twelfth year.”