RPM display issue after a longer flight during landing, displayed RPM at lower power settings are too high

[dj-tentacle]

Hi. I have the following issue with my EMS-D120 and my Rotax 912 ULS that appeared recently:

I went on a couple of longer cross country flights on hot summer days. During cruise I kept the RPM between 5000 and 5200 RPM as I have a very light airplane and sometimes have to reduce slightly when it get's a bit bumpy. During cruise the RPM is always accurate.

I usually land with the throttle set to almost idle most of the time and only add power when needed to stretch the approach.

With my carbs set to around 1700 RPM idle RPM I expect it to be there during landing too. However, based on my power setting the RPM shown was around 2500 to 3500 RPM. So the displayed RPM was generally too high and minor power changes had a huge effect on the RPM displayed.

When I go back to take off power or cruise it's getting accurate again. It does never exceed the RPM that is mechanically possible with the Rotax and that I'm used to at full power.

So the problem is limited to the RPM in the lower range only. After a short tank stop the RPM on the ground before the next takeoff has no issues anymore. So it looks like it only happened on a descent after long flights, maybe due to heat.

Never really looked at the issue during taxi after landing.

The 30 kOhm resistor is soldered into the signal line.

I have no erratic jumps of the RPM either that could indicate a grounding issue.

Does anybody have any idea? Should I try replacing the resistor with a new one or one with different specs?

The RPM is mechanically perfect, the carbs were synched recently. The engine runs perfectly and it's not a throttle issue. It definitely is a display/measurement/electrical issue.

 

[airguy]

Fixed pitch prop?

A fixed pitch prop will windmill considerably faster in idle-power glide than it will on the ground. Go fly again, note the rpm displayed during the idle-power descent to the runway, then as you land look at it again and as you slow down on the runway again, and then stop and look at it again. I'm guessing you'll find that once your airspeed decays into slow taxi speed your "rpm problem" will go away.

 

[dj-tentacle]

Fixed pitch prop?

A fixed pitch prop will windmill considerably faster in idle-power glide than it will on the ground. Go fly again, note the rpm displayed during the idle-power descent to the runway, then as you land look at it again and as you slow down on the runway again, and then stop and look at it again. I'm guessing you'll find that once your airspeed decays into slow taxi speed your "rpm problem" will go away.

Thanks for your reply but please believe me, I have this plane for a few years now and nothing was changed so that it would suddenly windmill. It's a Rotax with a Gearbox, there is not a lot of windmilling anyway.

As I said I can fully confim that it's a display/measurement/electrical issue only appearing on flights longer than 1,5h. It's an issue that seems to build up due to vibration, heat or electronic wear. The engine is not windmilling more after 1,5h than after 0,5h, the displayed RPM change though.