TBO or MTBF

[Bruce Tuncks]

What you just said Facthunter is the conventional wisdom which is intuitively right but which has been shown to be quite wrong in reality. Waddington had to fight against this idea too, but he prevailed and increased the reliability of "his" aircraft markedly.

 

I do have to agree the legal system would probably agree with you though.

 

You really need to read up on this stuff, I bet you find it interesting.

 

... Bruce

 

 

[facthunter]

Bruce I haven't, but I've been around long enough tho know some of what is being advocated lately borders on witchcraft particularly relating to piston engines which a have a fair bit to do with. I'm not saying your bloke is like that.

 

I do give a bit of credit for the general reliability of the Rotax to being it doesn't have to be routinely attended to as much as many other engines, so it doesn't get mucked up. People are used to not lifting their car bonnet till knocks, smoke or steam herald a big expense coming.

 

I get my opinions from first hand sources mainly since so much what is about is misleading at the very least I know from what I have seen that most light aircraft engines I have observed don't get the type of attention I think is optimum or even the minimum reasonably expected and needed. Nev

 

 

[jetjr]

How maintenance is managed is a profession, outside aviation theres more flexibility.

 

The methods used depends on cost, risk, repairability and consequential problems if failure occurs. Complicated plant uses a mixture of systems for each component.

 

In aviation i expect, many problems have major outcomes an as such moves to prventitive or programed replacement programs. TBO is an example.

 

Its prefered to move towards highly monitored on condition maintenance. Much of this is run through PLC and done with sensors and specific programs to keep track of variation

 

 

[Bruce Tuncks]

I reckon the most important thing with my Jabiru engine is to avoid flying with a non-seating valve.

 

Now this is an "on condition" example of where TBO and fixed-time schedules are irrelevant. In fact these schedules might do more harm than good if they stop investigations until the next scheduled service.

 

But what instrumentation could help spot a bad exhaust valve better than the turnover test and taking the head off? A few weeks ago, I had a soft cylinder on the turnover test and then measured an 80/20 leakdown on that cylinder.

 

Well I was too hasty, as there was nothing wrong to be found when I took that head off. I should have done the "soft hammer on the tappet" thing to clear what was probably a crumb of avgas residue.

 

.. Bruce

 

 

[facthunter]

Every engine should be considered "on condition". A pull through test cost little in time or money. Its easier than checking tyre pressures.

 

Warning.. Don't do this with engines with impulse mags as you get a hot spark at no revs as long as the mechanism trips if the earthing switches are faulty.

 

You were a bit unlucky, un your case but you did the right thing. . Probably just a speck of carbon residue. This can sometimes happen as a result of removing a spark plug, and a bit of carbon falls on the seat of an opened valve. Nev

 

 

[Yenn]

If you have CHT and EGT for each cylinder and watch them closely, Learn what they should show you and you will be well on the way to a better understanding of what is going on. 80/20 from a leak down test is not the disaster you may think. have a look at the valves at the rocker end and try to ease them, then go fly. try the test again and you may well get a far better result. Even if you still get a poor result, current thinking by the experts says you should use a camera inside the cylinder to have a look at the valves. If there is no sign of distress try flying it again and only as a last resort pull the cylinder off.

 

 

[Guest Maj Millard]

TBO means Time Before Overhaul....not time between overhaul. Time between overhaul is general expressed as expected engine life, or can be broken down further as low time , middle, and high time. Or simple as engine expected TBO....time before overhaul.

 

TBO ...'time before overhaul' can be used at any time in an engines life to express how many hours are left in the engines' total life. It is commonly used in used aircraft ads showing how much time remains before the engine requires overhaul. This is also expressed often as ETR...or 'engine time remaining ' or PTR...prop time remaining.

 

 

[Garfly]

Here's a discussion on the BMAA forum about Rotax 912 hours:

 

http://forums.bmaa.org/default.aspx?f=84&m=132786&g=132797#m132797

 

 

[dutchroll]

I consider Mike Busch (in the aircraft engine field) and Bob Nuckolls (in the aircraft electrical field) birds of a feather. They're incredibly knowledgable, but they're also idealistic and every now and again espouse principles which will only practically work in an ideal world comprising ideal people - which isn't this one.

 

 

[facthunter]

There does appear to be a lot of confusion about how TBO works and the "on condition" possibility further muddies the water.

 

Simply put it, is the time in hours when you HAVE to do something about your motor regardless of how well it appears to be running. It is illegal to keep using it in an aircraft with out a permit of some kind, (Running on condition I consider to be such a permit)

 

The annual or 100 hourly is such a limit too. You must perform the required work before the aircraft is used and have it signed off.

 

Neither limit guarantees the plane aircraft will not require some service prior to the stated limit. Experience with various engines dictate inspections /tests needed. Ie you do a MAG and idle speed check every flight. You do a controls free also. On a radial you turn the engine through before start. This is aviation, not your wife's 10 year old Hyundai you use for shopping.

 

An engine cannot be expected to run to the TBO without some maintenance. Your 100 hourly does compression tests which may dictate action is required . Doing a pull through before each flight is the best thing I can recommend. Get someone to show you what a good engine of your type should feel like.

 

Years ago when cars were greased every 1,000 miles people were more mechanically aware. They would talk with their mechanics even if they didn't do the work themselves. Today you get a bill.... Labour Parts. Cost.... and you just pay it and hope the kid who worked on it tightened the sump plug and actually put the right oil in it. Nev

 

 

[Guest Maj Millard]

The compression check on the 912 is virtually useless even though it is suggested as part of a normal 100 hrly by the manufacturer. You will always get good readings on a 912 until they get around the 2000 hr mark, and only then they may drop a little because a carbon buildup can be expected under the exhaust valve seats. The rings (square cut just like a VW) pistons ( forged alum Cima Mahle Porsche quality) and bore, ( all alloy - nikisil ) pretty much guarantee no loss of compression until very high times are reached.

 

Even then simply removing the heads ( no gaskets) for a standard valve grind will always restore compressions to standard value.

 

My 912 ULS has 900+ hours now and has never had a compression test done on it , nor do I intend to even think about doing one until it reaches TBO of 1500 hours. I'll bet you even then it'll still be ok, and well within range. Like any engine the best medicine is to use them often and don't baby them. I change my oil every 50 hrs ( Valvolene Durablend 15w50) and my oil filter every second oil change at every 100 hourly. I always do a filter cut and only use Rotax oil filters. The engine was run on Castrol GPS for its first 400 hrs until Rotax no longer recommended its use.

 

 

[David Isaac]

I wouldn't run Valvoline in any of my machines Maj. There are far better quality oils on the market IMHO and not just MHO.

 

 

[Guest Maj Millard]

Valvolene is one oil recommended by Rotax David.......It is doing just fine for me, over 500 hours now....wouldn't run a Shell product in my weed wacker personally.....

 

 

 

[ruffasguts]

The compression check on the 912 is virtually useless even though it is suggested as part of a normal 100 hrly by the manufacturer. You will always get good readings on a 912 until they get around the 2000 hr mark, and only then they may drop a little because a carbon buildup can be expected under the exhaust valve seats. The rings (square cut just like a VW) pistons ( forged alum Cima Mahle Porsche quality) and bore, ( all alloy - nikisil ) pretty much guarantee no loss of compression until very high times are reached.Even then simply removing the heads ( no gaskets) for a standard valve grind will always restore compressions to standard value.

My 912 ULS has 900+ hours now and has never had a compression test done on it , nor do I intend to even think about doing one until it reaches TBO of 1500 hours. I'll bet you even then it'll still be ok, and well within range. Like any engine the best medicine is to use them often and don't baby them. I change my oil every 50 hrs ( Valvolene Durablend 15w50) and my oil filter every second oil change at every 100 hourly. I always do a filter cut and only use Rotax oil filters. The engine was run on Castrol GPS for its first 400 hrs until Rotax no longer recommended its use.

Therefore u are not maintaning it in acodance with the maintenance manual

 

Mick

 

 

[Guest Maj Millard]

Therefore u are not maintaning it in acodance with the maintenance manualMick

That's entirely correct Mick. You will note that the maintenance recommendations by Rotax for non- certified engines are just that....recommendations not requirements....they may be requirements for certified engines however which mine is not.

Additionally we have the option in this country of nominating a maintenance system which may better suit our climatic or operating envoiriment.

 

I nominate a modified Rotax recommended system, which is what I use, and have used for many years successfully.

 

In respect to leak -down testing which I perform almost daily on GA aircraft engines by the way, my experience with many 912s over many years shows me it is of minimil use on a regularly operated 912. Many 912s that I have maintained have and are operating privately with well over the recommended TBO hours , with quite a few running perfectly still after 2000+ hours and some with way in excess of that up to the 4000 Hr region.

 

Rotax only recommends a leak down be performed around every 200 hrs anyway. This reflects the superior quality of 912 rings, valves/ valve guides and bores when compared to other like engines which require a leak down test every 25 hrs !.....and often shows poor sealing on some or all of the above on those particular engines.

 

The main purpose of a leak down is to ascertain the sealing capability of piston rings/ cylinder bores and the sealing of valves, in particular the exhaust valve and seat. Additionally it is a check that may also highlight any cracking of the cylinder or cyl head.

 

As I previously stated, the longevity of the 912 rings, pistons, Nikisil cylinders, cylinders and heads is well known and documented over many many thousands of hours of operational usage, and rarely do any low compressions eventuate when the test is performed correctly. (EG: always carried out with engine at operating temperature.) Even then it is usually the result of a carbon build-up on the exhaust valve seat, which is also common on similar style GA type engines.

 

Obviously on aircraft that are used for training or for hire and reward, I do perform my maintenance completely with Rotaxs recommendations.