Tachometer problems

In response to Sean:

"You have not received a response from the experts - so I will do what I can.

I doubt my response will be a of assistance, however you should know, I have never heard of a Rotax installation using propeller speed - it's always engine speed.

I understand this may require a mental shift, for those used to direct drive engines, however as far as I know Rotax engine management recommendation are all in engine speed (rpm).

Could your problem relate to the calibration of your electronic engine management system having difficulty (out of callibration) converting engine speed to propeller speed?😈"

Not impossible, that was one of my first thoughts when a pilot reported an overspeed that would show up during max take-off configuration.   I was able to duplicate that fault and with a calibrated optical sensor we determined there was a 30-50 RPM error that popped up but it was rare and with the prop sync "by ear" pilots knew they weren't getting the overspeed, so it took a while to come to our attention.   Problem sort of disappeared after a G1000 update and I thought that was the end of it.   

We have pulled the config module on a recommendation from the manufacturers avionics tech they sent out a few months back, not sure what they want us to do with it or if someone has brokered a deal to have a replacement sent our way, so that avenue is being explored (I think)

In response to Rotax Wizard:

"First, this is a certified aircraft, it is made to be as near to other conventional aircraft as possible that are for the most part direct drive.  (therefore crankshaft and prop RPM are the same}  All said the numbers are fine to use.  

The Rotax 912S, like the ULS experimental version, uses a signal from a trigger mounted at the flywheel.  The ground is via the bolts to the crankcase and the signal goes to the instrument.  The normal issue with errant signals is the signal wire picks up a high tension wire (spark plug lead) interference.  My suggestion is look at the routing and make sure that that it is not too near the plug leads.  Remember that each spark that occurs causes a field around the plug wire, this can be sensed in some cases and will lead to changes in the reading.  

Actual P leads (power from charge coil to module, the red ones) are not routed near the tach wire normally.  Check your plug to tack wire locations.  See what has changed.  There should be a small plastic sleeve on the tach wire from the trigger to the connector, be sure that is intact and not making contact with your high tension cables. 

Cheers"

Thanks again for your reply as well.  I did check for closeness of the ignition trigger coils as well and they are encased in a grounded shielding right up to the ignition modules.  I don't believe they are too close or any closer than the other engines and aircraft configurations.  That being said I agree that that absolutely must be the area where the interference is coming from.  

My next course of action (because lazy) is to simply change the routing in the engine compartment to the tachometer trigger coils wires which I will admit are new from factory as that was the first thing I changed before I tried messing with the "mag" switches.  I figure if I can isolate this wire engine side and possibly have them run towards the firewall in a tidy bundle it will alleviate some of the heavy presence of wires in and around my ignition units.

...I don't trust those solder sleeves that Tecnam used so I unfortunately cannot rule out that the P-lead wire might not have a great shielding on it either as I did ground that albeit to the engine due to the solder sleeves not looking like the solder was even in there when it was heated and shrunk

In response to Sean:

"".......it is made to be as near to other conventional aircraft as possible that are for the most part direct drive"

My completely biased opinion;

What a load of rubbish - If a pilot can not accomodate engine speed, as the appropriate parameter, for management, he/she should not be allowed in the cockpit.

The only reason for prop speed, as an engine management tool, is the predominance of direct drive ie prop/engine rpm the same.

The above statement panders to lazy inflexible thinking - not what you want in pilot, who should be able to accomodate atypical situations."

100% agree that its hogwash with Prop RPM but my reasoning is that pilots treat these things like Lycomings which is no bueno and I've already had to do a lot of extra work because of that.

Benefit of Prop RPM I guess is the simplicity😣

In response to Jeff B

"Paul,

I could not find wiring examples for connecting the GEA 71 to the Rotax tach coil. I suppose it uses one of the digital inputs. The only example shown is with a three wire transducer driving the tach signal.  

Because far more of these engines are installed in SLSA or ELSA, it’s  more common to see the Garmin GEA-24 or even the GSU-73 for engine function monitoring with the Rotax engine. There are wiring examples in the G3X installation manual for both of these modules connected to the Rotax tach coil. In both cases they require a 300 ohm resistor between the coil input and ground at the module, along with a diode and zener diode in the circuit. This acts as a buffer/regulator to the circuit. I don’t know if this is required for the GEA-71 but I would bet it is. I’m wondering if Tecnam may have overlooked this. You might call Garmin and ask them for a connection diagram and compare it to what you have."

Yup, you are correct it has that same "filter" that is used for the G3X and GEA24 systems.   It's weird because I have like 3 or 4 diagrams and most utilize two 110 ohm resistors whereas the newest one that I was unable to find last time had the single 300 ohm resistor.  

The GEA for the right engine which was my problem engine used the two 110 ohm resistors and the left engine GEA used the single 300 ohm resistor.   The board they had on there looked more like a chip the uni-bomber would use to make a remote detonator and not an honest to goodness Garmin part which I think was 011-02348-00.   I would reach out to Garmin but as I am not a dealer they don't want to talk to me and the nearest Garmin dealer makes me wanna scratch my forehead with a cheese grater.   But this move might be necessary if my relocation of these wires next doesn't yield results even though it seems isolated to a single ignition switch.

I did test and reconfigure the chip on the right engine GEA71 to match the functional one on the left with a single 300 ohm resistor which didn't make the problem worse, but it didn't improve it either

P.S. Tecnams av tech also suggested upgrading the GEA71 to GEA 71B so there's that as well I guess

James Ott wrote:

Even though its unusual, how are you to know what reasons someone may have for wanting to display prop speed. Your unthinking hasty and biased opinion is a load of rubbish; my opinion of course.

Thanks  for seconding my own "biased" statement.

As for "... how are you to know what reasons someone may have for wanting to display prop speed."

My challenge to you is;

What possible scenario can you envisage, where Rotax generated propeller speed, is of more/equal significant, compared with the normal engine rpm that most Rotax pilots manage their engine with? 😈