Is there a second engine hour meter?

Discussion in 'Flight Following' started by Chesterspal, May 14, 2019.

  1. Chesterspal

    Chesterspal Pre-Flight

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    The mechanic doing the pre-buy on the C150 I'm looking at says the time on that engine is 1850 hours. The hour meter in the cockpit, that I saw, said 1680 hours.

    Is there another hour meter and where is it and which is correct?
     
  2. Checkout_my_Six

    Checkout_my_Six Final Approach

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    not usually....but.
     
  3. Ryanb

    Ryanb Final Approach PoA Supporter

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    What hour meter in the cockpit were you looking at? The Hobbs is not related to engine hours and the Tachometer isn’t either. You’d have to look in the logs and see how many hours were on the tachometer when the engine was installed.

    Trust your A&P.
     
  4. chemgeek

    chemgeek Line Up and Wait

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    The tach has an hour meter, and there may also be a Hobbs meter. Engine time may not match the meter time, depending on when the engine was replaced or if it was not zero time when installed. Have to read the log books.
     
  5. Chesterspal

    Chesterspal Pre-Flight

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    I do but now I'm questioning the seller. Guy owns four planes, is selling two. Been flying for maybe 30 years so he should know what the engine time is or how to compute it.

    Mechanic says engine was "replaced" in 1994 and log entires prior are missing. Starts with this engine in 1994.

    Meter is DATCON on right side of panel, if that tells you something.

    Confused by what you say in red. If engine WAS replaced, as it was, would not the time be less than indicated as there were hours on the meter already from first engine?

    So does the Tachometer show engine time "off the ground" or just what does it indicate?

    Second meter, round one, indicating RPM reads 5785 3 at bottom. Assume this is TT on airframe, correct?
     
    Last edited: May 14, 2019
  6. texasclouds

    texasclouds Line Up and Wait

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    e n g i n e l o g b o o k s
     
  7. EdFred

    EdFred Touchdown! Greaser! PoA Supporter

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    My tach didn't match the time in the logbooks. Tach was probably replaced at some point. Or maybe when the engine was swapped years back.
     
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  8. MauleSkinner

    MauleSkinner Final Approach

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    Where does it say it’s “supposed to” show time off the ground?
     
  9. Capt. Geoffrey Thorpe

    Capt. Geoffrey Thorpe Touchdown! Greaser! PoA Supporter

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    The recording tach is actually an engine revolutions counter that displays "hours" instead of total revolutions.
    If you take the number of "hours" and multiply by something in the ball park of 150,000, it will tell you how many times the engine has gone around since the tach was installed. Given an accurate number to replace the aforementioned 150,000 you can get an accurate measure of engine revolutions You do not get an accurate measure of actual accumulated clock time.
    However, typically one uses the number on the tach as an indicator of time just because that's the way it's done.
    A "hobbs" (or in your case DATCON) type meter typically records the time that it has been turned on. How it is turned on depends on how it is wired. How it is wired depends on who wired it. So, it gives an accurate measure of clock time based on something. But one would have to dig around to find out what the something is. And, even if it is, say, connected to an oil pressure switch so that it gives an accurate clock time indication of how long the engine has been running, your A&P is likely to ignore it in favor of the tach time.
     
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  10. Chesterspal

    Chesterspal Pre-Flight

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    So, how does the mechanic (yes we will talk tomorrow on this... but) come up with the time on the engine "in air".

    Would there be a notation in the new logs that state the hours at that point according to some meter... perhaps the one on the round RPM gauge that now reads 5784, and use that for hours since installed?
     
  11. Capt. Geoffrey Thorpe

    Capt. Geoffrey Thorpe Touchdown! Greaser! PoA Supporter

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    He/she/it doesn't come up with anything related to actual time in the air.
     
  12. Chesterspal

    Chesterspal Pre-Flight

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    Sorry if I sound confused... 'cause I totally am.

    My reading tells me the engine hours meter only records the time "off the ground" by some type of pressure switch in the nose wheel. Is this wrong?

    But you folks say there is no such thing as an engine hours meter on a C150 so how can it be both ways?
     
  13. chemgeek

    chemgeek Line Up and Wait

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    The "Datcon" is a Hobbs meter. It starts recording time as soon as the engine oil pressure comes up. It would read zero when it was installed. The Hobbs is usually used to record "flight time" time for instruction. The Tachometer records "time" based on a predetermined revolution rate. In my plane I think it is set for 2300 rpm. It may or may not have read zero when the engine was installed or the tach replaced. Tach time is typically used in the engine logs. The total engine time should be meticulously recorded in the log book based on differences between tach readings since the last maintenance. Airframe time will be similarly recorded separately.

    Engine running = time accumulated.
     
  14. Eric Stoltz

    Eric Stoltz Line Up and Wait

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    Beatme towit. I have noticed on my aeroplane that the tach is usually .1 off from the hobbs per hour. I know the hobbs starts as soon as the engine starts by oil pressure switch and runs exactly by the second. Keep in mind that the tach time is only accurate at a specific rpm, like cruise rpm. When the tach is running slow, so does the meter in the tach. Spend a lot of time on the ground or in the patten (low rpm), that specific flight will have less tach time than hobbs time. My plane Currently has 5 hours less tach time than hobbs for the year. I log time in my pilot log book with the hobbs. I record airframe and engine time with the tach time.

    To compute engine time, find out what the tach says in 1994 when the engine was "new." Take today's tach time and subtract the tach time from the 1994 value and voila, there's the engine time. If the tach has been swapped out since 1994, there's more math involved, but use the same formula.

    Does that help?
     
  15. frfly172

    frfly172 Touchdown! Greaser! PoA Supporter

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    Hobbs and tach should read differently. For engine overhaul ,I would be using tach time.
     
  16. Chesterspal

    Chesterspal Pre-Flight

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    The tach meter now reads around 1680. The mechanic says engine time is 1850 so how can I subtract when his time is 200 hours more give or take?

    Just trying to figure out where he is getting those extra 200 hours from? No engine logs prior to 1994 engine replacement.
     
  17. cgrab

    cgrab Pattern Altitude

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    Looks like simple dyslexia to me.
     
  18. Chesterspal

    Chesterspal Pre-Flight

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    This is not the exact article I read, but says about the same thing.


    "Many aircraft, especially rental aircraft, have two hour meters (although you will probably see only one). Most pilots are familiar with the Hobbs meter, which is turned on and off by an oil-pressure switch or the master switch, depending on the airplane. Pilots use this meter to count their flight time and pay their rental fee.

    A squat switch, which is generally located on a main landing gear leg, activates the second hour meter. This hour meter runs when it senses that there's no weight on the landing gear, then accurately records maintenance time, which is used for inspections, scheduled maintenance, and tracking engine, prop, and accessory time.

    If an aircraft doesn't have this timer, it usually has an hour meter built into the tachometer (the aircraft may have both). This meter is designed to function accurately only within the rpm range of the engine at cruise power, so it will read low whenever the engine runs slower than cruise rpm."

    I assume this is what you were trying to tell me about the tach hours meter. So, that meter is reading 1680 (when you factor the lower revolutions) but the mechanic says in reality is is 1850.

    How does he get those extra 200 odd hours... how does he know how much time was at cruise and how much was not cruise time?

    Perhaps cgrab can answer that for us?
     
    Last edited: May 14, 2019
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  19. simtech

    simtech En-Route PoA Supporter

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    Was this a factory new engine or a zero time engine in 1994? I beleive new engines get a new log book so maybe thats why you are missing entries prior to that...?

    The hobbs meter you dont even need and usually runs off an oil pressure switch so its not consitient with true engine time. Tach time is what the log books will go by and unless the tach was changed when the new engine went in it wont be the same..Or if somewhere after the engine change the tach was replaced it obviously wont match engine time. The tach change will be in the airframe logbook as well and should indicate at what hour reading it was changed.

    Basically tach time will not reflect engine time unless the tach was changed when the engine was installed new AND the tach was never changed after the new engine was installed.
     
  20. EdFred

    EdFred Touchdown! Greaser! PoA Supporter

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    If you drive your car for 85000 miles and then replace the engine in your car with another engine that had 65,000 miles on it what does the odometer say?

    Why would you expect it to magically change when you put a new engine in it?
     
  21. Eric Stoltz

    Eric Stoltz Line Up and Wait

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    Makes me think that the Mechanic is using hobbs time to determine engine time?? 200 hours is about 10% difference, which looks about right for a tach/hobbs difference.
     
  22. Bell206

    Bell206 Pattern Altitude

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    The "official" time of an aircraft or engine or propeller or whatever is the time recorded in the logbook. Period. There are number of devices that can be used to record that that time in service (TIS). These include tachometers, hobbs meters and even a pilots watch--each with their own pros/cons. If the logbooks are not complete there is certain FAA guidance that provides a way to recreate those time/hour records. Have your mechanic show you how he determined the engines total time from the log book.
     
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  23. mondtster

    mondtster En-Route

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    People are getting awfully hung up with what little numbers on a gauge in the dash mean. There is no regulation I am aware of that requires any sort of time recording device in the first place, nor that that time recording device provide any meaningful number.

    One airplane I work on has logged over 7,000 hours and has never had a Hobbs or a recording tach in it. Times are all done by the pilot and the accuracy of their watch.
     
  24. chemgeek

    chemgeek Line Up and Wait

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    There is no "extra" 200 hours. The tach has 1680 hours on it since it was installed. That's it. What it indicates is not necessarily related to how many hours are logged on the engine. When the engine was installed, the AI will have noted the tach time in the engine log and notated that and the time on the installed engine (perhaps zero SMOH). At each maintenance interval after that requiring an engine log entry, the AI will notate the updated tach and engine time. IF the tach gets changed out, that will be notated, too, and the tach and corresponding engine and airframe total times appropriately notated.

    In my plane the tach reads something like 2800, the Hobbs something like 1600, and the engine has about 100 on it. (The airframe time different still.) You can only get the engine time by reading the logbooks. I'm on my third engine since new, and at least the second tach.

    ENGINE TIME IS IN THE LOG BOOK.
     
  25. luvflyin

    luvflyin Final Approach

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    Maybe a new tach was installed in the plane after the engine was. Maybe the tach was calibrated at the wrong RPM to yield one hour for the number it should have been for that engine. The average cruise RPM. Here's an example of how different tachs are calibrated differently

    https://www.aircraftspruce.com/catalog/inpages/mech_tachs.php
     
  26. Capt. Geoffrey Thorpe

    Capt. Geoffrey Thorpe Touchdown! Greaser! PoA Supporter

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    Not on a 150.

    You would have to ask the A&P where he/she/it got the 1850 - recording tach, hour meter, Log books, warm dark place, etc. No way for us to know.
     
  27. Chesterspal

    Chesterspal Pre-Flight

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    I will speak with the mechanic tomorrow and will get to the bottom of all this.

    The engine time IS important since the seller stated one thing and the mechanic is telling a different story.

    Going into this purchase I had around 120 hours and perhaps 2 years before TBO (1800 hours on a Continental 0-200A) and now I'm over TBO by 50 hours. Seems important enough to want to find out what's the truth, does it not?

    I think it does and I'm the one paying for it and will be flying in it.
     
  28. pigpenracing

    pigpenracing Pattern Altitude

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    Pretty simple... Was the tach time 0 when the engine was replaced? What was it?
    Was the tach replaced some other time? Read the logs.
     
  29. MauleSkinner

    MauleSkinner Final Approach

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    Depends what value you and your mechanic place on "TBO".
     
  30. Rgbeard

    Rgbeard Line Up and Wait

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    Let your A&P explain it. He has the data, we don't.

    And, no, there's not supposed to be any squat switch on the Hobbs of a C150.

    You're buying a run-out 150. Run it until you're not happy with it anymore and then overhaul it. Simple?
     
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  31. Capt. Geoffrey Thorpe

    Capt. Geoffrey Thorpe Touchdown! Greaser! PoA Supporter

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    Not really. There is nothing magic about the TBO number. You might get 0 hours and 0 years, or you might get another 1000 hours and 10 years before it actually needs an overhaul. Engines that run regularly often run well past TBO. Engines that do not run often are less likely to make it to TBO.
    120 hours short or 50 after TBO - no difference in any practical sense.
     
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  32. Ryanb

    Ryanb Final Approach PoA Supporter

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    I sense there’s a big lack of education in regard to aircraft ownership. Might be best to make sure you realize what you’re buying and the inflicting challenges that along go with it.

    You’re scaring me as a potential aircraft owner, based off this thread and a few in the past.
     
  33. Dana

    Dana Line Up and Wait

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    Most planes will have an hourmeter built into the tachometer. As others have said above, the hour recording speed depends on engine rpm. Since a slower turning engine, at least with a fixed pitch prop, is presumably not working as hard, the recorded hours are a good indication of effective time in service.

    A C-150 will usually also have a "Hobbs" (or other brand) hourmeter to record the actual time the engine was running. This number isn't used in the engine logs, but pilots use it to record flight hours. It can also be used to record airframe hours (I guess-- I've never owned a plane with a Hobbs), unless the plane doesn't have one, in which case the tach time may be used.

    If the engine was replaced, it's expected that you wouldn't have engine logs before that, since the logs go with the engine. You should still have the aircraft logs before then... but it's not unusual that engines are overhauled or replaced at the same time a plane changes hands, often after sitting neglected for awhile... where logs sometimes get lost.

    If the tach shows less time than the engine according to the logs, the tach was replaced. That's the case with my plane, and the actual numbers are clear in the logbooks. If the tach shows more time than the engine has, it may not have been replaced when the engine was replaced so is showing time from the old engine.
     
  34. chemgeek

    chemgeek Line Up and Wait

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    With a 1800 hr TBO, you are realistically buying a runout engine. You can certainly run beyond TBO at your own risk, but are looking at a potential OH or major rebuild in the near future depending on the state of the engine and operating history. Take care not to acquire a maintenance pit. A full OH will be a large fraction of your purchase price.
     
  35. Rgbeard

    Rgbeard Line Up and Wait

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  36. SToL

    SToL Line Up and Wait

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    Dang guys, I got so confused reading through this I had to stop...

    Let's make it real simple..

    1. A tach reads engine time. If the engine is running, the tach it running.

    2. An 'engine' hobbs works similar to the Tach in such, if the engine is running, the Hobbs is running. These are usually activated by the oil pressure switch.

    3. A 'flight hobbs only runs when the aircraft gear is not on the ground. These are typically connected via a squat switch in the gear.

    And it sounds like maybe your A&P, or you got the times backwards. The engine hobbs should always show more than the flight hobbs.

    PJ
     
    Last edited: May 15, 2019
  37. Ravioli

    Ravioli Final Approach

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    Not sure about this statement. IF the hobbs has a squat switch, but runs at 1.2hrs per tach hour, I can envision a plane that gets out of the pattern 'more often than not' still having a more hobbs than tach hours.
     
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  38. SToL

    SToL Line Up and Wait

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    That is a malfunctioning hobbs. A hobbs should read at 1.0 hrs per hour. I stand by my statement.

    PJ

    Edited typo... tach changed to hobbs.
     
    Last edited: May 14, 2019
  39. dmspilot

    dmspilot En-Route

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    Your statement is incorrect. Tach hours are proportional to the RPM.
     
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  40. SToL

    SToL Line Up and Wait

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    Opps, typo... that was in response to his statement about the hobbs, that should have said 'hobbs', not tach.
     
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