Tach verses Hobbs

[Roger_Lee]

Dynon,

With the D-120 does the Hobbs run when the master switch is on and the engine is still off or does the engine have to be running for the hobbs to track time? Dose the tach time run the same when the engine is at idle verses full throttle? (slower or faster)
My hobbs has 35 hours more than my tach after 160 hours of actual flight tach time.
Do you recommend using the tach time verses the hobbs for maint times?

 

[N930RV]

Hobbs time starts when oil pressure is greater than ~15 psi.  Tach time is calculated as a percent of cruise RPM.  You can set cruise rpm to what ever speed you wish.  As an example, if you choose 2000 rpm as cruise, when ever the actual engine rpm is 2000 the tach time will accumulate at the same rate as hobbs time.  When engine is running faster than 2000 rpm the tach time will accumulate faster than hobbs time and likewise when engine is running slower, tach time will accumulate at a slower rate than hobbs time.
  I use hobbs time to record flight time.  I think tach time is used to estimate engine time for maintenance purposes.  I'm sure someone on her can describe tach time usage better.

 

[dynonsupport]

You'll need to consult your neighborhood A&P and your engine manufacture for best maintenance practices for your motor, but N99WC has the explanation of hobbs vs tach correct. In fixed pitch prop aircraft (the only experience I have), you typically see tach time increment at somewhere between .7 and .8 of hobbs time, depending on how hard you're pushing the engine and what percentage of the flight you're spending at cruise.

 

[ctflyboy]

The question on Tach verses Hobbs has to do with the Rotax 912 mainly (I believe). Most mechanics (mine included) use the Tach time for maintaining most aircraft engines. I've been told because the 912 runs at a higher RPM and it's other peculiarities that we should only use the Hobbs time.

Question: the 120 comes with a Rotax custom set up which is great. But - are the percentages mentioned elsewhere in this post "setable" or because it's custom can it not be changed? (I know I can go look but I'm nervous about fooling with the custom setup)

I realize we need answers from our airplane manufactures as to what we should be doing but if in fact we find out with the Rotax we should be using only the Hobbs is it possible to have the Rotax custom setup remove the RPM timer?

Seems like there should be some more investigation into this between the Dynon, Rotax, and manufactures because there is a possibility of incorrect timeing for maintenance procedures.

While it's not a lot of hours difference at the low end it could mean 200 to 300 hours off at the life cycle end....

Roger H

 

[dynonsupport]

Roger,

The next firmware update (4.0) will delete tach time when Rotax is chosen as the engine. According to Flight Design, only hobbs is used on Rotax engines, and tach time should be ignored. We'll make this real easy to understand by not showing you the tach time at all

 

[josok]

Bad joke, and to my surprise i seem to have to explain why it is needed to have both. Rotax or other. Since tach is a function of cruise RPM and hobbs is approx the time the engine has been running, the hobbs/tach ratio reflects if the engine has been running faster or slower then cruise rpm.
That's why amongst others rental companies want to see both. In their billing software is this ratio comes up as a red flag for mis-treating the plane if it's over their limit.
For my experimental, that i do loan to friends, this ratio is known to be looked at, and prevents misuse.

Regards,

Jos Okhuijsen

 

[dynonsupport]

Jos,
Just don't choose "Rotax" as your engine type and you'll get the tach time. Choosing "Rotax" only turns off the tach time and forces some oil temperature settings on you. We felt this system still allows people to set up the monitor the way they want. We don't know you have a Rotax unless you tell us.

 

[PilotKris]

Roger,

The next firmware update (4.0) will delete tach time when Rotax is chosen as the engine. According to Flight Design, only hobbs is used on Rotax engines, and tach time should be ignored. We'll make this real easy to understand by not showing you the tach time at all

On the off chance that Flight Design doesn't know everything(and if it turns out the real reason Rotax doesn't use Tach time is simply that their tachometer doesn't record Tach time), how about with the new update, you get the choice of deciding if you want Tach time or not when you select "Rotax"?

Please tell me you're still recording Tach time even if you're not displaying it? It would be a real shame to not have this information if it turns out, 5 years from now, that Tach time really is the best indicator...

How can having more information available be a bad thing?

PilotKris

 

[dynonsupport]

It does. You can see it by either temporarily deselecting the Rotax pre-configuration, or alternately by going into the tach time edit screen under the global setup.

 

[easyhouse]

Are you telling me that if you select "Rotax" as your engine type, you can go into the Global Tach Time settings and enable it?   Or do you need to fool the Dynon that it's not a Rotax so you can see Tach Time?

 

[dynonsupport]

No. If you have the Rotax engine type selected, tach time is accumulating behind the scenes but is not displayed. If you go to the global setup, though, you can take a peek at the value by selecting 'set tach time' but not changing it.

Alternatively, if you temporarily deactivate Rotax mode, tach time will be displayed in the normal spot.

 

[dynonsupport]

Also, the ONLY thing choosing "Rotax" does is set up the custom oil temp gauge, limits the redline until the temperature comes up, and turn off the tach time (all per Rotax reccomendations). You're not messing up the EMS at all to just tell it "other." We didn't even have "Rotax" until the last update a few months ago.

 

[PilotKris]

No. If you have the Rotax engine type selected, tach time is accumulating behind the scenes but is not displayed.

!!THANKS!!

Personally, I think the REAL reason Rotax doesn't want us to use tach time is that they would sell fewer tachometers ('cause theirs doesn't record it).

PilotKris