I am looking to purchase an aircraft, and I've found one described as follows

  • Total time is 6300 hours
  • Engine time since major overhaul 2100 hours
    Engine runs strong and compression is all in the 70s
  • Has VAL Avionics all-in-one VOR/LOC/GS unit

The owner also states that the engine was rebuilt 556 hours ago, but this was never entered in the log books, so we're working based on 2100 hours since overhaul.

We have done a pre-buy inspection through A&P mechanic. On the report was mentioned,

TACH TIME ;8131.18
HOBBS ; 7141.9

Can you explain for me what these time figures mean and how I can judge the remaining life left for this engine and airframe?

  • 4
    $\begingroup$ Why wouldn't you just ask the mechanic for his opinion on the engine life? Having been the only one to actually inspect it, he's the only one qualified to answer. $\endgroup$ Nov 16 '14 at 8:19
  • $\begingroup$ The thing is I dont have direct contact with mechanic.I got only the inspection report.I am a biginer for the aircraft trade.hence I realy need to know the sellers mention of the aircraft detal which I had forwerded to U.thanks, $\endgroup$
    – Fernado
    Nov 16 '14 at 8:49
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    $\begingroup$ @Fernado Are you sure you want to spend that much money on something you don't seem to know much about? Holding back and doing some research sounds like a good idea. $\endgroup$ Nov 16 '14 at 12:39
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    $\begingroup$ TT vs tach vs Hobbs are all about a 1000 hrs different. Engine was rebuilt but wasn't entered in the maintenance logs? What else has not been entered? Don't go near it. $\endgroup$
    – Simon
    Nov 16 '14 at 13:20
  • 1
    $\begingroup$ Tach and Hobbs can often be 10-20% off, especially if Hobbs runs off the master $\endgroup$
    – rbp
    Jan 9 '15 at 23:03

I'm going out a bit on a limb here, and it isn't specifically an answer to exactly what you are asking (which is what Stack Exchange tends to strive for), but I think this is the answer you need, so at the risk of receiving a couple of downvotes, here goes.

engine was rebuilt 556 hours ago but was a problem because was never entered in the log books

I think that boldfaced text makes your entire question moot. In other words, the answer to your specific question doesn't really make any difference; the above tells you all you really need to know. Which is:

As Simon said in a comment, if the seller readily admits to not entering an engine rebuild into the log, then what else has not been entered into the log? Especially if you are a beginner, that's the sort of thing I would expect will come back to bite you hard.

Total time is stated as 6300, Hobbs time is stated as 7142, tach time is stated as 8131. And that's by the seller (including the mechanic's report provided by the seller). Does the difference between these figures inspire confidence in the airplane's state, the seller's honesty, and your ability to get an accurate picture of what you are buying?

As has been stated, the only person qualified to answer your question about the state of the engine in a reasonably objective manner is the mechanic who inspected the engine. The odds that this particular mechanic will stumble across this question is already quite low. What are the odds that the mechanic will not only stumble across the question, but also recognize the particular aircraft you are asking about and decide to answer the question with anything that is not already in the inspection report (which you already have received from the seller)?

More generally, with well-serviced aircraft, it isn't as easy as just saying "this aircraft has X number of hours of service left" even based on the current state of a single piece such as the (an) engine, because with good servicing it's perfectly possible to extend the service life of an aircraft for a very long time. A well-treated aircraft is a very different beast compared to perhaps a car (which might be lucky to get a checkup once a year and an oil change every few thousand miles), of course in part because the failure mode is much more serious. (If your car fails on the highway you pull over and call a tow truck; if an airplane fails in mid-flight then what do you do?)

Personally and for the specific situation you describe, I wouldn't go near that particular aircraft if I can avoid it, let alone buy it. At the very least buy something where the trivially obtainable numbers match.

  • 1
    $\begingroup$ A difference between tach, hobbs, airframe, and engine time is not at all unusual in older aircraft where components have been replaced. Case in point, mine has 4000 airframe hours, and about 425 since the last engine overhaul. The tach reads around 2000 hours (because it was replaced and they didn't run the new tach up to match the airframe time), and if I installed a Hobbs meter today it would start from 0 hours. $\endgroup$
    – voretaq7
    Jan 9 '15 at 22:52
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    $\begingroup$ @voretaq7 I can certainly see the point of your comment, but there is also the "little detail" that the engine rebuild (whatever exactly that means) was never entered into the log. That would be the major killer in my book. $\endgroup$
    – user
    Jan 10 '15 at 12:33
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    $\begingroup$ Agreed - an engine rebuild is something that would have had to be done by the factory and would certainly be in the logs (the engine log would have been reset), so the fact that it's not "in the logs" is a HUGE red flag. $\endgroup$
    – voretaq7
    Jan 11 '15 at 4:56
  • $\begingroup$ @voretaq7 There is also the (perhaps likely given the display name) possibility that English is not the OP's native language, and it really was "just" an engine overhaul done by a local mechanic. Even so, that too should be in the log, so the question posed in the 4th paragraph is valid either way. Hence, even in that case, I still stand by this answer. $\endgroup$
    – user
    Jan 12 '15 at 8:30

I'll echo many of the sentiments that have already been expressed: There's a discrepancy between what the ad describes and what the mechanic's prebuy report describes, which indicates that either the ad is "hastily put together" or intentionally misleading (neither is good). Also the idea that an engine was "rebuilt" but never logged is a huge red flag.

Were I in your situation (and I was not too long ago) I would only consider purchasing this aircraft if you could talk to the inspecting mechanic in person (preferably while seeing the aircraft), and I would ask their opinion on the condition of the airframe, engine, accessories, instruments, and avionics.
In fact that is very good advice no matter how experienced you are at purchasing aircraft: Nothing beats an expert evaluation.

To answer your question about the remaining life on an airframe and engine requires a deeper understanding of the plane you're looking to buy (again, talk to your mechanic about this - I really can't emphasize that enough).

You'd be best served by first understanding what Tach and Hobbs hours mean, and how engine time is described. Armed with that knowledge, let's forge on and figure out what times are important for this particular plane.

First you'll notice that the Tach and Hobbs times in the mechanic's report are different -- that's OK, they're probably never going to be exactly the same. You'll also notice that none of these numbers match the "Time Since Major Overhaul" for the engine -- that's also normal (TSMOH is a derived number - the current time minus the time when the last overhaul was done - so there's almost never a direct representation of it in the cockpit).

You'll also notice the Tach and Hobbs, and Total Time (airframe time) don't match up with the total time for the airframe. That's a bit of a red flag: Normally one of these is what's being recorded as "Total Time - Airframe" (TTAF), and if none of them match either the tach / hobbs meter was replaced at some point and not "run up" to match the airframe time, or there's a error in the ad as I mentioned earlier.
Your mechanic should be able to tell you which situation you're in by a careful examination of the logs, and that should also reveal how many hours are on the engine and airframe.

For the sake of argument I'm going to assume the Tach time above is the Total Time - Airframe (about 8100 hours), and we'll go with 2100 hours since overhaul on the engine because that's what the logs apparently say.

So we've established the Total Time for the airframe (about 8100 hours), and the time since the engine was last overhauled (2100 hours) -- what does that mean? How long will this plane last?

The short answer is there's no way to know just based on hours -- this is why we pay mechanics to do a thorough prebuy inspection. A 20,000 hour airframe that has had excellent maintenance could easily go another 20,000 hours, but a 2,000 hour airframe that has been abused and neglected may be ready to be scrapped.

For the engine, most GA Piston engines have a 2000 hour Time Between Overhaul (TBO) recommended by the manufacturer. Overhauling at this point isn't mandatory for most part 91 operations and well-cared-for engines can continue to operate for hundreds of hours beyond the recommended overhaul point with no problems, but generally if you're looking to buy an airplane an engine that's got 2100 hours on it will be considered "run out". Consider your mechanic's opinion of this engine carefully, and have a plan in your mind as to what you're going to do when the engine needs to be torn down.

For the airframe 10,000 hours is usually considered to be "high time" (often these have been "working airplanes" - flight school trainers, etc.) - This doesn't mean the plane is worn out, but it bears special consideration in the inspection.
Remember that nothing can ever "reset" airframe time the way overhauls or engine replacement can for an engine: The parts on that airframe have their accumulated time (and accumulated fatigue cycles) forever.

A 10,000 hour airframe can continue to give years of excellent service, with one major caveat: Life-Limited Parts. Some aircraft have components which have a maximum service life limit (for example, the wings on a Grumman AA-5 have a 12,000 hour life limit. Once reached the wings are no longer airworthy and must be replaced, and as Grumman is no longer making those planes you'd be hard pressed to find a replacement wing).
In consultation with your mechanic you should consider the time remaining on any life-limited parts as well as the overall time on the airframe and its condition in order to ascertain how many more hours the plane can realistically fly for.

Sometimes the numbers can be mind-boggling: When I was plane shopping I looked at a Cherokee 140 with 10,000 hours on the airframe. It was a flight school trainer, but somehow managed to get through its life with no major damage and had been meticulously maintained by the schools which owned it over the years. My mechanic's opinion was that it could easily fly another 20,000 hours or more if it continued to get the same kind of lavish care, which is more hours than many GA pilots will ever have in their logbooks. (Piper says the spar is good for 30,000 hours per Service Bulletin 886, after which they want it inspected).
Other times the numbers can be soberingly obvious (like Grumman AA-5 with 11,500 hours - obviously a plane in the twilight years of its service life).

So what's the bottom line?
The aircraft you're describing has mid-to-high time airframe by most standards.
The engine is "run out" based on the time since it was last overhauled.

Both could continue to give years of excellent service, or they could fall apart tomorrow: The only way to have confidence either way is to find a mechanic who knows that particular type of aircraft, hire them, and have them go over the plane and logs. Then sit down with that mechanic and go over their findings.
Your mechanic will tell you if the plane is a gem or a lemon, and if they know the aircraft type they've got a better idea what to look for than you, I, or most folks kicking around on the internet would.

  • 2
    $\begingroup$ TL;DR: sit down with the mechanic who inspected the plane and go over their results with them. You'll be glad you did! $\endgroup$
    – voretaq7
    Jan 9 '15 at 23:37

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