How long should I go on engines?

[Rickskymaster]

I have 1200 hours SFReMan's on both engines.
Just came out of annual with no problems, using a quart of oil every 10 hours.
The question I am starting to think about is how long should I go before Overhauling them.
I have heard that the Air Force went 2,000 hours and then changed.
What is really confusing is the TBO. My recommended TBO is 1400 hours, yet the same turbo charged engine on a Seneca gets 1,800 hours TBO.
My thought right now is to run them to 2,000 and continue oil analysis.
Thoughts?
Rick

[kevin]

Rick,

Here are some threads where this subject has been discussed before:

http://www.337skymaster.com/messages...light=overhaul
http://www.337skymaster.com/messages...light=overhaul
http://www.337skymaster.com/messages...light=overhaul
http://www.337skymaster.com/messages...light=overhaul
http://www.337skymaster.com/messages...light=overhaul
http://www.337skymaster.com/messages...light=overhaul

Everyone's opinion is different. One thing that I would consider is how many years old are the engines? If you have been running them 150+ hours per year since birth, making them about 10 years old right now, I would feel comfortable running them to the Seneca TBO, and in fact that was my plan on my airplane when I owned it. If they are 20 years old, on the other hand, I would overhaul them fairly soon.

The Air Force TBO is not really relevant to us. That was for airplanes that flew 2 to 3 times per day, 5 to 7 days a week. A 2000 hour engine in the Air Force could be barely six months old...

The one thing everyone agrees about is that these engines last longer the more frequently they are used...

Kevin

[Ernie Martin]

I think your course is reasonable. TBO is very meaningful for carriers (like Part 135) who have to abide by them. For Part 91, there is significant merit in using "on condition" criteria similar to that used by major airlines in the maintenance of jet engines. Basically, I monitor the engine as you do -- oil consumption, oil analysis and, for older engines, perhaps double the frequency of compression checks to twice a year -- and fly it as long as all parameters look good.

I do this, in part, because it's a twin. If I had a single engine, I might do things differently, especially if I do a lot of night, IFR or over-water flying.

I don't know exactly what the failure distribution of IO-360 is, but for properly maintained engines and after discarding "infant mortality" events*, it might look like the figure below. 1500 hours marks the point at which failures start becoming frequent, but most engines last for much longer, and mine could be one of those, so I keep them running until there are signs of problems.

Ernie

________________
* Defective parts or improper assembly will cause some engines to fail very early -- perhaps just hours after overhaul. These failures are due to "infant mortality" and are not considered above. Incidentally, I'd much rather be flying a half-time engine with no sign of problems than one with 20 hours out of overhaul.

[SkyKing]

Ernie,

I'm curious where you've obtained your 1500 hour statistical criteria, that failure rates start becoming frequent within and beyond this time frame.

I look at it this way: If the engine has performed well within the last hour, it's most likely to perform in the same mode during the next hour. Trend monitoring of the engines, how they sound, what they look like under the cowl at Cessna maintenance recommended checks (like the exhaust system), as well as the look of the oil and burn rate would seem to be better indicators of what to anticipate as far as overhauling an engine. In fact, in may be a situation that the power producing end may be the only items that need to be taken care of, i.e., the cylinder assemblies.

With the new turmoil on the VAR cranks, those that have a good running engine should think twice about splitting cases.

I see no statistical probabilities or mortality rates for an engine that is 10 years old versus one that is 20 years old or older.

Nice graph, but I don't think it's very realistic.

As to Rickyskymaster's question, my thought is run them until there is a problem... but do the preventive maintenance as recommended in the Cessna service manual. As long as compressions, oil usage and all other parameters are within acceptable ranges... if it ain't broke don't screw with it.

SkyKing

[Eustacio-Chachi]

I just had a problem with an engine with 34.6 hours since OH (done in Florida, will not name the shop to protect my rights). The reason I had both engines overhauled even though there was no indication of problems, was because in 19 years I had only put about 1300 hours on them.
I could not make the planned trip to Canaima in Venezuela and hope to make it to Key West, the engine should be back next week.

[SkyKing]

I think I made my point!!

But the question is, 'WHY' did you overhaul them in the first place, if there was no indication of any problems?

SkyKing

[Eustacio-Chachi]

Skyking, I usually make long trips in the region and there are not that many facilities if you have a major problem on an engine. Besides the Aviation Authority in my country based on a bulletin which I have seen by Lycoming not Continental is requesting that you go for a complete overhaul after 12 years. I do regret having to send my engines for OH. I would have felt a lot more confortable flying across the Caribean on my engines before the overhaul.
Let me tell you I am one hundred percent for what you say " if it ain´t broke don´t screw with it "

[Ernie Martin]

Skyking:

I think you misread my message. My main point is, and I quote, that you should "fly it as long as all parameters look good". Let Part 135 operators worry about the 1500 hours. I then went on to say that I don't know what the failure distribution looks like. The sole purpose of the graph was to show that more frequent failures may start appearing at 1500 hours, but that for most engines there are still lots of hours left -- absent, of course, poor maintenence and infant mortality (see Kim Geyer's examples in his message on the thread entitled "IO360 - Factory new vs Factory rebuilt" and Chachi's example above).

In short, I agree with you and Chachi: keep a good engine running.

Ernie

[kevin]

Although I am in the minority, maybe a minority of one, I have to disagree with the "keep a good engine running" idea. Here are my reasons.

First, please read Gmas old message for some of the reasons why you might be concerned about an "old" engine. Here is the reference again:

http://www.337skymaster.com/messages...light=overhaul

Second, if you just keep running the engine, you have an increasing chance of having a catastrophic failure. If you do have such a failure, you cannot get credit for the core any longer, as core credit is only given for engines that were running when removed. So you run the engine longer, take the additional risk (see next paragraph), end up spending the same or *more* money in the end.

But third, most importantly to me, although we have twins, there is on every takeoff a period of time, albeit short, when an engine failure is a true emergency, and can result in an accident even with the best technique in a centerline thrust aircraft. An example is a short strips with obstacles, or even just a normal length strip with obstacles on a hot day. Another example is, since most twin pilots approach with power, an engine failure on final can result in an undershoot. And people do freeze up, or feather the wrong engine, or whatever. We all think that we will do better, but my position is that it is best to avoid the risk inthe first place as much as is reasonable. There is an old saying that a superior pilot is one who uses his superior judgement to avoid having to use his superior skills.

But the catch here is what each of us individually decides is a reasonable tradeoff between costs and avoidance of risk.

For me, if I have been running an engine from birth, and know how it has been maintained, and know that it has been run frequently through its life, I will run it to the TBO established for the same engine on a Seneca. If any of those ifs are missing, I will overhaul sooner. I would not risk my life, the lives of my passengers, or even just the airframe, just to avoid doing an overhaul a year or two sooner. In my life, I have experienced three actual engine failures, two in singles, one in a P337, and that is 3 more than I would ever like to experience again. (The P337 failure was on an 800 hour engine 200 hours after I bought the airplane, and I would have experienced that failure regardless of what we are discussing here.) If it costs me three or four thousand more to have it overhauled early, it is worth it to me, and I recommend the same policy to others who operate aircraft under Part 91.

If all that sounds holier than thou, I am sorry, but I am passionate on this subject. I don't want to loose any of you...

Kevin

[Ernie Martin]

Kevin:

I understand your position. For certain engines (one where maintenance is unknown or one which mostly sat without flying for years) I would take your side, but let me play the role of "run'em while they hum" advocate for engines of known good maintenance which haven't been sitting.

In GMAs message he's concerned about wear and other factors which can lead to catastrophic failure, and you point out the impact on core value and the in-flight emergency. My rebuttal is this: and this happened on an engine where I'm carefully tracking oil usage, compression and oil-change analysis? Not likely. Remember also that doing an overhaul is no guarantee of another 1500 hours of trouble-free flying, as your 800 hour failure and Kim Geyer's experience with recently reman engines suggest. In fact, because of infant mortality, I'd much rather be flying an engine at TBO with no sign of problems than one with 20 hours out of overhaul. This is a statement I made earlier but now strengthen to include a TBO engine.

My view, for what it's worth.

Ernie

[kevin]

I agree with most of what you say Ernie. But what I am trying to point out is that:

1.) There are reasons to be concerned about the overall age of an engine.
2.) That just because you are not making metal, your oil consumption is normal, and your compressions are OK, that does not mean that you will not have a failure. Engines wear out, and the longer you run them, the more chance of a failure there is. In fact, my engine that failed met all three tests before the failure.

So yes, I do agree with you, I would rather fly behind an engine with 1500 hours than I would one with 20, but I also would rather reduce my risk by NOT flying behind a 2000 hour engine. The risk period for infant mortality is limited. The risk on a high time engine just keeps getting worse.

Kevin

[stackj]

Sigh... I'm replacing an engine now with a factory zero time engine. (Where did all the money go?).

The engine I am replacing came from a boneyard and was placed on the airplane with 850 hours plus based on the log. (Removal time was not noted in the log). I have put over 900 more hours on it since hanging in on my baby (The airplane - not Jenny).

Compressions are pretty low all around on this engine. I get an intermittent vibration which I think is coming from the front engine.

We have decided the risks are getting too high to continue operating this engine for an extended time. We will probably put about 20 more hours on the engine before our appointment to replace it (Mar 7, 2005).

I didn't need that money anyway... After all, debt is really better!

[Walter Atkinson]

An interesting issue to consider is the fact that top end problems are seldom catestrophic (rare cases are, but they are the exception) and they telegraph their impending failure for many hours if one knows how to listen to what they are telling you.

Bottom end problems are quite different. They tend not to telegraph their impending failure and they tend to be catestrophic. These are the ones to be concerned about in high-time engines. AND, there is a very big difference in a 2000 hour engine that's 4 years old and a 2000 hour engine which is 25 years since overhaul.

These issues are what make this question so hard to answer with any degree of accuracy.

[WebMaster]

AND, I had/have a crack in my case, and was talking to someone a few months ago, that got a REMAN engine, had less than 300 hours on it, and developed big cracks.

Case cracks, at least on the 2 that I am aware of aren't catastrophic if caught in time, but I can just see the whole engine come appart on you. it would be a really bad time.

[Jerry De Santis]

I have a 1975 P337G with A.C., Riley intercoolers, spoilers etc. Engines are TSIO-360 CB and I fly it between 250 and 350 hours per year. Almost 900 hours over TBO, 2300 hours on both engines. Compressions still good. Oil always changed between 25 and 50 hours. I tend to operate ROP. Changed some accessories such as turbo charger on each engine, alt, starters. Engines still Run good...will keep flying it.

Jerry
N34EC