This has already been posted by someone else but the Rotax 915 manual now has a note explaining acceptable timing methods. Hopefully this can be applied to other engines too - I do not know how to contact anyone int eh factory to ask!
DEFINITION OF TERMS
OPERATING HOURS
Definition All of the maintenance intervals, such as the 100 hr. inspection and the engine TBO, relate
to the number of operating hours of the engine.
General Note • In aviation there are different measurement methods used as an acceptable means to
record operating time elapsed.
• BRP-Rotax does not mandate a specific method to record time elapsed as a basis for
maintenance and overhaul intervals.
• BRP-Rotax therefore relies on common practice and defers to the aircraft original equipment
manufacturer and/or local regulations.
In order to prevent misunderstandings and to ensure safety, BRP-Rotax accepts any of
the following methods to record operating hours:
• mechanical hour meters such as an oil pressure switch, etc.
• electronic hour meters such as BRP-Rotax ECU, TCU, FlyDat, etc.
• "flight Hours" entered in the aircraft's Journey Log or Technical Log.
• operating hours determined by "Hobbs" or "Tach" time (if the aircraft does not have
Journey or Technical Log)
NOTE
Maintenance and overhaul intervals are always dictated by the relevant method
used.
NOTE
The planned inspections to be performed at certain intervals are based on experience
from long test runs and field observations. They are intended as precautionary
maintenance measures in order to ensure continued trouble-free operation of
the engine.
