Tach Time?
- Thread starter So ILL
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JeppUpdater
Well-Known Member
Assuming you're talking about a piston engine:
The Hobbs typically runs anytime the engine is running, operated by an oil pressure switch. Occasionally they'll be wired directly to the main bus and come on when power is applied to the aircraft, whether the engine is running or not.
The tach time is generated using the tach signal, which means it's directly affected by engine RPM. The only time one Hobbs hour/one real hour equals one tach hour is when the engine is at redline. This is useful for maintenance as it recognizes different stress levels on the engine. It's also useful as a renter when you can pull the power back and rack up less time.
The time is NOT necessarily the time on the engine. Depending on engine overhauls and gauge replacements it could be any value. Maintenance can look at the current time shown in the airplane and convert that to engine time for maintenance purposes.
When you get into larger and turbine aircraft the tach time goes away and the Hobbs only runs in the air via an airspeed or weight-on-wheels switch.
The Hobbs typically runs anytime the engine is running, operated by an oil pressure switch. Occasionally they'll be wired directly to the main bus and come on when power is applied to the aircraft, whether the engine is running or not.
The tach time is generated using the tach signal, which means it's directly affected by engine RPM. The only time one Hobbs hour/one real hour equals one tach hour is when the engine is at redline. This is useful for maintenance as it recognizes different stress levels on the engine. It's also useful as a renter when you can pull the power back and rack up less time.
The time is NOT necessarily the time on the engine. Depending on engine overhauls and gauge replacements it could be any value. Maintenance can look at the current time shown in the airplane and convert that to engine time for maintenance purposes.
When you get into larger and turbine aircraft the tach time goes away and the Hobbs only runs in the air via an airspeed or weight-on-wheels switch.
JeppUpdater
Well-Known Member
As a general rule no, it would not. As I said, maintenance could read X tach time in the airplane and coincide that with Y engine time.
Say for example your 150. It's a trainer and has 8100 hours on the tach. The TBO on the O-200 is 2000 hours from what I recall. So at minimum, it's had four overhauls in it's life before you're flying it (well...hopefully). The tach could say 8100 hours, but that doesn't mean the engine has 8100 hours on it, and certainly doesn't mean it has that much since overhaul.
With any engine you have multiple times:
Time since new (since the engine itself was actually manufactured, aka zero time).
Time since major overhaul (a complete overhaul).
Depending on the engine and inspection results it could also have had a top overhaul, bottom overhaul, a time since factory reman, and if a turbine, a time since hot inspection.
The tach is just a gauge. The only purpose of the tach time is to record time that is specific to the engine and RPM-dependent. It has zero definitive correlation with any particular time on the engine other than when the airplane is brand new. Eventually the gauge will die, the engine will require overhaul, or the engine may be swapped to another airplane. All those will change the value and/or it's relationship with the engine installed.
In those case, you would see in the logbook "installed overhauled/remanufactured engine at tach time X,XXX.X" or similar.
Here's an example courtesy of controller.com: http://www.controller.com/listingsd...ROKEE-140/1964-PIPER-CHEROKEE-140/1207785.htm
Airframe hours: 6970
Engine hours: 2665 since major overhaul (SMOH)
Tach hours: 0878.7 I believe
z987k
Well-Known Member
Under part 91. But yes, as long as it's not making metal and the compressions are good, keep flying it. Most piston airplane engines are very lower power and very low stress, they'll run forever.
Blackhawk
Well-Known Member
And also under part 135 if approved.
Roger Roger
Bottom of the list
There are very few piston engines I can visualize wanting to run much further than TBO. The lyc 540k series would be a good candidate, TCM just upped TBO on frequently flown factory IO520s to 2100 hours and I feel that's too long. Actually I feel that any hours is too long for that catastrophe of an engine design that for some reason is one of the most popular in the state.
z987k
Well-Known Member
You'll need to look at the logbooks.
Blackhawk
Well-Known Member
Ah, the old "it's not safe to go beyond TBO" old wives tale. As with most OWT, no historic or factual basis.
http://www.avweb.com/news/savvyaviator/187037-1.html
Roger Roger
Bottom of the list
I make my living managing a fleet of big piston engines. I've seen enough to know that many of them shouldn't go beyond TBO. Like I said there's a few out there that are good candidates but the big bore contis should just go straight in the trash at TBO. Or when they come out of the factory.
O320s on the other hand? As long as they're flown regularly, run em forever.
z987k
Well-Known Member
Well there has to be something to it or it wouldn't be regulatory for 135/121.
z987k
Well-Known Member
QFT.
Oh and by big bore, you mean anything bigger than the O-200 right? Although I've had good luck with the O-470, in general I think I'd rather run a franklin than a continental.
Roger Roger
Bottom of the list
I'm not real familiar with the 470s, they seem to have an ok record on the 182 etc, but the 520F in particular is my nemesis. Knock on wood I've had better luck with the 550F but still...I know...let's put pushrods, intake, and exhaust all on the same side of the cylinder, then design them so that cylinder problems are endemic to the species, and finally mount it in an airplane with no removable lower cowl...
Then there's starter adapters, finicky fuel systems, and did I mention cylinder problems? And lack of accessibility of anything?
Especially fun when #6 goes....try dynamite or a backhoe. I've had the headache of being involved in one banana engine swap. That's a life experience I care to never repeat. Fwiw I've not seen a 520 hit tbo in the few years I spent around a shop. And the bastards load up and shut off once their warm and you idle them. Pos, not sure what bonanza was thinking.
Blackhawk
Well-Known Member
Actually it is possible for 135/121 to exceed TBO. That is another OWT. Since you did not bother reading my link I'll quote it for you:
"Not so. Both Lycoming and TCM publish engine TBOs in the form of non-mandatory service bulletins. Some Part 121/135 operators have Operations Specifications that require them to comply with all manufacturer's service bulletins (even non-mandatory ones), while others have Op Specs that require compliance only with mandatory service bulletins. Those in the latter group are no more obligated to comply with published TBO than are Part 91 operators. Those in the former group might theoretically be required to overhaul at published TBO, but most such operators request TBO extensions from their FSDO and these are routinely granted, often for as much as 50% over the engine manufacturer's published TBO. So, in actual practice, published TBO is hardly ever compulsory for any operators -- commercial or non-commercial."
Blackhawk
Well-Known Member
Another Mike Bush article on the subject. Since most here don't seem to bother reading the links I'll paraphrase.
"Dr. Ulrich analyzed five years’ worth of NTSB accident data for the period 2001-2005 inclusive, examining all accidents involving small piston-powered airplanes (under 12,500 lbs. gross weight) for which the NTSB identified “engine failure” as either the probable cause or a contributing factor. From this population of accidents, Dr. Ulrich eliminated those involving air-race and agricultural-application aircraft. Then he analyzed the relationship between the frequency of engine-failure accidents and the number of hours on the engine since it was last built, rebuilt or overhauled. He did a similar analysis based on the calendar age of the engine since it was last built, rebuilt or overhauled. The following histograms show the results of his study:"
If you go to the link you can see the chart. Basically, it says the following:
While much has to do with how you operate and maintain an engine, the most likely time for an engine to fail is the first 500 hours. This likelihood decreases steadily until the 3000 hour point when it again begins to increase.
The same is true for time in service. An engine is most likely to fail at the 1-4 year mark. This decreases over the years. At the 13-16 year mark there is a slight uptick, but then the numbers go down again until the 25 year mark.
"
What Dr. Ulrich’s research demonstrates unequivocally is striking and disturbing frequency of “infant-mortality” engine-failure accidents during the first few years and first few hundred hours after an engine is built, rebuilt or overhauled. Ulrich’s findings makes it indisputably clear that by far the most likely time for you to fall out of the sky due to a catastrophic engine failure is when the engine is young, not when it’s old.
(The next most likely time for you to fall out of the sky is shortly after invasive engine maintenance in the field, particularly cylinder replacement, but that’s a subject for a future blog post…stay tuned!)"
http://blog.aopa.org/opinionleaders/2014/03/13/do-tbos-make-sense/
"Dr. Ulrich analyzed five years’ worth of NTSB accident data for the period 2001-2005 inclusive, examining all accidents involving small piston-powered airplanes (under 12,500 lbs. gross weight) for which the NTSB identified “engine failure” as either the probable cause or a contributing factor. From this population of accidents, Dr. Ulrich eliminated those involving air-race and agricultural-application aircraft. Then he analyzed the relationship between the frequency of engine-failure accidents and the number of hours on the engine since it was last built, rebuilt or overhauled. He did a similar analysis based on the calendar age of the engine since it was last built, rebuilt or overhauled. The following histograms show the results of his study:"
If you go to the link you can see the chart. Basically, it says the following:
While much has to do with how you operate and maintain an engine, the most likely time for an engine to fail is the first 500 hours. This likelihood decreases steadily until the 3000 hour point when it again begins to increase.
The same is true for time in service. An engine is most likely to fail at the 1-4 year mark. This decreases over the years. At the 13-16 year mark there is a slight uptick, but then the numbers go down again until the 25 year mark.
"
What Dr. Ulrich’s research demonstrates unequivocally is striking and disturbing frequency of “infant-mortality” engine-failure accidents during the first few years and first few hundred hours after an engine is built, rebuilt or overhauled. Ulrich’s findings makes it indisputably clear that by far the most likely time for you to fall out of the sky due to a catastrophic engine failure is when the engine is young, not when it’s old.
(The next most likely time for you to fall out of the sky is shortly after invasive engine maintenance in the field, particularly cylinder replacement, but that’s a subject for a future blog post…stay tuned!)"
http://blog.aopa.org/opinionleaders/2014/03/13/do-tbos-make-sense/
Roger Roger
Bottom of the list
Part of this is blatantly false. It's not a matter of whether you have service bulletin compliance written into your ops specs, OpSpec D101 requires a list of TBO limited items and their intervals, and at least at my FSDO they are very reluctant to grant anything other than what the manufacturer has published.
I've read all mike's stuff, and while it sounds great in theory, the things I've seen in the air taxi biz make me a little skeptical of his "never overhaul anything" claims. Maybe it has to do with the absurd amount of cycles per hour we run.
Stuff like multiple cracked crankcases, spun and loose small end rod bushings, leaks leaks leaks galore, bad cams (almost every time a lycoming gets torn down), bad counterweight bushings leading to throwing an alternator belt every couple dozen hours, etc.
Also I think some engines are better cut out for long life than others. Though even on my relatively bulletproof little O320 there were enough issues when I tore it down that I was glad I did (case fretting, bad cam, worn bearings)
SlumTodd_Millionaire
Most Hated Member
While there's certainly nothing unsafe about flying an engine past TBO as long as it's still producing good compression and not showing signs of internal wear, it's pretty unlikely that your engine will make it much past TBO before those problems start coming up. And when they do, it's time to overhaul, especially on a piston single. You don't mess around with a single engine airplane showing signs of engine wear.
In reality, most people fly their airplanes so irregularly and know so little about how to operate the engine to keep it in top shape that most of them don't even make it to TBO. In order to get past TBO, you really need to be flying it weekly and operating it for peak cooling. If you're not, you're probably going to be overhauling sometime between 1600-2000 hours.
In reality, most people fly their airplanes so irregularly and know so little about how to operate the engine to keep it in top shape that most of them don't even make it to TBO. In order to get past TBO, you really need to be flying it weekly and operating it for peak cooling. If you're not, you're probably going to be overhauling sometime between 1600-2000 hours.