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4 Answers

ADC/AHRS operating with master switch on in G1000 C172SP

Asked by: 4323 views Aircraft Systems

The electrical diagram for the C172SP shows that the ADC/AHRS are on avionics bus. Then, why does it come up and operate when I turn on only the master switch during preflight?

"The primary buses are supplied with power whenever the MASTER switch (BAT) is turned on, and are not affected by starter or external power usage. Each primary bus is also connected to an avionics bus through a circuit breaker and the AVIONICS BUS 1 AND BUS 2 switches. Each avionics bus is powered when the MASTER switch (BAT) and the corresponding AVIONICS switch is in the ON position." (emphasis added)

Shouldn't it show a red X on all displayed flight instruments if the avionics switch is off? Does the line coming from buses 1+2 with black arrowheads pointing to the essential bus mean it's powering the essential bus? I've been taught it's a diode which allows flow of electricity in only 1 direction (to the essential bus) to charge it; does it also mean that it's powering it at the same time?

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4 Answers



  1. Dan S. on Apr 08, 2015

    Take a look at the electrical diagram on page 7-49. You’ll notice that the ADC and AHRS are also powered by the essential bus. Any time the master battery is on, the essential bus will be powered, or in its absence, the standby battery will supply power to it. This is for safety reasons so that in the event of electrical failure you would still have essential instruments for flight.

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  2. Drew on Apr 08, 2015

    That makes sense, but that begs the question: why have equipment on the avionics bus 1 when they’re already on the essential bus, which is automatically powered whenever the master switch is on without the avionics being on? When using the standby, you get ADC+AHRS, and when using the main batt without standby or the avionics switch, you still get ADC+AHRS. There is no scenario where you would need to turn on the avionics bus 1 to obtain ADC+AHRS.

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  3. Best Answer


    Dan S. on Apr 08, 2015

    You are correct that you still get AHRS/ADC when only the essential bus is powered up. Could you go fly without the avionics bus activated? Of course you could. But what would happen if the essential bus got fried? Having multiple buses gives the luxury of redundancy. So if you lost the essential bus, the avionics bus will still keep your essential instruments operational.

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  4. Drew on Apr 08, 2015

    Ah, thank you, Dan.

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