Engine Dependability

On Mon, 19 Apr 2004 15:37:43 -0500, O. Sami Saydjari wrote:



Greg Copeland wrote:

On Mon, 19 Apr 2004 13:53:36 -0600, Newps wrote:


"Greg Copeland" wrote in message
news


Well, I guess that puts us back to the original question. I, like the
orginal article's author, thought historical statistics were used to
establish TBO numbers. If no one is tracking this information, where do
the TBO numbers come from? Insurance liability statistics from wrecks?

TBO comes from two places.

1) Marketing

2) A Guess


LOL! Is anyone else bothered by this? I guess it's not really saying
MTBF, it's just saying, your engine is ganna be tired when it hit this
number. So, I guess that really isn't all that bad after all.

I, for one, prefer to base my decisions on facts instead of
speculations. How do they know the engine will be tired after x hours?
What does tired mean? The only thing that would be meaningful to
owners is probability of failure at X hours.

It does not seem that collecting the data and calculating MTBF would be
that hard.

I sure hope the engineers who designed the engine did not use the same
attitude with respect to the components they used! "Crankshaft A is 20%
cheaper than Crankshaft B? Well, who cares, everyone knows that
everyone just makes the reliability numbers up anyway. Let's use
Crankshaft A." Arg!




Well, there's no doubt that having an MTBF number with each engine would
be nice to know, but I doubt you're going to get enough sampling from this
group to even begin to eliminate noise. So, until someone is able to
create a meaningful MTBF number for us, the TBO is the best we have.
Which means, exactly what it says. It says, after x-number of hours, you
should be considering an overhaul of the engine. I don't believe it
speaks to anything else. In other words, one can assume it means, should
you actually reach TBO without requiring an overhaul, at x-hours, you
should be considering it.

On Mon, 19 Apr 2004 23:52:14 +0000, EDR wrote:

In article , O. Sami Saydjari
wrote:

I do not mean to beat a dead horse, but I really think it would be a
good idea, from an engineering perspective, to collect and report this data.

Yes, but unless you know how the engine was consistantly operated, you
have no understanding of why one engine goes 2000 hours and another
only 150 hours.

I understand what you're saying. Just the same, I believe with enough
samples, you could get a meaningful MTBF for non-utility and utility
categories. Furthermore, given enough samples for each category and using
the mean (not the average -- though ATBF may be interesting as well), as
it should provide a fair idea of the MTBF is for a given engine.

If one makes some assumptions, you can reasonably assume the MTBF is going
to be less than or equal to the TBO for a given engine. What percentage
of people run beyond TBO? Would it be fair to include these samples in a
MTBF set? After all, according to the manufacturer, you're already on
borrowed time when running beyond TBO.

In other words, one can assume it [TBO] means, should
you actually reach TBO without requiring an overhaul, at x-hours, you
should be considering it.


Well, OK, it sure would be nice if the industry defined TBO similarly to
the words you use above. Something like: "Warning: In the rare and
miraculous event that your engine actually reaches this number that we
pulled from some random bodily orifice, we sure will be surprised...oh,
and, unless your life insurance is up to date and you seek a big pay-off
to your beneficiaries, go do an overhaul."

OK. Just so you know, I am tongue in cheek here. I do know that some
engines make it to, and even past TBO. I just find it interesting that,
on average, mine made less than 50% of advertized TBO.

Yes, I do realize that a sampling from this newsgroup would not be
scientific, but I do think it would be interesting to share experiences.
Maybe everyone else is getting 90-110% TBOs and this is anomaly. That
would be nice to know.

-Sami

On Mon, 19 Apr 2004 21:40:27 -0500, O. Sami Saydjari wrote:

Yes, I do realize that a sampling from this newsgroup would not be
scientific, but I do think it would be interesting to share experiences.
Maybe everyone else is getting 90-110% TBOs and this is anomaly. That
would be nice to know.


Fair enough.

On Mon, 19 Apr 2004 21:40:27 -0500, "O. Sami Saydjari"
wrote:

snip

Yes, I do realize that a sampling from this newsgroup would not be
scientific, but I do think it would be interesting to share experiences.
Maybe everyone else is getting 90-110% TBOs and this is anomaly. That
would be nice to know.

Have personally seen 8 or 9 "make it" to from o-haul to factory TBO.
Were installed in T-Arrow's, T-Dakota's and a couple of Senecas. Only
one that didn't make it was due to massive internal corrosion
(discovered during a mid-life crankcase repair).

Aside from day-to-day operation, frequency of use/proper storage
during periods of non-use is a big factor.

TC

O. Sami Saydjari wrote:
I talked to TCM today to register that I am the new owner of one of
their engines. Just in passing, I mentioned that their engine failed
recently within a few hundred hours after major overhaul. They seemed
completely uninterested in knowing this fact. I asked if they kept
actual statistics on actual dependability of their engines. She said
that they did not, to the best of her knowledge. That seems quite odd.

Where do they get TBO numbers from. I always assumed there was some
serious historical statistical date to back these up. Does anyone keep
these statistics? Perhaps A&Ps report such failures? Overhaul shops
maybe? I sure hope someone is tracking the information.

-Sami
N2057M, Piper Turbo Arrow III

See http://www.avweb.com/news/columns/187037-1.html for a recent article
on TBO.

--
Jim Pennino

Remove -spam-sux to reply.

On Tue, 20 Apr 2004 03:38:01 +0000, jimp wrote:

O. Sami Saydjari wrote:
I talked to TCM today to register that I am the new owner of one of
their engines. Just in passing, I mentioned that their engine failed
recently within a few hundred hours after major overhaul. They seemed
completely uninterested in knowing this fact. I asked if they kept
actual statistics on actual dependability of their engines. She said
that they did not, to the best of her knowledge. That seems quite odd.

Where do they get TBO numbers from. I always assumed there was some
serious historical statistical date to back these up. Does anyone keep
these statistics? Perhaps A&Ps report such failures? Overhaul shops
maybe? I sure hope someone is tracking the information.

-Sami
N2057M, Piper Turbo Arrow III

See http://www.avweb.com/news/columns/187037-1.html for a recent article
on TBO.


Great link! Thanks!

"BTIZ" wrote in
news:kHZgc.13759$432.5418@fed1read01:

did they do the overhaul?

if not.. then why should they care about the failure... it goes back
to the shop that did the overhaul

U assure you they have no apparent interest in the ones they do
themselves either. Our field has at least three TCM factory reman
engines in various planes, all of which broke in with the classic TCM
failure mode (zero oil usage), and all of which show the classic failure
pattern (starting to use oil at 400 hours, followed by compression
drop).

Both owners and shop have tried to get the local TCM rep to even come
out and look at the problem. Phone calls go unreturned. Letters go
unanswered. Compresses on all these engines are effectively 0/80, but
if you play with the prop long enough you can get compression readings
up in the 60's and 70's. The one time I managed to talk to the rep, his
response was "Well, hell. If you can get it to pass the annual, why do
you care?"

After four years the final response from TCM was to issue a new Advisory
Circular defining what counts as airworthy. Now readings as low as
26/80, with air hissing past both rings AND exhaust valves is perfectly
satisfactory, even for a brand new engine.

Sorry, but if Superior made jugs for my engine I would have chosen them
in a heartbeat.

-----------------------------------------------
James M. Knox
TriSoft ph 512-385-0316
1109-A Shady Lane fax 512-366-4331
Austin, Tx 78721
-----------------------------------------------

"O. Sami Saydjari" wrote
Where do they get TBO numbers from. I always assumed there was some
serious historical statistical date to back these up. Does anyone keep
these statistics? Perhaps A&Ps report such failures? Overhaul shops
maybe? I sure hope someone is tracking the information.

First off, nobody is tracking this information. The engine
manufacturers don't want it tracked - the statistics would reveal how
unreliable these engines really are. TBO's are usually about 2000
hours, but I don't actually know anyone with more than 2000 hours of
piston GA experience who has not had an engine failure.

Second, the TBO numbers are not MTBF numbers. In theory, they are
based on estimated corrosion and wear of critical components. For
example, improved lubricants have had a massive effect on TBO's
without the engine manufacturers doing anything. TBO is not how long
the engine is expected to go without failing catastrophically, but how
long it's expected to go before some components are no longer within
service limits. This can cause the oil consumption to be excessive,
rated power not to be delivered, etc.

Third, catastrophic engine failure is not generally the result of the
bottom end (camshaft, crankshaft, case) though it can happen. When it
does, it generally occurs earlier rather than later, unless of course
something ugly happens (like a prop strike). Usually such failures
are in the jugs (valves are most common, though I have seen engines
literally lose a jug) or the accessories.

Fourth, the serious engineering talent at places like TCM and Lycoming
has been gone for a long time. They don't offer top pay, interesting
work, or a minimally bureaucratic work environment, so they don't
attract the top engineering talent. Their TBO's are mostly a guess,
and a marketing rather than an engineering guess at that.

Fifth, the important TBO is not hourly but calendar. I don't often
see engines go much past the calendar TBO (usually 12-15 years)
without significant top end work. I've seen lots go past hourly TBO
when flown lots - as much as 3000 hours when flown 500 hours a year.

Michael

Just a tidbit - I purchased a 1969 Arrow (Lycoming IO360) a few years
ago.

There's a logbook entry at 100 hours that the engine was replaced. No
stated reason was given, nor are there any airframe entries to suggest
an accident.

TBO is not anything like MTBF. It's the manufacturer's estimate
assuming ideal operating conditions. In general, this means very
regular use with good control of temperatures. The Turbo Arrow has a
history of problems because of heat buildup.