Originally Posted by czechsix
Just curious what you plan to use the switch for? Traditionally they are often used to run the Hobbs, but G3X (and presumably all the other EFIS systems) will track engine hours without needing the switch. This eliminates a couple pieces of hardware and a potential failure point that could result in dumping all your oil overboard...
I also wanted an independent means of tracking time. My RV already had a Hobbs meter in the panel but was unfortunately wired to start running whenever electrical power was turned on ( ie battery turned on). You can imagine the difference in “hours” that quickly developed. When I bought the RV from its builder the “tach” time was around 160+ however the Hobbs read over300.
When I checked the log book sometimes tach time was used, sometimes Hobbs and sometimes both. You could literally see the disparity grow over time. I decided to bring them back together. I purchased a new Hobbs meter, an oil pressure switch, ran the Hobbs to match current tach time, and installed both new switch and new Hobbs. Of course I noted in my log book the changes.
Now after a year and a half, time tracking difference is again occurring but it’s less than two hours difference for over 100 hours of use. This difference is due to how both are powered. Tach time begins at 650 rpm and Hobbs begins at sensing of oil pressure. For most Flight tracking I use Hobbs because it’s visible after engine shutdown. For log book entry I use both. However when someone else logs work on my RV I’ve noticed that they usually only record Hobbs - which incidentally was what occurred when the builder owned it. At least now when that happens the two are close to each other and time differences are immaterial.
I also discovered during this process that “Tach Time” could be changed by just an EIS entry. Hobbs time changes take a lot more time and effort (see above). That’s one reason complete tracking of work in a well maintained set of log books is important. Not all owners keep good log books. Not all aircraft mechanics will turn on power to check tach time when they see a Hobbs meter.
Jim Harris, RV7A, 2nd owner, N523RM
IO-360, Hartzel CS prop, Older Aerotronics built panel Dual GRT Horizon WS, GRT EIS, Garmin 340, 335, Dual 430s, TruTrak 385 A/P with auto level, FlightBox hard wired to WS, Dynon D10A with internal battery (backup EFIS)
Retired - Living the dream - And going broke!