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8 Answers

How does an odometer differ from tach time?

Asked by: 12423 views , ,
Aircraft Systems, General Aviation

Lots of questions here...

-What is the gauge called that "tach time" is registered on?

-Pilots use "tach time" in order to plan for inspections such as 100 hour inspections.  Tach time is linked to RPM of the engine. 

Cars have an odometer which is also linked to the RPM of the engine. 

However, the odometer is used to calculate distance (or, milleage) whereas airplanes use "tach time" to calculate engine time.

Does anyone know how these devices differ mechanically?  You wouldn't say a plane for hire/rent/instruction needs a 100 mile inspection; they require a 100 hour inspection (even though it seems the tach time meter and the odometer operate mechanically in the same way). 

-The "tach time meter" on the plane I rent from my FBO is scratched off from the bottom of the tachometer.  Instead, it has a "maintence hobbs" gauge underneath the regular Hobbs meter.  Why?

-Why does the Hobbs meter count every tenth of an hour (6 minutes) instead of each minute?  If someone shut down after 5 minutes, then 1 minute into your flight you already have .1.

-Also, does anyone know where in the POH I can find information concerning the the Hobbs meter?  I would like to determine if it run off of the oil pressure switch or if it runs off of the battery master.

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8 Answers



  1. John D. Collins on Feb 20, 2011

    Don’t expect to find much if any information on a Hobbs meter in your POH, as many of these are added after the aircraft left the factory and are just used for keeping track of time.  They operate anytime they have electrical power applied to them.  Depending on what controls the power will determine what time is being recorded.  Some are hard wired to the master switch, so as soon as you turn on the battery, they start counting time.  More common is to use an oil pressure switch to provide power, so as long as the engine is running, they count the time.  Others are more sophisticated and will only be active when the landing gear is retracted, thus they count air time.  Still others are wired to components that have maintenance time requirements, for example the janitrol heater can have a hobbs that counts the time it is used.
     
    In some aircraft, there is a tachometer with a time meter that counts time on a 1 to 1 basis only at a specific RPM, such as 2400 RPM. At RPMs lower than 2400, it will be slower than real time and at RPMs higher than 2400 it will be faster than real time.  Many of the newer aircraft don’t have a mechanical tach time meter, in which case a Hobbs meter is often used for this function.
     
    Why does a Hobbs meter count tenths of an hour instead of minutes, it is cheaper and easier and counts with sufficient precision and accuracy for the application.  Even the mechanical tach counts hours and tenths/hundredths of a minute, not minutes.  Sure, you can end up paying for an extra tenth if you are renting, but half the time you will end up better off, so it all washes out in the long run.
     
    One last minor point, you said “You wouldn’t say a plane for hire/rent/instruction needs a 100 mile inspection; they require a 100 hour inspection.” is partially correct, a plane that is used for instruction requires a hundred hour inspection, but one that is used for rental does not.
     

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  2. Matej Dostal on Feb 21, 2011

    A little sidenote: odometers in cars are connected to wheels (after the clutch and the gearbox get involved). You can park your car and rev your engine all you want, but,  unless the wheels start turning, the odometer won’t move at all.

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  3. Wesley Beard on Feb 21, 2011

    Quote John: “One last minor point, you said “You wouldn’t say a plane for hire/rent/instruction needs a 100 mile inspection; they require a 100 hour inspection.” is partially correct, a plane that is used for instruction requires a hundred hour inspection, but one that is used for rental does not.”
     
    I don’t necessarily agree with the 100 hour inspection not being required for rental aircraft.  I’ve read 91.409(b) and in part states “No person may operate an aircraft carrying any person (other than a cremember) for hire, and …”  It sounds like to me that the aircraft is for hire during a rental session and thus requires a 100 hr inspection.  Is there something I am missing?

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  4. Flight Student on Feb 21, 2011

    Thank you guys for all of the information.
    However, does any one know why the tach time meter is scratched off from the tachometer and replaced with a seperate gauge that been labeled as a “maintence hobbs?”  I’ve asked a flight instructor at the FBO and he didn’t seem to know.
    Anyone have any ideas? 
     

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  5. John D. Collins on Feb 21, 2011

    I assume the tach hour meter stopped working and a clock that records time such as the hobbs meter was installed for maintenance purposes.  One could question if this is a proper repair of the tach, but a separate hobbs could be installed in any aircraft and used for maintenance purposes.  Many aircraft have both a tach hour meter and a hobbs meter, either can be used for maintenance purposes, one just has to be consistent.

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  6. John D. Collins on Feb 21, 2011

    Wesley,
     
    This rule applies to the person, a renter is not for hire.  Just flying for fun or travel and carrying passengers who share a pro rata cost of the rental with the pilot isn’t considered “for hire”.  A scenic flight flown by an authorized commercial pilot would be for hire. Flight instruction for hire is specifically included in the 100 hour requirement, rental is not.  For hire means the pilot is being paid for his/her services, not that the pilot is paying a rental fee for the use of the aircraft.
     
     

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  7. Aviatrix on Feb 21, 2011

    John,
     
    A tach time meter OR a Hobbs meter can be used for inspections/maintence if one is consistent?  Are you saying if I started my insepctions in reference to the Hobbs, I would have to stick with the Hobbs throughout the inspection/maintence requirements of my aircraft?
     
    I thought one was supposed to use the tach time meter for regular inspections/maintence because this measures engine time.  The Hobbs meter measures clock time.  It seems engine time is what we’d want for inspections/maintence b/c this is a beter of wear and tear on the engine.
     

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  8. Wesley Beard on Feb 21, 2011

    It’s an interesting topic because the regulations are specifilcally talking about the airplane maintenance requirements and not about pilots.  To say the least the FAA agrees with your point of view.  Learn something different every day.
     
    May 3, 1984Mr. Perry RackersJefferson City Flying ServiceDear Mr. RackersThis is in reply to your request of May 1, 1984, that we render an opinion regarding the applicability of the 100-hour inspections requirement of Section 91.169(b) of the Federal Aviation Regulations to rental aircraft. Section 91.169(b) of the Federal Aviation Regulations provides that, except as noted in Section 91.169(c), a person may not operate an aircraft carrying any person, other than a crewmember, for hire, and may not give flight instruction for hire in an aircraft which that person provides unless, within the previous 100 hours of time in service, the aircraft has received either an annual or a 100-hour inspection.If a person merely leases or rents an aircraft to another  person and does not provide the pilot, that aircraft is not required by Section 91.169(b) of the Federal Aviation Regulations to have a 100-hour inspection. As noted above, the 100-hourinspection is required only when the aircraft is carrying a person for hire, or when a person is providing flight instruction for hire, in their own aircraft.If there are any questions, please advise us.Sincerely,Joseph T. BrennanAssociate Regional Counsel

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