Hobbs meter

J

Jeff Nunan

New member
I have a Garmin 250 GPS which records flight time, that is, once the wheels leave the ground to when they touch down. I have noticed this recorded flight time is longer than my Hobbs time.
Doesn't the Hobbs begin recording as soon as the prop turns until it ultimately stops turning and therefore should indicate a longer period? I frequently observe a 10 to 15 minute discrepancy depending on taxi time, time to take-off between the two instruments. Is the Hobbs in need of adjustment or ?
Thanks for your help.
 
Jeff Nunan said:
I have noticed this recorded flight time is longer than my Hobbs time.
Doesn't the Hobbs begin recording as soon as the prop turns until it ultimately stops turning and therefore should indicate a longer period?
Click to expand...

well, it depends on what you mean by "Hobbs" time. A Hobbs meter is a meter which records actual clock time, activated by a switch. The most common installation starts the Hobbs meter with an oil pressure switch, so it starts recording time as soon as the engine starts. Other installations connect the hobbs meter to the master switch, or sometimes to an airspeed switch. It that's what you're referring to as Hobbs time, then something is wrong, either your hobbs meter or your GPS is lying to you.

Now, if by "hobbs" time, you mean the numbers on the little window in your tachometer, then no, there is no problem. That is not Hobbs time, it's tach time and isn't actual time. those numbers are driven by the rotation of the prop, and match actual time at some RPM, usually a cruise rpm. If you sit on the ground idling for an hour at 1000 RPM, the tach time may only register a half hour. SO depending on how you run your engine it is entirely possible for the tach time for a flight to be less than actual flight time.
 
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