Can someone explain in simple words why we make sure electronics are off before we start up/ shut down our Cessna172 engine?

  • $\begingroup$ You could install an avionics master switch to avoid this. $\endgroup$ – Michael Hall Jan 14 at 6:17

You shut the components off to protect the sensitive electronics from electrical surges that may occur during start up of the engine.

Circuit breakers are designed to trip open if too much current in amps is drawn from the power source by the electrical components or a short circuit downstream of the circuit breaker. They do not necessarily protect the components from too much amperage and/or voltage generated and pushed through the circuit breaker from upstream of the circuit breaker.

To be safe, disconnecting the components from the power source via a physical switch that can be opened Is prudent to mitigate the risk.

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    $\begingroup$ The wording here is a bit confusing. "Upstream" means "towards the source" - I think you mean downstream (ie: circulating reactive currents on the load side network, etc). In any case, breakers and protectors only trip on sustained overcurrent conditions and the risk during ignition is reactive/inductive voltage spikes generated in the starter that can be large enough to damage electronics without necessarily going into overcurrent conditions, or for long enough for breakers to trip. $\endgroup$ – J... Jan 14 at 14:19
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    $\begingroup$ The electronic components may or may not draw enough current to trip the breaker at a higher voltage, while still potentially being destroyed. This is the exact same concept as using a surge protector for devices even when a house has circuit breakers. $\endgroup$ – trognanders Jan 16 at 2:23

The main problem lies in the behavior of the coils inside the starter. As all inductors, they build up a magnetic field - this stored energy will cause a high spike in voltage while disconnecting. Though it can be dissipated, there is still a slight chance of damaging sensitive electronic devices. And in aeronautics one does not take chances...

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    $\begingroup$ +1 for mentioning the phenomenon better known as inductive kickback, which if not properly controlled can generate high voltages. These days wiring of large inductors such as motors that are connected to power supplies (including batteries, alternators, magnetos, etc) common to other equipment have "snubber" diodes that are supposed to harmlessly short the voltage spike to ground. The problem is that snubber diodes sometimes burn out or otherwise fail. $\endgroup$ – rclocher3 Jan 14 at 18:00
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    $\begingroup$ From Edsel Murphy's Laws: "A transistor protected by a fast-acting fuse will protect the fuse by blowing first". One would then suppose that a fuse attached to a whole bunch of transistors, resistors, and etc would be the safest electrical component in the world. :-) $\endgroup$ – Bob Jarvis - Reinstate Monica Jan 15 at 17:29
  • $\begingroup$ Not familiar with aviation electronics, but can't you just use a reverse-biased diode to get protect from voltage spikes when you have an inductive load? $\endgroup$ – forest Jan 16 at 23:11

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