A common maintenance issue the older aircraft

[Stephen Furner]

A common maintenance issue for older aircraft that are privately flown is that they reach their calendar life before getting anywhere near their TBO hours. Suppose you are coming up to the calendar life on an engine but are only about half way through its TBO hours. What would you do?

[Gold Miner]

If the aircraft is flown privately and not used for training etc, then you can continue to run it beyond the calender life.

My PA-28 has an engine that was overhauled to zero-time 14 years ago and has done around 900 hours since. I take care of it as much as possible but do not intend to overhaul it again any time soon, if I can avoid it.

[Silvaire1]

I'm currently flying behind a Lycoming that was assembled in the spring of 1971 and has never been apart. Runs well, doesn't leak. It may get pulled apart someday.

[Pilot DAR]

If it runs well, and running over is okay in terms of your ops conditions, keep running it. Pay extra attention to non-metallic components (hoses, "O" rings, and gaskets) for deterioration though....

[Stephen Furner]

Many thanks for sharing your thoughts and experiences. Interesting to see that no-one supports an overhaul or new engine unless needed for conformance to operation regulations for commercial work.

[Pilot DAR]

On condition has a lot of merit, as long as you monitor the condition of the engine.

My O-200 had at least 3700 hours since major when I decided to overhaul it. It had started to make ferrous metal. Once I had it apart, I found that the metal was from the crimped on alternator drive coupling, and not serious in the slightest. You just don't know until you get it all apart, and see that everything else is in great shape. I did have one crank throw just a little worn on the power side, so I had the crank ground .010 under, and used the appropriate bearings. The engine was otherwise in excellent condition, and would have run a long time yet,...

Mogas gets a little of the credit!

[IO540]

I know people will differ on this but I am not so keen on running an engine "on condition" for so long.

The reason is that the monitoring is on very narrow parameters. Most people do it on the compression ratio, but that only tells you the piston rings have not broken apart, and the valves are still closing OK when the engine is not running.

Almost nobody does oil analysis, and it appears clear from some accident reports that some firms do not even cut open the oil filter (or they cut it open but disregard the contents on orders from a regular customer).

The biggest thing which people do not realise is that the cost saving from running "on condition" is very small indeed. Let's say an overhaul costs £20k (and you should get a gold plated one done in the USA for that, including shipping both ways by DHL). That comes to £10/hour. Let's say you run to 3000 hours. That comes to £6.60/hour. So you are saving just £3.40/hour and for what? For running the most important bit of your plane into a relatively unknown region. This is against say £70/hour spent on fuel. Even 50hr checks cost way more than that, for most people in Europe.

On top of that, most people are flying planes they did not buy new and they have not had the engine opened up under their ownership, so they have no idea what abuse it has seen. There are loads of muppets flying around who know zilch about engine management. You are taking a risk on this unknown history, every time you fly over mountains, etc.

It doesn't make any sense to me.

[maxred]

Concur with that. There is 'on condition', and running till the end. Comps are only one part, and, as stated, oil analysis. I think both checks are a must, and I have them both checked at each annual. My engine is still to reach TBO, however, a top end o/h some years back, whilst not zero timing the engine, at least gave some confidence that it had been stripped and checked. I am out of time on calander, therefore I run on condition.

However, it is also in my budget to replace, which, of course brings the usual debate of buy another????, however, if you are current with the aircraft you own, and have followed all the maintenace though, then the on condition should not be a major issue.

My problem is more other timed life parts, airframe, which also run on condition, and can be more difficult to access for checks. Motors etc.

It again boils down to getting a 'shop' that you are confident with, a sign off AP/IA, that you have confidence in, and if in doubt, do not fly. I an currently watching a case with interest, the aeroplane seems to be falling to bits, they keep fixing it, moaning about high maintenace costs, and guess what I am not going to fly again. It is a club aeroplane.

[IO540]

Going past the 12 years is not the same thing (in risk terms) as going past 2000 hours.

There is nothing in an engine which is strictly "lifed" in an engineering sense. There are oil seals which won't last 100 years, of course, but if you have oil pouring out that will be kind of obvious... But most of the highly stressed parts are steel and steel does not have (AIUI) a fatigue life (aluminium has). Conrods for example will run "for ever" unless there is an inherent defect to start with.

For an engine to reach 12 years before reaching 2k hrs it would need to be doing 166 hours/year, which is about 5x the PPL average, and is more than I have ever done (max was about 140 I think, pure airborne time). There is no safety data suggesting that going slightly past the 12 years is a problem, on the old piston engines. The question is ... how far past?

I think most people would be suprised at just how knackered an engine is allowed to be before it is rendered unairworthy. I don't recall the Lyco oil usage limits but they are about 5x higher than what a normal engine should be consuming; on a long trip you will be carrying a boot load of oil bottles :) Similarly with compression figures.

[Rod1]

I think many aircraft get into the “on condition 3000 hours” by simply putting off the inevitable for another 200 hours, then another and another. Many group owned aircraft only do 150 hours a year so extending by a few hundred hours seems a good idea at the time. Many engines will only be ½ way to tbo by 12 years and are run for 20 ish before significant work is done.

Rod1

[AdamFrisch]

This is why a twin sometimes gives you that buffer. I won't think twice about running an engine beyond TBO if it's still got a pulse and behaving. In fact, my left engine now has pretty low compression on two cylinders, but it runs fine so it doesn't make sense to pull those two cylinders. It burns no oil. They'll probably make it to next years annual and if they don't, well, I have a second engine and have that extra safety margin. Even if they pack it in completely at the same time (which is very unlikely), the aircraft will produce power on that side.

On a single, this might be to close of a margin/comfort. I'm basically a nervous flier, so had this been a single I would have addressed those cylinders before I did much more flying.

[IO540]

If I was a nervous flyer I would not have two clapped out engines because one of those is a lot more likely to go bang than a newish one on a single, and when one of the two does go bang, and if it does it on takeoff, then unless you are very current you are going to kill yourself :)

I don't buy the "I carry a spare engine so it's OK for both to be clapped out" argument, which is put out by a good number of twin engine pilots (I don't mean yourself, necessarily).

If I was a nervous flyer I would have a SE turboprop, which statistically beats any piston twin into the ground. If I wanted a piston twin I would have two immaculately looked after engines on it (and marry a rich woman to pay for the fuel :) ).

[Silvaire1]

I approach risk management for the 12 year recommendation differently than I might for operating TBO. The limiting cases are so much wider apart: if the engine was stored in cool dehumidified air or dry nitrogen it might be as good after 12 years as it was on day 1. Conversely, if it is stored outside in a very humid environment it could corrode fast. And where the engine sits within that spectrum is important, because you take a risk disassembling and bolting together a perfectly good engine - mistakes can be made, and parts can be damaged.

I know a guy with two newly overhauled radial engines in storage, waiting for use on his biplane someday. They are stored in cooled, dehumidified air 24 hrs/day. He bought them because the expert who built them wasn't going to be around forever and he was apparently the best with these engines.

The 40 year old original build Lycoming I mentioned above has mid-time operating hours, and I know where it was stored (and by whom) for most of that time... It doesn't leak a drop or burn much oil, and it was inspected pretty closely when I bought it. It has a newish carb and mags. That doesn't reduce the risk of operating it to zero, but the risk is balanced against the risk of overhaul, as well as the cost. For now I'm happy to fly it, and "for now" could last for years (or not, if things change).


I do find the orange pushrod tube seals on that engine curious... on other Lycomings I've seen they are always green. Maybe this is because I haven't seen many that were installed in 1971!

[englishal]

There are so many variables here, and I reckon the biggest variable is the quality of the initial build / zero time.

I know people running an IO540 with > 2000 hrs and it runs sweet as a nut. I also know people who have had to rebuild after 1000 hours. I know people who pull cylinders to reach beyond TBO.

I think that an Engine at TBO needs to be monitored closely, and as soon as there is one problem, rebuild the whole engine. Otherwise why not run it on and on. I think the problem comes when people try to patch up old engines and keep them alive...a bad idea in my mind. Also applies to less than TBO - Ours had 1600 hrs on it but someone before us had pulled a cylinder. In this instance we decided to bite the bullet and paid for a 28k rebuild even though we could have patched it up and continued for another 400 hrs.....

[IO540]

Actually you would rebuild an engine if you found anything significant at 1500 hours or so.

Just not worth "repairing" it.

Unless it is a turbocharged one in which case cylinders get changed rather more often :)

[englishal]

You and I would rebuild at 1500 hrs, but I BET there are people out there who would patch the thing up until it disintegrated at 4000 hours and then wondered why they wrote their plane off...

[Silvaire1]

My observation is that engine failures resulting in off field landings are not strongly correlated with engine time since overhaul. I think that's more strongly related to basic maintenance. Burning/dropping valves might be the exception, but they typically give warning in low compression at annual, and are reworked during any top overhaul too.

Performance does degrade with many hours SMOH but those high time engines do not frequently quit as a result of their time in service unless oil levels are run low or they are otherwise ill maintained. I tend to check power occasionally on mine with that dynamometer mounted on the front of the engine (full power run-up with known prop)

Most of the total engine failures I've been aware of resulted from unpredicted fatigue failure of some part - a part which may well have been "pre-broken" at the factory and/or put back in service after an overhaul: cylinder base flanges, crankshaft, crankcase, rods. Crack inspection & detection helps, but is imperfect. A guy I know recently found a long crack in his Continental O-360 crankcase at 200 hrs SMOH. Doubtless it was there before, they missed it. The real world is a b*tch eh?

[Shunter]

I think every situation is different to be honest. Our otherwise solid engine (2200hrs) started to use oil and made some metal in 2010 so we pulled the cylinders at the annual. It was a piston pin end-cap trying to grind itself into oblivion. We fitted 2 recon cylinders and she's been good as gold ever since. All compressions are >75psi, burns no oil and pulls like a train. Everything else, cams etc, were in good condition.

So in our case, after much discussion with our maintenance company, we decided it would be premature to tear it down. If we'd discovered for example the cams were on the way out aswell then it would have been a different story, but when everything else is in great shape one can argue a very good case for carrying on with it.

I'd certainly rather have a well maintained 2200hr engine whose internals I'm sure of as opposed to a rarely used 1000hr engine with rusting cams and cracked cylinders ready to pop its clogs.

[englishal]

Does it make economical sense to replace cylinders on a 2200 hr engine? How much would something like that cost to carry out and how many more hours (realistically) could one expect from the engine before MO is required?

Just curious?

[ShyTorque]

Most people do it on the compression ratio,

IO540, that was presumably a typo error. The CR of an engine doesn't ever change unless it's stripped and mechanically altered. You really meant say the actual measured compression readings, which may reduce with wear of the bores/rings and valves/seats.

[IO540]

Yes, of course, I meant compression tests. You pressurise the cylinder from an airline and measure the rate of leakage (in essence).

[gasax]

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.

[Silvaire1]

Yes, of course, I meant compression tests. You pressurise the cylinder from an airline and measure the rate of leakage (in essence).

Its easy to be casual with engine testing terminology, because most everybody uses the same tests on aircraft and they'd know what you mean. In fact saying "compression" isn't quite right either, despite my using it in another post above. The correct term for the test which checks cylinder sealing would be 'Differential Pressure Cylinder Leakage Test'

[AdamFrisch]

Compression alone is not a sufficient indicator. Engines can have great compression and a leaky o-ring pushing oil out the exhaust.

[Gold Miner]

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.

[ShyTorque]

Engines can have great compression and a leaky o-ring pushing oil out the exhaust.

O-ring?

You mean "oil control ring".... allowing oil to be blown out of the exhaust ;)

[Silvaire1]

Compression alone is not a sufficient indicator. Engines can have great compression and a leaky o-ring pushing oil out the exhaust.

Which is why the diagnostic test use for aircraft engines measures total cylinder leakage flow, and has nothing to do with compression. Then you listen for flow out the crankcase breaker relative to exhaust pipe and you learn a little bit, and go in the appropriate direction.

[stickandrudderman]

And even a cylinder leak test isn't definitive.

[jxk]

And even a cylinder leak test isn't definitive.

Very true the differential pressure test will not for instance show camshaft wear on the Lycoming.

[IO540]

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.

[Silvaire1]

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.

[IO540]

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.

[Silvaire1]

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.

[IO540]

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".

[IO540]

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".

[Stephen Furner]

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?

[Rod1]

“why do engine manufactures set a calendar life limit such as 12 years?”

To sell more engines.

Rod1

[stevef]

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.

[Stephen Furner]

A Google search revealed Lycoming Service Instruction No. 1009AU

http://www.lycoming.textron.com/support/publications/service-instructions/pdfs/SI1009AU.pdf (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.”

[stevef]

That's the one! I was only looking at the hard copy the other day when updating manuals.

[IO540]

Inactivity, sure. But you don't need 12 years of that to trash an engine :)

There is no obvious engineering reason for the 12 year life limit on an engine that is running regularly.

Hoses are a separate issue. They are all external and yes they can and usually are lifed. But again there you can have Teflon hoses which have no inherent degradation mechanisms. On my plane (TB20) Teflon hoses are not lifed.