J.P. Instruments » Tach Time and HOBBS

Tach Time and HOBBS

Introduction to Tach Time


Tach Time is calculated based on the average cruise RPM for your aircraft.  Tach Time is accumulated once per second, using the following calculation:
Tach Time = Tach Time + (Current RPM / Average Cruise RPM).
Tach Time is true (counting at 1:1 with HOBBS), when your current RPM is at the Average Cruise RPM setting in the EDM.
  • When your RPM is less than Average Cruise RPM (when taxiing, decent, etc), Tach Time will count slower than HOBBS Time.
  • When your RPM is higher than Average Cruise RPM (during takeoff, climbing, etc), Tach Time will count faster than HOBBS Time.
Calibrating Tach Time


When shipped from the factory, your J.P. Instruments EDM will be set with Average Cruise RPM = 2400.
If your aircraft’s average cruise RPM is not 2400 RPM, follow these steps to set the Average Cruise RPM value.
  • With the engine off, enter Pilot Program Mode by holding STEP and LF for 2 seconds.
  • Tap NEXT repeatedly until END? is displayed.
  • Tap the FACTORY button, or hold STEP and LF for 2 seconds.
  • Tap NEXT until AVERAGE CRUISE RPM is displayed
  • Hold the left 2 buttons for 2 seconds to allow modification of the Average Cruise RPM.
  • Tap PLUS or MINUS to modify the RPM value.
  • When the value is properly set, hold the left 2 buttons for 2 seconds to save changes.
  • Tap NEXT until END? is displayed.
  • Tap YES to restart the EDM.
Important: Be sure to set AVERAGE CRUISE RPM in the first 10 hours of operation.  After 10 hours of operation, the Average Cruise RPM setting will be locked down to prevent tampering with Tach Time values.

HOBBS Time is a time-based counter. Once activated, the EDM HOBBS Time counter accumulates 1 hour for every hour of elapsed time.

Tach Time is an RPM-based counter.  It is activated whenever the EDM senses RPM.  High RPM values cause Tach Time to count faster. Low RPM values cause Tach Time to count slower.

EDM730/830 – Version 3.33 and later

EDM930 – Version 1.20.477 and later

EDM960 – Version 1.20.477 and later

If you have an older EDM730/830, EDM930, or EDM960 and need Tach Time, contact JPI regarding returning your EDM to the factory for update.