A good article on engine airworthiness

I may be blind, but SB388C does not seem to mandate valve lift measurement, but valve stem clearance (wiggle room) measurement

I can only call the view that something that is not certified as airworthy is not airworthy a bureaucrat’s view – I’ll just say there is a lot of stuff certified as airworthy that isn’t, and a lot of stuff manifestly airworthy that is not certified.

This is only semantics. When we talk about airworthy, what we mean is the engine is deemed suitable for operation in an aircraft by an aviation authority. This IS what this discussion is all about, how to verify the airworthiness of an engine to the authority. This is dead simple with a TBO system, a date in a log book (plus the usual annual etc). To keep an engine functional and operational in an aircraft is not a problem. A system with TBO is not needed to do that, it is one method of several, but by far the cheapest and simplest. A Lycoming is designed from the ground up to be easy to maintain, even complete overhaul in the field. It is designed so a person can keep the engine in tip top shape indefinitely, and he can do all necessary maintenance in the hangar or at home or wherever. So a very simple system of overhaul (complete tear-down, inspect and replace/repair if necessary) every 2000 h or thereabout and much simpler intermediate inspections and maintenance has developed. A system that requires zero bureaucracy, it requires no online monitoring equipment. In fact it is as down to basic and simple as it is possible, also for the authorities, because they don’t need to bother about details of the “condition” of every single engine out there.

Yes, you can monitor this, sample that, endoscope, analyze etc. All laborious stuff. And none of it really work without databases, without people analyzing the data, making statistics. None of it work without online logging of how the engine is operated, plus the complete history of everything about the engine (every single detail) from when it left the factory, at least not as a verification of airworthiness.

Yes, it’s another SB, don’t have the number right now. It gives the minimum valve travel for each engine. German approved maintenance programs refer to it.

Yes, you can monitor this, sample that, endoscope, analyze etc. All laborious stuff. And none of it really work without databases, without people analyzing the data, making statistics.

There is a much simpler system: you appoint people with a certain qualification to make that call, simply because they are qualified. That qualification would be an A&P or maybe an IA. Eurocrats believe in paper and rules and CYA. Smarter authorities believe in qualified field personnel.

Yes, it’s another SB, don’t have the number right now.

If you remember the number, I’d like to know, because I recently tried to find such an SB and failed. Both SB369C (engine inspection after overspeed) and this suggest to measure all valve lifts and compare them. I haven’t found any absolute limits (not even relative limits).

German approved maintenance programs refer to it.

German maintenance programmes seem to be very detailed, my approved maintenance programme just states that the owner and the maintenance company need to agree on which SB are to be performed.

There is a much simpler system: you appoint people with a certain qualification to make that call, simply because they are qualified. That qualification would be an A&P or maybe an IA. Eurocrats believe in paper and rules and CYA. Smarter authorities believe in qualified field personnel.

I just think that every other system will create more bureaucracy, not less. It will be more expensive, not less. People are mixing together things here. Airworthiness in the eyes of the authorities is not the same as an engine being fit for flight (of tip top condition, or “airworthy” in a general sense). All the authorities care about is that you can verify that the engine is maintained within their approved system. A system they have made together with the engine manufacturer. The actual condition of the engine is of no interest to them. This system consists of dates in a log book. How can it be simpler than that? It cannot.

The minute you start with a condition based system, then how are you going to verify airworthiness? It must of course be verified based on the actual condition of the engine. Who decides which conditions are OK and which conditions are not? some field personnel? No. It is the engine manufacturer together with the authorities, the field personnel is only a data collector. This means the condition of each and every engine suddenly becomes of interest to the authorities. Which again mean that they would have to create a system for which the condition of each and every engine at any given time is a known and stored quantity for the authorities to look up. This is the only way they can verify continuous airworthiness. The military use systems like that, the airlines use systems like that. They are much better, much more accurate than a dead simple TBO-based system, but they are also much more expensive and much more bureaucratic. In my opinion I cannot see how it even is possible to create such a system without rather advanced on board monitoring equipment. Equipment that do not even exist for Lycomings.

I am starting to build my engine for the experimental Onex now. A VW engine that I am going to put together from individual parts Clearly there will be no such thing as TBO on this engine. Not because I wouldn’t rather have a TBO, but because it isn’t possible. This means I have to rely on data acquisition and manual inspections to have at least some idea of the condition of the engine. Data acquisition is made very simple with the glass engine monitoring equipment, but I can only measure simple things as temperatures and pressures (in addition to rpm). Most people “create” their own “personal TBO” on these engines. It may be around 200-500 hours or something, and a total re-build is easily done within a week working on it in the evenings, or on a week-end. Others have run them well past 1000 hours with no problems. So even TBO-less engines receives a TBO from their owners. A Lycoming is just as easily maintained as a VW.

In my opinion, the real reason for wanting to go to condition based maintenance is the perception that it will save money. That perception is wrong. Also, when you get past that erroneous perception, you will be reasoning that you would gladly do a overhaul, if for no other reason than to have a better flying experience the next 2-5-10-30 years.

It doesn’t make much sense what you are writing LeSving. An ever simpler system would be to ban all aviation.

On-condition maintenance has been the standard in every relevant country for decades and there is no way around it. Fixed TBO does not work for aviation as a whole and it is a stupid thing because it is wasteful and there are many situations where these simple rules do not fit . The authorities don’t have to verify airworthiness based on documentation, they have to make sure the people determining it in the field are qualified. We require doctors to have a degree, we require lawyers to have a degree and we require aircraft inspectors to have a degree. That is enough.

LeSvig,

I think most people who have experience with engine overhauls in Europe would also argue that high cost is also combined with poor quality and increased risk (regardless of what the pristine paperwork says), given high failure rates after the overhaul. [cf. “Bathtub Curve”].

The largest bugbear is the 12 year life, not the hours since last overhaul – the 12 year life is not justified by anything except for corrosion risks, which can easily be mitigated by inspection. A large chunk of running-time related risk – wear and tear – is also more easily dealt with; the only one that is not is material fatigue, which is much more difficult to inspect for.

I just think that every other system will create more bureaucracy, not less.

How do you explain the FAA system, then?

Who decides which conditions are OK and which conditions are not? some field personnel? No.

Who decides the weather is suitable for the flight?

Sorry, at the risk of Peter donking this post, this “Authorities-know-best” attitude is depressing to see in a fellow aviator. Manufacturers err on the side of revenue and covering their backs [and making money, jet engines make money in maintenance, not when sold]. Civil aviation authorities are hardly a reservoir of highly competent engineering experts. Fortunately, they have started to realise that they have overdone the maintenance regs for light aviation and are going the other way now, even if you seem to be happy with the Part-M, manufacturer-recommended-is-mandatory, paper-centric approach to maintenance.

Sorry, at the risk of Peter donking this post, this “Authorities-know-best” attitude is depressing to see in a fellow aviator

Then you have not read my post. Weather you like it or not, certification is up to the authorities, not each individual “field personnel”. And really, that is my main point. With the TBO based system, the authorities don’t have to deal with the condition of each engine, they only have to deal with log books being up to date. Is overhaul done? yes or no, that is all. It is about verifying airworthiness. With a condition based system, it is no longer enough to verify the log book of the engine, the log book does not say anything meaningful about the condition. You have to report the condition to the authorities to verify airworthiness. It will be like these car controls we have, only 100 times more bureaucratic and detailed. I don’t think you understand the consequences of what you are asking for.

Me, I am building two experimental aircraft, a Onex and a RV-4. Both are well outside any EASA rule, and I have to create my own maintenance system from scratch. TBO or condition based, it is up to me to decide.