Tachometer problems

Paul,

The fact that this has always been a problem makes me think it’s systemic rather than a part failure. I believe you need to know if Tecnam and/or Garmin has seen this on other Rotax powered aircraft using the GEA 71.  So, you may have to engage your Garmin dealer or Tecnam - as painful as that may be!  

With both engines now exhibiting this behavior this can’t be isolated to your aircraft.  The number of Rotax engines installed in certified aircraft with the G1000 must be very small, so this kind of thing has not had much chance to get worked out.  There are some folks on this site that are quite good with electronics, can you post a simple diagram of how the coils are wired to the GEA 71?  Maybe just markup one of the diagrams in the GEA 71 installation manual.  Good luck with this Paul.  

 All

First, the topic is the tack picking up interference.  The issue is not with the tack signal wire picking up the other trigger wire signals, it is the tach wire picking up the "secondary" high tension, read spark plug wires, field when they fire.  This pulse give a false signal to the tach reading.  It is understandable that when one of the ignitions is off, hence stopping some of the high tension firing from the coils, that the signal is fine.  Check the route of the wire in relationship to the spark plug wires themselves.  

As for prop speed vs crank speed.  The desire for many flight schools for years was to have a common understanding of engine speeds to the pilot.  In any direct drive engine we think of this as prop speed.  If the Rotax can give you prop speed then all these different aircraft in a training fleet will have a common thread for the most part.  

Example here is let's look at the 912ULS, 5800 WOT take off power.  divide by gear ration this is about 2400 prop.  Checking for mag drop, Rotax is 4000 and divide by 2.43 is about 1650.  Rotax idle, recommended for most at 1800, divide by 2.43 is about 740.  For a student pilot these numbers are very similar to each other and it starts to make it easier to switch between direct drive engines and geared ones.  Rotax used to made mechanical tack drives as part of the certified engines back in the 1990s, now it is done by just using an instrument that can calculate the gear ratio electronically.  It is simply a matter of choice, I don't believe it is more than that.

Cheers

Thanks RW,

I understand the rational  however would ask one simple question;

Is the pilot of a IC powered aircraft managing propeller or engine speed, as an indicator of system condition & performance?😈