Technically, neither...
In Part I, Definitions,
"Time in Service, with respect to maintenance time records, means the
time from the moment an aircraft leaves the surface of the earth until
it touches it at the next point of landing."

As a practical matter, for Part 91 operations, I've never heard of
anybody using anything but tach time.

Rich



O. Sami Saydjari wrote:
OK, dumb question. For 100-hour inspections (or 50 hours until oil
change), is this measured in Hobbs time or Tach time?

-Sami
N2057M, Piper Turbo Arrow III