shutdown sequence - master switch

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.

Cheers

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. 

I forgot and / or failed to post a thank you to the Wizard and Jim!!

On a related point;

Gearbox clutch wear, at engine shutdown.

My first 912ULS had a very light 2 blade composite prop. My current 912ULS has a relativly heavy 3 blade CS prop (11kg+). This advice relates to my experience with the heavy prop but is good practice for all.

To minimise shutdown wear on the clutch mechanism - The engine should be idling at about 1700 rpm, before turning of the first ignition. Engine rpm will drop on just one ignition. Allow engine rpm to stabilise/smooth for a brief moment ( about 2 seconds will usually do it). Turn of second ignition. This strategy is intended to reduce the energy/momentum in the prop rotation at shutdown, thus minimising the effect on the gearbox clutch mechanism.

I've been using Sean's shutdown technique for over 10 years with no gearbox/clutch issues at all.