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What is an Essential/Crossfeed Bus? C172S

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Aircraft Systems

In the skyhawk 172s poh on th electircal diagram there is an Essential/Crossfeed bus.   What does this do?   I know that it provides power for the alternator and annunciator panel,  but I would like to know why it is different from everything else.

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1 Answers

  1. Wes Beard on May 09, 2012

    I have a G1000 C172S POH for use with serial numbers 172S9810 and ON.  This answer may apply to your aircraft and it may not.  If my POH the essential and crossfeed buses are separate buses and not one bus.  I can’t tell from your question if you have two separate buses.  
    The essential and crossfeed buses get their power from Electrical Bus #1 and #2.   Even though the essential and crossfeed buses receive power from Electrical Buses  #1 and #2 they cannot trun around and give power to them.  On the crossfeed bus has the alternator field circuit breaker and the warning circuit breaker.  The alternator circuit breaker will sense a over voltage situation on either electrical bus and remove the alternator from the system.  The warn system measures the voltage of both electrical buses and reports its findings to the GIA.
    The essential bus is powered separately by a standby battery that will last “for a limited time” in the event of alternator and main battery failure.  On this bus, there is enough equipment for the pilot to make an immediate landing without incident.
    To answer your question, it is different from everything else because the manufacturer has determined that the equipment powered by the essential bus is essential for flight and has provided an alternate source of power for that equipment. 

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