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5 Answers

Starting Engines w/ Electrical System Off

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Aircraft Systems, General Aviation, Student Pilot

Why is it necessary to turn of the avionics PRIOR to start the engines as at engine shutoff?

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5 Answers



  1. Wes Beard on May 27, 2014

    There is a starter on the airplane that draws a lot of power. If the avionics are also turned on, there may not be enough power for both the avionics and the starter. If that is the case, the starter will not turn the prop over and the avionics may have issues when the voltage drops to such a low level.

    The starter turns the propeller. The propeller shaft starts the movement of the magnetos. The magnetos supply power to the spark plugs so the engine can run independent of the electrical system.

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  2. Cadu on May 29, 2014

    Beard, thanks for the explanation, however, I’ve been heard several reasons for that as In older planes with generators and bad batteries if you were to just shut down the engine you would get an electrical output that matched what the engine did, however, at actual airplanes it’s unlikely you’ll be able to have that kind of problem, but it’s still a very good habit to get into.

    So, do you agree?

    I have the same question if it’s also necessary turn off the avionics before magnetos check and before you shut down the engine and why? Would be the same reason before start up the engines? There is a lot of discussion about this too.

    Regards,

    Carlos

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  3. Wes Beard on May 29, 2014

    There are differences between a generator and an alternator. First off, a generator supplies DC power and the voltage is a function of the generator RPM (which is attached to the propeller shaft). So a low power setting, the generator is not really outputting much power.

    An alternator outputs AC power and goes through a transformer usually inside the alternator assembly. It is not prone to outputting low voltage at low power settings.

    The only reason to turn off the avionics before the magneto check is to not subject the batteries to a sharp increase in amperage load if the engine dies. I have never had an engine die during a magneto check. If the magneto is bad, usually you can catch it before the engine fails completely. The generator / alternator will still be outputting power as the propeller shaft winds down.

    In every airplane I’ve flown it is customary to turn off the avionics before the engine is cutoff due to not wanting an excessive load on the batteries.

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  4. Chris Polek on May 30, 2014

    If I may add to the conversation. The primary reason to turn off the avionics prior to engine start is as stated earlier the starter draws a lot of power when it turns especially when it first starts to turn over. If the avionics are on they will lose power since the starter will take almost all the power to operate. When the engine starts and the starter is shutoff, there will be a power spike which the avionics don’t take to kindly too. This spike will cause damage to the delicate circuitry in the radios. It may not cause a problem right away but it will definitely cause a decrease in there life span. As for shutting down the avionics prior to engine shutdown in most of the plans I have flown and worked on this step is called out in the checklist. But the best reason I can think of is so you remember to turn off the avionics master switch. I don’t know how many airplanes I have gotten into and found the avionics master switch left on or if I was in a hurry a flip the master switch on to do a simple lighting check and all the avionics come to life.

    As far as when doing a mag check. There is no issue since you are only checking the magnetos and your engine should only loose a little rpm which at your mag check engine speed the alternator is at a speed known as on speed which is the speed at which the alternator is putting out its maximum and little rpm changes will not affect its output.

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  5. Cadu on Jun 02, 2014

    Guys, thank you for all explanation. Everything it’s more clear from now on.

    Regards,

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