The ol' hobbs or the new-fangled SFS timer? Or are both equally as good?



Hello Ron,

The hobbs meter is usually controlled by an oil pressure switch. It
basically tells you how long the engine has been running. This, in itself,
is somewhat useless information. Time In Service is what is used to keep
track of engine and airframe time. T.I.S. is defined as the time the
aircraft leaves the surface of the earth until it touches down again, the
time that the airplane is actually in the air. Most GPS's have this feature
and it is based on the time that the unit reaches a certain speed (usually
60 knots) until it slows below this speed. This feature is an extremely
close approximation of Time in Service.

Flight Time is defined as the time the aircraft moves under it's own power
FOR THE PURPOSE OF FLIGHT until it comes to rest again after landing. This
includes taxi time. Flight Time is the time that you put in your pilot
logbook. Hobbs time is a very rough approximation of flight time but
probably no better than estimating your taxi time and adding it to your
T.I.S.  There is no other use for hobb's time unless you're renting your
airplane out which is, as we know, not allowed for experimental aircraft
under almost all circumstances. Also, when you start the airplane up just to
taxi over to the gas pump, the hobbs runs. This time is neither Flight Time
nor Time In Service.

My opinion is that, in our non-rental environment, the hobbs meter is
probably not worth the expense, weight, panel room, installation effort or
electricity that it uses. A rough estimate of your taxi time added to the
SFS timer's number is probably closer to the "official" flight time
definition for your pilot log book. Assuming that the SFS timer retains a
cumulative total, it should be extremely close Time In Service. That should
be all you need.

Excuse my long winded answer to your question. Hope that this helps.

Bill Harrelson
5zq@cox.net   N5ZQ   LNC2  Dogwood Airpark,  Fredericksburg, VA