Attn: Genghis...Question about engines and overhauls...

[Double_r]

Hi Genghis,

I have a quick question about engines and overhauls. In an aircraft systems class we are discussing recip. engines. The question was brought up on how many times an aircraft engine can be overhauled. My professor didn't have a definitive answer because he does not have an A and P. I know that in general Continential and Lycoming are the two basic engines and may be diffrent. I guess what I am looking for is on average how many times the average engine is overhauled. I understand that the inside of the cylinder is ground out, thus resulting in a layer of metal to be removed. I posted over on the tech forum, and I was told that you are the man to ask.

Thanks for your help!!!

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Take care and safe flying!!

Double_R

[Genghis the Engineer]

I've never come across an absolute limit, obviously as you remove material from the cylinder you eventually reach the point where you can't get a big enough piston. For a cheaper engine such as a Rotax you'll end up scrapping it, but for something more expensive (a Merlin for example) you'll probably re-line the cylinder if you really have to - but that isn't going to happen very often because it's a very expensive operation.

So, no absolute, basically you'll scrap the engine when you get to the point that it's cheaper to replace it than the alternative. I've certainly come across engines that have had at least half a dozen overhauls.

G

[Double_r]

Thank you very much sir, I appreciate you thoughts on the matter. ;-)

Double_r

[Lu Zuckerman]

I have to harken back about forty years for this so I can't say it is accurate. The overhaul manual for a specific engine will show the limits of bore out on a cylinder. On large radial engines they required that a colored band be painted on the cylinder just above the bottom flange that the retention bolts pass through. If I remember correctly if a cylinder had an orange band on it the cylinder would be scrapped at the next overhaul or if that specific cylinder had to be changed. It was not uncommon on larger engines (1830/1820 and 2000/2800) to see cylinders with different colored bands and some cylinders with no colored band which indicated it was new.

[Lu Zuckerman]

Since I don't know how to edit my message I will post now what I forgot to incorporate in the above posting.

The overhaul manual will also specify the life of the major parts of the engine. In some overhauls,the manual will specify that certain parts must be replaced with servicable parts that will hopefully last until the next overhaul. The manusl will also pecify the rework limits for various parts inclucing the crank case and the crank and connecting parts. The manual will even specify what kind of reqork can be done on a piston and under what conditions the piston(s) must be changed.

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The Cat

[Genghis the Engineer]

It does remind one of the hundred year old broom does it not.

G

[Lu Zuckerman]

To: Genghis,

To my knowledge, on small engines used on general aviation aircraft, they do not reline the cylinder. They bore it out to remove the damaged surface and then replate with a propriatary metalic coating system and then hone it out to the original bore size. That way, they can utilize the original pistons or, new pistons of original size. The ammount of times this can be done is limited by the manufacturer.

By the way, Being from the colonies, I don't understand your comment about the hundred year old broom. If you you were alluding to the fact that I date back to ancient aviation history you should have correctly said 70 year old broom (in December). But as the commercial on the telly says I keep on ticking. The last two aircraft I worked on in a consulting capacity were the A340 and the A3XX. In two weeks I will most likely be working on constant speed drives used to supply electrical power to jets just like the ones' you fly.

Cheers.

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The Cat

[Genghis the Engineer]

Nothing offensive meant about the broom, the hundred year old broom is a British Simile - it's 100 years old, but had twenty new heads and fifteen new handles! It applies to any "very old" piece of machinery which has had virtually all of it replaced at some time or another.

I don't disagree with you Lu, read carefully what I said about economics. The point was about general principles, which you seem to back-up. As it happens I've dealt with both Merlins and Rotaxes, but only the odd excursion in Lycontinentals which you probably know far better than I.

Sadly I haven't professionally been near a jet for 4 years (last one was a Saab Gripen) - and although I fly 60-100 hours per year, I am primarily an engineer by profession.

G