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5 Answers

Question regarding C172 G1000 Electrical System

Asked by: 7019 views Aircraft Systems, General Aviation, Instrument Rating


I am a private pilot currently working on my instrument rating. This question is regarding the electrical system of a 172 fitted with g1000.

The power supply to the avionics bus 1 and 2 is delivered from the main electrical bus 1 and 2 respectively. These two avionics buses connect circuit breakers for various components such as the Transponder, AHRS and ADC. In fact, all the components of the G1000 system are connected to either one of the avionics buses. All except the two Garmin Integrated Avionics units. So this has led me to believe that the GIAs are infact the two avionics buses, but it still doesnt add up for me as that would make the G1000 schematic I have (apparently supplied from Garmin) highly inaccurate. Where do the GIAs fit into the system? There doesnt seem to be any connection to them from either buses.

I havent really been able to get a conclusive answer from an instructor.

Thanks for any help!


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5 Answers

  1. Jim Foley on May 31, 2012

    The only thing I can find on this is from Max Trescott’s G1000 Handbook, page 191 paragraph 3, but it is under the heading “Cessna 182 Electrical System.”  I can only assume this is the same, or at least similar to how the C172S is wired.  (I highly suggest you purchase this book.  It is extremely informative): 
    The essentials bus powers the PFD, ADC, AHRS, GEA 71, Engine/Airframe Unit, one GIA 63 that provides GPS1 and COM1 and the standby instrument lights.  It recieves power from Bus1, Bus2, and Avionics Bus#1.”
    “Nonessential avionics, such as the Stormscope, MFD and second GIA 63 are powered through the Avionics 1 and 2 switches, which can both be turned off to conserve power in an emergency.”

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  2. Wes Beard on May 31, 2012

    Jim is basically correct.  I double checked in my C172 G1000 PIM.  Both GIAs have integrated navigation, VHF and GPS processors.  If you trace these items back to the electrical schematic, you will see that GIA #1 is powered by the essential bus (which is powered by both electrica buses) and Avionics Bus #1.  The second GIA is powered by Avionics Bus #2

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  3. Jim Foley on May 31, 2012

    Hey Wes, were in the PIM is it?  I briefly looked before going to Max, and didn’t see anything.

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  4. Max King on Jun 01, 2012

    Thanks for the answers guys! When I look at the schematic in my PIM, the only things connected to the Avionics bus 1 is the PFD, ADC/AHRS, NAV1/ENG, FIS and ADF/DME. And the essentials bus has (much the same) the PFD, ADC/AHRS, NAV1/ENG, Comm 1, Stby batt and Stby ind. lights. It doesnt say anything about a GIA. Could it be that the schematic doesnt show the GIA as a single unit (like I have been looking for), but rather shows where the components that make up the GIA are connected? In that case I think I get it 🙂
    But that makes me wonder why the Comm 1 is the only unit on the essentials bus not connected to both buses, like the NAV 1.

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  5. Wes Beard on Jun 03, 2012

    In my PIM, the GIA is detailed on page 7-63 while the electrical schematic is on 7-46.  The electrical schematic doesn’t say a GIA but we can infer where it is by looking at the components that make up the GIA.  
    About the comm/nav situation.  If you were only on battery power, which would you rather have: a way to talk or a way to navigate.  Cessna choose a way to navigate in this emergency situation. 

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