"EDR" wrote in message
...
In article ,
wrote:

The hobb's is a clock that is started and stopped by a engine oil
pressure switch on most aircraft.

Some are connected to the battery Master switch. Hobbs starts when the
Master is turned on, stops when it is turned off.

And on some retracts it's connected to the squat switch so that it only
measures
"flight time." This saves you on maintenance a bit.

But the heart of the matter is that the hobbs is just an electrically driven
hour meter.
It has two terminals (hot and ground) and whenever voltage is present (from
whatever
source) it runs.

On my plane it is a seperately fused line (not through the master bus)
through the
oil pressure switch. This for rentals cuts down on any clock shenanigans
by running
with the master off (so a lot of rentals have it this way). My favoritle
rental was the
170 I used to fly that had the hobbs installed in where you could actually
see the
slide on contacts to it and pull it off if you were so inclined.