RPM's - Rotax Tach vx GRT EIS Tach

Guday Tim,

I think the question I would ask is - Can the Rotax tachometer be adjusted/calibrated????

I expect your EFB/EMS can be calibrated.

One other thought - When I did my engine / EFB calibrations, I triple checked, using a mower style (induction tach), a laser tach and a propeller balancing tach ( gave me prop rpm). The prop rpm directly relates to engine rpm. I managed to get all three to agree within a few rpm  of the EFB- good enough for me.😈 

I just did an annual on a Sting Sport that had this exact problem of a 200 rpm difference.

"I just did an annual on a Sting Sport that had this exact problem of a 200 rpm difference."

AND?????😈

I’m wondering if the Rotax Tach is affected by the Grand Rapids EIS. They both must use the same rev counter coil. I know some tach connections require a dampening circuit so maybe the EIS has requirements not fully compatible with the Rotax tach.  If this is the case, then the Rotax tach would be accurate in an aircraft without a GRT EIS installed. I can’t imaging Rotax selling a tach that was off that much, especially considering that their engine power limitations are based on RPM.  

It seems silly though for a manufacturer to build an aircraft with this known issue, this would have been disqualifying for a certified aircraft and probably is for an SLSA. The operation manual statement is down right amusing. They might as well have said “we could not get the tachometer to work right, so don’t pay attention to it”.  CFR 91.205 lists a tachometer as required equipment, so this makes me think that the GRT EIS did not qualify to meet this requirement so the Rotax tach had to be installed… working or not. Or maybe the Rotax tach is there as a backup? 

The only reasonable way forward is to make readings with rev counters/tacho's that are not connected to the aircraft systems.

This will establish which (or both) installed tachos are wrong.

From this point corrective actions may be postulated.😈