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Thread: [BAC-Mail] Flight Hour Meter Pressure Switch

  1. #1

    [BAC-Mail] Flight Hour Meter Pressure Switch

    > Greetings All,
    > I was browsing through my maintenance manual (Sport 150 MB-901) and
    > came across an electrical drawing for the Cigar Lighter and Flight Hour
    > Meter circuit (Page 4-76 169-590015G5). It shows a "Flight Hour Meter
    > Pressure Switch" I was just wondering what pressure (oil, Pitot
    > Static?) is this needing to make the circuit. I was under the
    > assumption that our Hobbs runs as soon as the master is turned on. Not
    > a big deal, seems to be clocking OK. Just curiosity as I hardly use
    > the thing anymore.

    *** Usually oil pressure. When you have oil pressure, you know for sure
    that the engine's running. So you can use it to schedule maintenance,
    charge for run time, etc.

    - Jerry Kaidor

    Added by Mike Rellihan-
    It is fairly common to encounter Hobbs meters that have been wired to the buss, rather than through an oil pressure switch. This is particularly true if the plane was used in training service, or as a time-builder. It inflates the rental revenue to the FBO, and inflates the logs of the time-builder, if the meter runs any time the Master is turned on, rather than just while the engine is running.

    As an aside, many modern hand-held GPS units start the trip time when the plane starts moving, when it exceeds a certain speed, or some combination. Some complex aircraft have the Hobbs wired to a squat switch on a landing gear leg, which turns on the meter when the plane lifts off.

  2. #2
    I think my Hobbs is just wired to the bus but I've never looked. My Garmin 327 has a flight timer that auto-starts when it detects a climb. I think most of the digital transponders with a flight timer will do this (Narco and King).

  3. #3
    Wes K wrote in MM,

    "The stand alone Hour meter or Hobbs Meter is an electrical gage that records operating time that occurs while the meter has electricity flowing through it. These are also setup to be accurate at a specific voltage. lower the voltage and it becomes slow, raise the voltage and it runs fast. It can be wired direct to the buss. It can use a mechanical switch to feed electricity to it. There are several type switch installations. Some are setup on a landing gear safety switch so the meter will only record time when the aircraft is off it's wheels. Some are setup on oil pressure switches and the meter will only run when the engine has a least 2 to 5 PSI oil pressure. Some are connected to airspeed switches which will feed electricity to the meter when the airspeed is above a set speed. All are not suppose to be able to be disable by the operator. This means they are to be fused in a location that cannot be accessed during flight. Many FBO's use these hour meters to gouge the most money they can from renters and charterers. Clubs use them often."
    full text:2003-11-18 16:47:47,

    or much easier to find with a search of hobbs,
    in BAC, under MM archs

  4. #4
    From Bob Steward, A&P-IA:
    Oil pressure. It was used to run the Hobbs to clock rental time.

    Some sneaky FBOs ran the Hobbs off the Master, and got to charge you for
    the time you listened to ATIS and ran your ground checks before starting
    the engine!

    Bob Steward, A&P IA
    Birmingham, AL

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