I vaguely remember reading that there is a recommended sequence to shutdown a 912 - but I can't find it now.  Am I imaging things?  The question is: turn off the ignition first, then the master switch?  Or, the other way around?  Or, does it even matter?  In my recollection the technical reason why was related to stopping the engine (meaning alternator) before taking the battery out of the circuit (master switch).  Did I read it (where?) or am I suffering from an active imagination?  Thanks.

  • Re: shutdown sequence - master switch

    by » 3 months ago

    Hi Darrel

    I can only speak for myself in how to do the standard 912/914 system.  Normally you have 2 toggle switches or one rotary switch to shut off the engine.  In each case bring the engine back to idle speed and allow it to stabilize, oil temps mostly need to be normal and not elevated from flight.  To allow for a smooth shut down switch off one ignition, this is easy with a pair of toggle switches but in the case of a rotary switch drop one side only.  This will drop your engine speed a little bit and give a slight cooling inside the cylinder head to avoid any hot deposits firing unspent fuel on shutdown.  When stable (should only be a few seconds normally) drop the other side and then switch off the master.  

    The engine is self generating in that as long as it is running the stator will generate power.  Depending on how you have the airframe wired any aux electric pump will run on the battery.  In the case of the 914 there are 2 pumps and in some cases the pumps are both wired to the aircraft buss so turning off the master will kill the engine.  (not the best way to wire it I might add) In the case of the 912 we have a mechanical pump on the engine, you could turn off the electric pump first but that should not make a difference.  The electric in series, as shown in the installation manual, will be off when you turn off your master.  The fuel system will simply circulate some fuel back to the tank via the return line from the engine when on and that is not a bad thing as it is cooling the fuel while the engine is bleeding down in heat from flying. 

    Just my opinion.


  • Re: shutdown sequence - master switch

    by » 3 months ago

    Hey Darrel,

    Yes, shut down each ignition circuit individually, pausing momentarily between circuits as The Wizard 😊 states, then turn off your battery (I’m assuming that’s what you mean by the Master Switch). A quick review of the electrical portion of the current Installation Manual will help you understand why. The IM states “A capacitor of at least 22000 μF/25 V is necessary to protect the correct function of regulator and to flatten voltage. The regulator is not designed to store any electrical charge. If for any reason the battery or bus system is disconnected from the regulator while the engine is running (i.e. the master switch is shut off) the capacitor will safely absorb and dissipate the electrical charge produced by the generator. Otherwise the regulator would be damaged.” So you can first disconnect the battery prior to shutting off the ignition circuits and the capacitor (you have one installed correct?) will prevent damage to your regulator/rectifier- assuming you’re just sitting there at a steady idle rpm- but by disconnecting the battery with the engine running (remember the battery is also a surge protection device) you are leaving the capacitor as the lone device to protect the regulator/rectifier by its ability to absorb and discharge excess voltage from the engine. 


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