Originally Posted by iahflyr
Thank you rickair.
I have just a few specific follow up questions (since you know this stuff).
1. When will the hobbs start counting and when will it stop? (i.e, when the master is turned on/off, when the ignition is turned on/off, or when the engine is running (oil pressure in the green)).
2. You said an a/c tach can be calibrated to a normal operating rpm, say 2300. So if you go less than 2300, the tach time slows down. But, if your RPM's are above 2300, does the tach time count faster than actual time??
Thank you
Some operations like to rip you off, and will have the hobbs wired to the master...but some pilots use this to their advantage on cross-countries in non-radio / non-xponder airspace...
Normally, the hobbs should be running only when oil pressure is present, ie the engine is on. You can turn on the master and check atis, clearance, etc before start.
If the engine runs faster than the calibrated RPM, tach time would run faster than actual clock time...but since this normally only happens for a few minutes during takeoff and climb, tach time usually totals less than hobbs due to the VERY low rpm on taxi and descent.