TBO or MTBF

[Chird65]

In all the talk about the CASA action and reliability in general the term TBO - Time Between Overhaul is used and if a failure happens before this we feel cheated. is the term MTBF - Mean Time Between Failures a better measure for reliability. The vendor may not like to show this in their marketing but it would be more insightful for operators.

 

I base this on a comment I read elsewhere;

 

"Given that term,

 

TBO

 

, stands for time between overhauls, I would state that this term relates to the

 

end of life failure modes

 

. This interval relates to the time when wear-out of items start to occur. The MTBF [mean time between failure] term relates to the probabilistic condition of failure. That is,

 

MTBF is related to the number of chance failures in a given period of operation

 

. The probabilistic situation is one that failure could occur at any time during the operating period or not occur at all where end of life failures are predictable at a specific space and time. The TBO value has no direct relation to the chance failure rate as its function is to determine the time to renew the unit from wear-out failure modes."

 

What are the thoughts on this;

 

• Should we expect no failures before TBO or;
   
   
  
• Should the MTBF be reported by RAA or ATSB to give a better yardstick on products?
   
   
  

 

 

Chris

 

 

[turboplanner]

It would be extremely difficulty to set standards with any certainty for low-volume aircraft engine production.

 

If we look at production rates over 100,000 engines, misuse and other peripheral factors blend in to form a more reliable prediction.

 

MTBF is where the reasonable cost of Scheduled Maintenance becomes a cost so high that the Total Cost of ownership of the vehicle is blown to pieces and you might as well have paid maybe 40% more for a bigger or more expensive vehicle. For this reason, it's not smart to run a product to MTBF because resale value is virtually parts value. The Optimum selling point in Total Cost of Ownership is where there's enough life to MTBF that the potential buy can do what he wants to do with it then scrap it. (A lot of people buy cheap earthmoving equipment, pay for it with a contract and sell it again at the end).

 

However, people still run to MTBF and beyond. I sold a lot of trucks against a competitor whose engine had a life cycle of 80,000 km. Mine was about 450,000 km, but my price was $5,000.00 more than his, and I had to work for every sale. However, in fleet operations I was able to increase market share because the cost of a replacement engine on the competitor's truck was $18,000.00, so I was $13,000 ahead on Total cost of life after the first 18,000 km, and $85,000 ahead at my recommended sell point of 400,000 km. You would think that cost factors like that would stand out enough so people could see for themselves, but one fleet was racking up half a million dollars a year in engine replacements before the penny dropped. The competitor's truck was taken off the market.

 

For a hobby, I don't think you want to be playing Russian roulette by going anywhere near MTBF, unless the engine is so cheap that you just pull the old on out, and put a new one in, and there have been times in the past with engines such as the Perkins 6/354 diesel where they all ran comfortably to 100,000 miles and replacement engines which ran to another 100,000 were cheaper in cost and labour than rebuilds. That model isn't available with today's economics, primarily due to multiple margins.

 

For an operator TBO can provide the opportunity of budgeting costs out through the lifetime if there is an expectation of no failures/unscheduled maintenance within the period. Even though there may be known component lifecycles shorter than MTBF, if these components are replaced as part of the TBO programme there is no catastrophic shock load, just perhaps product A having a higher maintenance cost than product B.

 

So I would disagree with the above quote because TBO really means what it says - time between overhauls, and there can be several overhauls before there's noting you can do but buy a new assembly.

 

In answer to your questions:

 

• Should we expect no failures before TBO? TBO should be set, and adjusted to try to achieve this.
   
   
  
• Should the MTBF be reported by RAA or ATSB to give a better yardstick on products? No, because the causes are way too diverse and the operations vary way too much under RAA regulations
   
   
  

 

 

 

 

[Chird65]

It would be extremely difficulty to set standards with any certainty for low-volume aircraft engine production. ...In answer to your questions:

 

• Should we expect no failures before TBO? TBO should be set, and adjusted to try to achieve this.
   
   
  
• Should the MTBF be reported by RAA or ATSB to give a better yardstick on products? No, because the causes are way too diverse and the operations vary way too much under RAA regulations
   
   
  

Thanks for your take; so if I understand what your saying is we should expect no failures due to "wearing" out components before TBO?

 

 

[turboplanner]

That's a leading question in the current environment, and I did say "It would be extremely difficulty to set standards with any certainty for low-volume aircraft engine production." right at the start of my post, so I'm not applying this to Jabiru.

 

In the big picture, where enough statistics are available I would set the TBO to catch all parts which might wear out before the scheduled service, so that you as a customer would not have to bring a product in for unscheduled maintenance, such as a "worn out" component.

 

So for example, some manufacturers service intervals might by 20,000 km, some 10,000 km, and mine in my Nissan are 5,000 km, and what they replace at that interval is their business except where it starts to cost me big money.

 

 

[Chird65]

Sorry didn't mean to lead you anywhere.....

 

But I got the answer I was looking for. i have always assumed MTBF "could" be less than TBO.

 

thanks for your thoughtful reply.

 

 

[dazza 38]

I would expect MTBF to be a lot higher figure than TBO. When talking aero engines.

 

 

[turboplanner]

Sorry didn't mean to lead you anywhere.....But I got the answer I was looking for. i have always assumed MTBF "could" be less than TBO.

thanks for your thoughtful reply.

It can be, and manufacturer's have been in that position with serious market share results, unless they managed the situation.

 

An example of that was the Caterpillar 3406B (from memory) where main bearings failed at around 600,000 km (half life). They just set up a proactive changeover where the bearings were replaced before that time, usually in conjunction with a scheduled service and at no cost. Customers stuck to them like glue.

 

 

[Keith Page]

Turbo that is a very polite way of expressing how they stuck there.

 

KP.

 

 

[facthunter]

To expect to go through to TBO without some work is not realistic. There is also a checking/monitoring function justified in aviation on safety and a lot of other applications on economics alone..

 

Retensioning and adjustments, Oil changes, oil filter monitoring, exhaust system repairs etc. With the nature of the Jabiru and most other aircooled, piston engines, in aircraft, a TOP would be regarded as normal. I doubt I would try to run a jabiru to TBO without a TOP overhaul. It would be false economy.

 

I regard the TBO figure as the LIMIT on hours run without rebuild unless there is an extension permitted. The engine may also reach a TIME ( Years in service) limit . The engine is then regarded as TIME EX. as far as aviation is concerned. Getting an engine to be regarded as zero time is another contentious story.. No rebuilt engine is NEW. Nev

 

 

[jetboy]

TBO is a nominal figure that the manufacturer decides for maintainence schedule purposes and as time in service of the design increases they gather enough data to revise that figure. For example, Continental O-200 came out with 600 hr TBO which was extended to 1800 hrs eventually. Mine had never got more than 900 hrs on a cylinder without removal and rework, over its 30 yr history. The average time for a pot to be removed was 500 hrs, according to the logs.

 

MTBF is unrelated and based on an actual failure rate experienced by something, averaged over the number of those items in service.

 

It could be quite a large time, but for any individual engine might be a very short time.

 

If the MTBF is getting shorter, there has to be an explanation as to the changes that have been made to the product or its operation.

 

 

[dazza 38]

Another way of putting it, Rotax 912 engines that are maintained IAW the maintenance manual, regularly make and exceed the TBO. That engine would have a MTBF rate in hours above TBO.

 

Brand B however has a fair few engines that seem to implode well below the manufacturers nominated TBO, even though they are maintained IAW the manufacturers maintenance manual. That would indicate to me that their TBO is a bit too high in hours.

 

 

[facthunter]

Implode is a bit emotive. In Accordance With the manufacturer's manual may be a bit elastic. Specifically I've seen few who retension the heads correctly. as an example. It is a bit fiddly, but there you go. Nev

 

 

[Yenn]

There are a lot of aero engines going past TBO. Mike Busch who writes in the EAA magazine and has a book out on the subject, suggests that TBO means very little. His engines on his 310 Cessna have gone way past TBO and are on condition.

 

If you can legally run past TBO and you have a LAME on side, in my opinion you would be mad to do anything at TBO, except monitor the engine. For those of us running RAAus rego planes TBO means very little, so we can do our own maintenance, keep good records, monitor engine condition and save money safely.

 

 

[dazza 38]

Also in private ownership, a lot of engines will reach calendar life before they get anywhere near TBO because of engine hours.

 

As Yenn has mentioned above, just run on condition.

 

 

[facthunter]

It's a free country (so they say). I've heard over the years that running engines over TBO often means they have suffered and damaged themselves making rebuilds more costly. In other words it can be false economy. It isn't just a matter of wear. Things fatigue and bearings sometimes move in the tunnels, and cracks happen, valves stems erode etc... It's an aeroplane not the farm tractor.. .Nev

 

 

[dazza 38]

It's a free country (so they say). I've heard over the years that running engines over TBO often means they have suffered and damaged themselves making rebuilds more costly. In other words it can be false economy. It isn't just a matter of wear. Things fatigue and bearings sometimes move in the tunnels, and cracks happen, valves stems erode etc... It's an aeroplane not the farm tractor.. .Nev

True, but with the 912, it isn't worth rebuilding as it is too expensive to rebuild. I would run it on condition and when the time comes, I would replace it with a brand new engine.

 

 

[turboplanner]

If you run it until something breaks, first you have to pull off the forced landing without killing someone, then you may have tens of thousands of dollars of frame damage.

 

I agree there are some engines where the cheapest TBO is to put in a new engine, but as Fachunter says running after TBO can have catastrophic consequences.

 

I can remember a major truck fleet which had a semi with a Cummins VT300 engine which had a Repair and Maintenance cost average of 8 cents per kilometre throughout its life. The State Manager decided to run it past TBO, and it threw a rod through the side. The cost of the towing/downtime/new motor ran the R&M average up to 42 cents per kilometre, which pushed it into a loss performance for its whole life. The company never made a cent's profit over about ten years from thaqt truck.

 

TBO is usually met well after the warranty period is over into an area we describe as "Policy" If you have a problem, even though it's way out of warranty, the first step in deciding to make a Policy decision is to look at the service history. If that shows casual compliance with scheduled maintenance or operation after TBO, the decision's a very easy one. As Facthunter said, it's a free country.....but you'll pay for your decision to ignore sound management practice.

 

 

[dazza 38]

If you run it until something breaks, first you have to pull off the forced landing without killing someone, then you may have tens of thousands of dollars of frame damage.I agree there are some engines where the cheapest TBO is to put in a new engine, but as Fachunter says running after TBO can have catastrophic consequences.

 

I can remember a major truck fleet which had a semi with a Cummins VT300 engine which had a Repair and Maintenance cost average of 8 cents per kilometre throughout its life. The State Manager decided to run it past TBO, and it threw a rod through the side. The cost of the towing/downtime/new motor ran the R&M average up to 42 cents per kilometre, which pushed it into a loss performance for its whole life. The company never made a cent's profit over about ten years from thaqt truck.

 

TBO is usually met well after the warranty period is over into an area we describe as "Policy" If you have a problem, even though it's way out of warranty, the first step in deciding to make a Policy decision is to look at the service history. If that shows casual compliance with scheduled maintenance or operation after TBO, the decision's a very easy one. As Facthunter said, it's a free country.....but you'll pay for your decision to ignore sound management practice.

I am not saying run a aircraft engine until it breaks, obviously more maintenance checks would have to be carried out eg- compression checks etc.

As a example - the Rotax 582 can easily be ran passed its 300 TBO.

 

 

[Guest Andys@coffs]

Implode is a bit emotive. In Accordance With the manufacturer's manual may be a bit elastic. Specifically I've seen few who retension the heads correctly. as an example. It is a bit fiddly, but there you go. Nev

Nev, when you say they retension incorrectly, what do you mean? I cant see how you can get it wrong....but I don't know what I don't know?

 

Andy

 

 

[coljones]

If you run it until something breaks, first you have to pull off the forced landing without killing someone, then you may have tens of thousands of dollars of frame damage.I agree there are some engines where the cheapest TBO is to put in a new engine, but as Fachunter says running after TBO can have catastrophic consequences.

 

I can remember a major truck fleet which had a semi with a Cummins VT300 engine which had a Repair and Maintenance cost average of 8 cents per kilometre throughout its life. The State Manager decided to run it past TBO, and it threw a rod through the side. The cost of the towing/downtime/new motor ran the R&M average up to 42 cents per kilometre, which pushed it into a loss performance for its whole life. The company never made a cent's profit over about ten years from thaqt truck.

 

TBO is usually met well after the warranty period is over into an area we describe as "Policy" If you have a problem, even though it's way out of warranty, the first step in deciding to make a Policy decision is to look at the service history. If that shows casual compliance with scheduled maintenance or operation after TBO, the decision's a very easy one. As Facthunter said, it's a free country.....but you'll pay for your decision to ignore sound management practice.

However in this case it was an unsound management practice that ignored a sound engineering practice. We need less managers and more engineers - an informed client.

 

 

[Downunder]

I am not saying run a aircraft engine until it breaks, obviously more maintenance checks would have to be carried out eg- compression checks etc.As a example - the Rotax 582 can easily be ran passed its 300 TBO.

Yes, obviously an accelerated maintenance regime in place after TBO (time) has been reached. Off the top of my head, maybe doing 100hr 912 services at 75hrs/9 months to 2500hrs, then at 50hrs/6 months after that etc........

 

Personally, I would probably increase servicing/monitoring/maintenance before actual TBO has been reached.

 

Anything out of the ordinary, and it's time to bite the bullet for a new engine.

 

 

[facthunter]

Andy Can't give a "complete" coverage here but, as the hot areas may be affected by distortion and the threads may have binded it is necessary to slack them off and retension, or remove and wire brush in some cases. Not all just the one(s) near the exhaust port.. The head metal can collapse onto the bolts and require very careful "easing" of the hole to ensure a free fit, In essence it is not enough to just check the bolt with a tension wrench, as you might do if the engine had never been run. Nev

 

 

[jetjr]

Isnt there an issue with 912 on having timex situation well before many will reach TBO?

 

 

[Bruce Tuncks]

I've just read Mike Busch's book "Manifesto" and he convincingly makes the case that much ( not all) of our "preventive maintenance" is actually counter-productive and in fact increases the risks.

 

This was discovered by Waddington in WW2 but was classified until 1973. Before 1973, it was rediscovered by a pair of US airline boffins and now everybody is doing RCM that is, reliability centered maintenance, except our lot. RCM is higher-tech than our methods.

 

We are about 50 years behind the times here, mainly because fear of litigation can prevent rational change.

 

...Bruce

 

 

[facthunter]

TBO is taken by some, as time without being touched, which it isn't. Inspections and checks, are part of all aircraft ops. You don't wait for things to break. You find the problem before that happens. Because all aircraft are built as light as possible. They are somewhat unique in being fragile if not used right. You can't bash a plane against another in a hangar like you might a car in a supermarket car park and just drive it home. If you overspeed some engines above a certain figure it's a throwaway. Can't be rebuilt.. Aircraft aren't lawnmowers. Nev