Operating an engine on condition, is it safe?

[dazednconfused]

I fly at a flight school and regularly use a 172 with an engine on condition, where the oil usage is monitored, i assume, before deciding it needs an overhaul. I'm wondering if this is common and more importantly safe? Are there not other components that should be replaced at a certain interval regardless of what the oil usage and compression is? I have no background in this topic whatsoever. Thank you for any feedback.

[Colonel Sanders]

"On condition" is predicated upon TBO (Time Between Overhaul) which is established by the engine manufacturer as a purely statistical exercise.

TBO is not only determined by flight hours, but also by calendar. After 12 years, officially your engine is past TBO, even if it only has 100TTSN on it.

In fact, what kills most privately-owned aircraft is lack of use leading to internal corrosion. They almost never make TBO.

Frequently-flown aircraft can often go well past TBO. A long time ago, I flew a little twin that had one engine 5500 SMOH, which is about 3x TBO. They just kept changing accessories and jugs. The bottom end was still fine because it flew frequently (no corrosion), they had no cold starts, etc.

Another engine was removed from an air combat airplane at 2600 SNEW. Conventional wisdom would tell you that flying an engine hard would wear it out sooner. Wrong. This engine was flown very hard - wide open throttle nearly all the time - and it was running fine at 2600 hours when they got nervous and pulled it for overhaul. Every since internal part of that engine was still within service limits, which is what you get AFTER a field overhaul.

Let's say you take a brand new engine and fly it 100 hours in the first year, then park it without inhibiting it for 2 years. Well, at 3 years and 100 SNEW, it's junk - it's internally corroded out, and it's nowhere near TBO.

TBO is a statistical value. Statistics lie. My favorite statistic is that a man with his head in the freezer and feet in the stove is, on average comfortable.

[cgzro]

yes. properly monitored there should be lots of time to spot problems.
infact new engines are sometimes more trouble as are low time infrequently run engines.

its likely the only 'old' parts are the crank case cam and bottom end stuff. All the top end has likely been rebuild once anyway as have the other do dads.

I was recently flying a brand new overhauled engine (1 hr since major) and it blew 2 gallons of oil overboard through an incorrectly sealed case.. soooo

Anyway airplanes always demand vigilance and careful preflights can eliminate most problems early .. old or new.

[Colonel Sanders]

Absolutely! Stuff that has just been serviced is
the most unreliable. The more serious the maintenance,
the more likely you will have serious problems with it.

Decades ago, the military used to require maintenance
test flights. Maybe they still do, who knows. Often they
could get very exciting. Pilots were specially designated
for this duty. They learned to look for tools left in the cockpit
which would jam flight controls, etc.

[Cyclenut]

Dazed,
"On Condition" means more than just dropping the oils. If you review AN B041, it gives you a better idea what's 'supposed' to happen to an engine in the on-condition program. Deteriorating performance should show up before the 'fly to failure' thing.

Also, in Canada, there's no "12 year" thing for engine life for a private operator. I think the U.S. still have a 12 year calendar life for private piston powered aircraft, but not sure. CAR 625 Appendix C states:

"Information Note: No hard time, including calendar time, between overhauls need be observed in the case of small aircraft reciprocating engines in non-commercial private operation."

[Colonel Sanders]

Uh ... he's flying at a flight school, so TBO - both
flight and calendar time - certainly does apply to
his commercially-registered aircraft in Canada!

I think the U.S. still have a 12 year calendar
life for private piston powered aircraft, but not sure

Umm ... they never did. For part 91 ops, engine
manufacturer's TBO - both flight time and calendar -
does not apply.

Heck, you don't have to overhaul constant-speed
props under part 91!

[Cyclenut]

Uh ... he's flying at a flight school, so TBO - both flight and calendar time - certainly does apply to his commercially-registered aircraft in Canada!

Colonel, my humble apologies. My reference to "private" was mis-placed and not apropriate to an FTU question. Guess I should read more before opening mouth.

BUT,
from his original post, DAZED stated this engine was operated "on condition". If this is the case, the Maintenance Schedule Approval will show NO hard time OR calendar time. It will show "On Condition". (But still not Fly to Failure, as some think)

If the engine was NOT on condition and still commercially operated, then I agree TBO time certainly applies (plus whatever tolerance the MSA allowed, if any) but I believe the "12 year thing" is only via an SB from the manufacturer which means it isn't mandatory, at least not in Canada. (not sure about U.S.) If I recall correctly (and my memory's not as good as it once was), both Lycoming and Continental tried to impose this limitation by way of SB's only , and that doesn't impose an airworthiness limitation, even for a comercially operated engine. ("Manufacturer's recommendations" still doesn't require an operator to comply with SB's) I'd be interested to see if this could be tied into something concrete, 'cause that would help clear things up!

Hey man, just trying to keep things lively!

[Bede]

Col,

Correct me if I'm wrong, but technically the TBO for a commercial operator is set by the maintenance schedule, not the manufacturer's recommendations. It's just that TC approves maintenance schedules that are often the same as the manufacturer's recommendations.

[Colonel Sanders]

It's just that TC approves maintenance schedules that are often the same as the manufacturer's recommendations

That's kind of a lawyer's answer ... if you start up an FTU and ask for a TBO of 20,000 hours for your Lycomings, I will be able to hear the laughter from the TC office from here.

Yes, you can run engines "on condition" on an FTU OC. Is it worth the hassle and the extra expense of doing oil analysis every 50 hours since overhaul, as I was told by TC I would have to do? I didn't think so, so in my six year MCM journey, I left the TBO at the manufacturer's. If I asked for something else, I could see it taking eight or ten or twelve years to get the MCM approved. With no AMO.

[Bede]

Col.,

You're right it's a lawyers answer. When I did PRM duties I successfully increased the intervals for a few items above the manufacturer's recommendations. I was almost able to get the control cables, aircraft structure, etc. on the 172 go from 200hr to 400hr. but at the last minute, our PMI balked and said we couldn't do it. It would have saved us a lot of time ripping out the interior every 200 hr.

[Colonel Sanders]

the 172 go from 200hr to 400hr

I know exactly what you're talking about.

Some FTU's do the Cessna 50/100/50/200 cycle - several
times a year, which is insane. There is no need to rip out
the interior 5 or 6 times a year at the 200hr inspection.

What many flight schools do is 50/100/50/100 etc and
only do one or two full teardowns per year. I suspect this will
come as a shock to some TC types whom are unaware what
is happening in other regions.