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Thread: Tach time to Clock time ratio

  1. #1

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    Tach time to Clock time ratio

    I'm thinking my tack time is way too slow in relation to clock time. If I start the engine (C-80-12), idle for 5 minutes, taxi for 10 minutes at 1000 rpm (float plane), take off and fly around at 2350 rpm, land, and taxi back all in 1 hour actual clock time my tach time will about 0.50 1) Is there a rough guideline as to what the ratio should be--what is normal? 2) What would cause the tach to run slower than "normal"? Thanks, Ron

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    mvivion's Avatar
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    Tach time can be all over the place. My cub's tach is almost perfectly aligned with the stopwatch I start and stop at engine start and shutdown. The numbers you cite are low, but think of it this way: You'll get a lot done between 100 hour inspections......

    Mechanical tachs are often significantly in error, both in total time and, more importantly, in rpm. Legally, a tach can be 100 rpm in error, and just be placarded as such.

    Id borrow or buy a digital tach checker, go fly and verify that the tach is accurate in rpm as a first step. If it's in error in the "wrong" direction, you may not be doing your engine (or your neighbors) any favors. In your case, it old suggest that you may not be making max rpm......

    Start there. I love electronic tachs, personally, for the reason stated above. Digital tachs, however, record time like a Hobbs meter. Now your 100 inspections come closer together......

    MTV

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    Mush's Avatar
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    Tach time is only accurate when when the engine is running at the specific RPM the tachometer is rated at. Say for instance you have 2400 RPM tach. It will only show one hour when you are running the engine for an hour at 2400 RPM. If you ran the engine at say half that (1200 RPMs) for one hour it would only record .5. This is one of the main reasons why rental planes use Hobbs meters. The Hobbs starts when the engine is running and records exactly 1 hour (in tenths) after an hour based on clock time no matter what the RPMs are. The flight school I teach at charges 1.2 times the tach time if the Hobbs meter is inop or not installed.
    Last edited by Mush; 08-30-2016 at 10:03 AM.

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    Quote Originally Posted by mvivion View Post
    Tach time can be all over the place. My cub's tach is almost perfectly aligned with the stopwatch I start and stop at engine start and shutdown. The numbers you cite are low, but think of it this way: You'll get a lot done between 100 hour inspections......

    Mechanical tachs are often significantly in error, both in total time and, more importantly, in rpm. Legally, a tach can be 100 rpm in error, and just be placarded as such.

    Id borrow or buy a digital tach checker, go fly and verify that the tach is accurate in rpm as a first step. If it's in error in the "wrong" direction, you may not be doing your engine (or your neighbors) any favors. In your case, it old suggest that you may not be making max rpm......

    Start there. I love electronic tachs, personally, for the reason stated above. Digital tachs, however, record time like a Hobbs meter. Now your 100 inspections come closer together......

    MTV
    Already checked the tach -- it's right on the money.

  5. #5

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    Quote Originally Posted by Mush View Post
    Tach time is only accurate when when the engine is running at the specific RPM the tachometer is rated at. Say for instance you have 2400 RPM tach. It will only show one hour when you are running the engine for an hour at 2400 RPM. If you ran the engine at say half that (1200 RPMs) for one hour it would only record .5. This is one of the main reasons why rental planes use Hobbs meters. The Hobbs starts when the engine is running and records exactly 1 hour (in tenths) after an hour based on clock time no matter what the RPMs are. The flight school I teach at charges 1.2 times the tach time if the Hobbs meter is inop or not installed.
    1.2 x the tach time seems reasonable. I'd have to charge 2 x tach time to be equal to actual time the engine is running.

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    It is supposed to be that way. Logbook time for you starts when you release the brakes and stops when you shut down, including probably 13 minutes of taxi time. Logbook time for your airframe starts when the wheels leave the ground, and ends when they touch down. The recording tach is a good approximation of that. Some say it is not a legal approximation, but everybody uses it.

  7. #7
    wireweinie's Avatar
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    If you're worried about it, install a digital tach or connect your hobbs meter to an air switch installed in the pitot line. The hobbs won't start counting until the airspeed is above the rated setting.

    Web

  8. #8
    skywagon8a's Avatar
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    Tach time is not time. It is actually an rpm counter which is calibrated to indicate one hour at a certain rpm. That rpm is noted on a sticker which is on the side of the case of the tach. It is only accurate when run for the entire time at that rpm. Since wear and tear on an engine is directly proportional to how many times the crankshaft turns, the FAA has accepted the tach hour-meter as as usage meter for the engines. That is all that they are intended to indicate.

    In the "old" days before these meters were installed on tachs it was the responsibility of a person to keep track of the actual time and record it in the log book. Somehow there were a lot of engines which were worn out with very low time on them. Tach hourmeters are a good thing which keeps the record keepers honest.
    Last edited by skywagon8a; 08-31-2016 at 05:32 AM.
    N1PA

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    Quote Originally Posted by bob turner View Post
    It is supposed to be that way. Logbook time for you starts when you release the brakes and stops when you shut down, including probably 13 minutes of taxi time. Logbook time for your airframe starts when the wheels leave the ground, and ends when they touch down. The recording tach is a good approximation of that. Some say it is not a legal approximation, but everybody uses it.
    What about oil changes? My mechanic recommends 20 hrs (I don't have an oil filter). And what about when you buy a plane -- what time is used for TSMOH -- I assume everyone uses tach time. TBO same thing?

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    mvivion's Avatar
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    Quote Originally Posted by rsalar View Post
    What about oil changes? My mechanic recommends 20 hrs (I don't have an oil filter). And what about when you buy a plane -- what time is used for TSMOH -- I assume everyone uses tach time. TBO same thing?
    Tach time is what goes in the engine logs, so the answer is yes, basically. As I noted, if you're working Part 135, you'll get a lot of work done during between 100 hour inspections.... But, oil changes you can do whenever you want as long as you're not operating under a certificate of some kind which specifies. So, if your clock says it's at 20 hours, but the tach says 15, change the oil.....nobody cares but you there.

    MTV

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    Look up Mitchell tachs on Air Spruce. They have a chart showing the different models and the average cruise RPM for which they are calibrated. Your aircraft might have a tach that is calibrated for a average cruise RPM that is significantly higher than your aircraft. Also, if you are doing a lot of low RPM flying the tach time is going to be much less than the clock time.

    Though your stated ratio of 0.5 seems really far off from reality for a flight with considerable amount of cruising.

    Looks like a Mitchell model RT-9 would be right for you cruise RPMs, since it is calibrated for 2300rpms. You can find used tachs on Ebay for under $50. A new Mitchell is $200.

  12. #12
    behindpropellers's Avatar
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    Lots over overthinking going on here.

    You might look at this as crank revolutions until overhaul instead of time. Essentially, the "hour meter" on the tach is giving a ratio of total RPMs.... So.... an oil change is required after XX revolutions.

    Tim

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    The real problem is there is no true standard for how to record an "engine hour". Paraphrasing... Continental SIL98-9C states that a Hobbs meter is commonly used acceptable device in the aviation industry. But then they warn that Hobbs meter recording methods widely vary (oil pressure vs master switch vs air switch, etc.). Continental summarizes it all by saying they don't specify a method and its up to you to decide.

    Hobbs meter or tach recorder seem like acceptable methods. But a tach recorder that is off by a factor of 2 from clock time seems too much.

    In the past I flew a good number of club planes that had Hobbs meter and tach recorder. The planes used for a lot of training with low rpm pattern work typically had tach-to-clock ratio of around 0.75. Our cross country planes were closer to 0.90 to 1.0.
    Last edited by Deaner; 09-01-2016 at 03:58 PM.

  14. #14
    cruiser's Avatar
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    MY EI tach records two times, tach time, engine time over 1300 rpm and flight time when rpm is over 2000. Apparently running time under 1300 is not recorded.

  15. #15
    Eddie Foy's Avatar
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    Quote Originally Posted by cruiser View Post
    MY EI tach records two times, tach time, engine time over 1300 rpm and flight time when rpm is over 2000. Apparently running time under 1300 is not recorded.
    I have the CGR-30P. I cant find where the manual states how flight time is calculated.

    Logbook FAA flight time is from the time you move the aircraft with the intention of flight until you come to rest at the point of landing.

    Military time is from take off to land.

    I havent flown with it yet to test this out. I use a cheap hackable timer stuck to the dash.
    "Put out my hand and touched the face of God!"

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