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5 Answers

Master Switch

Asked by: 5465 views ,
Aircraft Systems

My checklist for the Cessna 152 (1980) says to listen for the avionics cooling fan. I always thought the noise I hear when I turn on the Master is the avionics cooling fan, but another pilot told me it was not, 152's don't have cooling fans, that I'm hearing the gyros. 

Can you tell me what I'm hearing? It sounds like a fan, but I guess it could be the electric gyro for the turn coordinator?

Thanks!! Doreen Yost

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5 Answers

  1. John D. Collins on Aug 18, 2011

    Each airplane is different, but often the avionics are protected by a separate Avionics Master Switch.  Usually the avionics fan, if one is installed, is on its own circuit breaker.  I don’t think most C152 have an avionics master switch or an avionics fan. Usually you have to turn the individual Nav Com and transponder radios on separately.  I suspect what you are hearing when you turn the master switch on is the turn coordinator which is electric and will make a distinct high pitch whine as it spins up.

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  2. Scott on Aug 18, 2011

    All of the 150, 152’s and 172’s I’ve flown have quite an audible gyro, and no cooling fan that could be heard.
    Easy way to check – when you shut down the aircraft, can you still hear the same noise, but slowly spinning down over serveral minutes? If so, it’s the giro.

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  3. Nathan Parker on Aug 19, 2011

    Later model 152s had an avionics cooling fan as standard equipment.  Might have been an option earlier.  Check the equipment list of the airplane.

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  4. James MacGregor CFI on Aug 20, 2011

    Trying to discern between a fan and a gyro can be tough, the big thing is to listen for any binding or squealing, anything that doesn’t sound smooth (if that makes sense).

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  5. Jake on Oct 16, 2011

    i FLY A ’77 152, and my checklist or any other paperwork for that matter, does not mention an avionics fan, and no such switch is on the panel, there is radio and master, but no ‘avionics’.  on power up, what you hear are the gyros spinning up, that’s about it.

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