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5 Answers

Why we requierd to track HOBBS AND TACH?

Asked by: 4380 views General Aviation

To track one of them is not enough?
When the school charging as they always go by the Habbs which is more than the Tach

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5 Answers

  1. Lucas on Aug 26, 2012

    If you Idle the engine for 1 hour the tach will show close to 20 minutes of time so you would only pay for 20 minutes instead of 1 hour. Vice versa if you keep the RPM at maximum you would probably end up paying for 1.5 hours.
    The hobbs on the other hand is an actual clock and that should be precise.
    In general, most schools that have an airplane where the hobbs is not present, will multiply the tach * 1.3 to try an get an accurate timing to charge the student.


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  2. Wes Beard on Aug 26, 2012

    For more information…

    The tach time measures the number of revolutions the propeller makes. Typically, the tach time is set so that it will read accurately for one hour when at a cruise setting of 2400-2500 RPM. Each manufacturer sets it differently. In essence, if you measured the accuracy of the tach time against a clock, it will be most accurate when the RPMs are somewhere in that range.

    The tach time slows down with a slower RPM and speeds up with a faster RPM.

    I generally agree with the tach time conversion of 1.3 hours for a training aircraft. Be advised that during cross country flights that conversion is too great. It may be closer to a 1.1 factor.

    The other issue is with a constant speed propeller (CSP). Since tach time measures RPM, a CSP will remain within that envelope much longer and the conversion factor is going to be less. I almost lost a good friend who I fly with due to this issue. I ignorantly relied on the 1.3 factor as our flight time and when we measured against an actual clock I was off by a half hour on some of the cross country flights we did. Lesson learned.

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  3. Alex Clark on Aug 26, 2012

    Hobbs = Billing and logging of time. Mostly billing…

    Tach = Engine maintenance, oil changes, time to 100 hour inspection and so on.

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  4. David A Hatcher on Sep 03, 2012

    When you rent an aircraft you are paying for “flight time”. From 14 CFR Part 1; Definitions and Abbreviations Flight time means: (1) Pilot time that commences when an aircraft moves under its own power for the purpose of flight and ends when the aircraft comes to rest after landing
    For maintenance purposes flight time beginning when all the stress for flight is placed on the airframe and engine.
    So taxi time (for the purpose of flight) is considered flight time (you’re paying for!) while that time would not be (fully) considered for overhaul, oil changes, inspections etc.
    By “logging both” as a rental pilot you are just helping a little administrative work for the flight school/FBO

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  5. Sam Dawson on Nov 05, 2012

    Just to add to what David wrote. Though Hobbs time is often used in logging flight time it is not the legal definition of flight time and may not be accurate. I have seen GA airplane Hobbs meters use oil pressure, indicated airspeed, airflow, vibration, and electrical power to record “flight time”. As David pointed out none of these are the definition of flight time. As a student you may be getting robbed of valid a valuable flight time if you are not using the legal definition; as a commercial pilot you may be in violation of the regulations by not using the FAA definition of flight time.

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