Welcome Guest. Sign in or Signup

5 Answers

Master Switch

Asked by: 10051 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

Ace Any FAA Written Test!
Actual FAA Questions / Free Lifetime Updates
The best explanations in the business
Fast, efficient study.
Pass Your Checkride With Confidence!
FAA Practical Test prep that reflects actual checkrides.
Any checkride: Airplane, Helicopter, Glider, etc.
Written and maintained by actual pilot examiners and master CFIs.
The World's Most Trusted eLogbook
Be Organized, Current, Professional, and Safe.
Highly customizable - for student pilots through pros.
Free Transition Service for users of other eLogs.
Our sincere thanks to pilots such as yourself who support AskACFI while helping themselves by using the awesome PC, Mac, iPhone/iPad, and Android aviation apps of our sponsors.

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.

    +2 Votes Thumb up 2 Votes Thumb down 0 Votes



  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.

    +3 Votes Thumb up 3 Votes Thumb down 0 Votes



  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.

    0 Votes Thumb up 0 Votes Thumb down 0 Votes



  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).

    0 Votes Thumb up 0 Votes Thumb down 0 Votes



  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.

    0 Votes Thumb up 0 Votes Thumb down 0 Votes


The following terms have been auto-detected the question above and any answers or discussion provided. Click on a term to see its definition from the Dauntless Aviation JargonBuster Glossary.

Answer Question

Our sincere thanks to all who contribute constructively to this forum in answering flight training questions. If you are a flight instructor or represent a flight school / FBO offering flight instruction, you are welcome to include links to your site and related contact information as it pertains to offering local flight instruction in a specific geographic area. Additionally, direct links to FAA and related official government sources of information are welcome. However we thank you for your understanding that links to other sites or text that may be construed as explicit or implicit advertising of other business, sites, or goods/services are not permitted even if such links nominally are relevant to the question asked.