Master Switch

"Michael" wrote in message
ups.com...
....snip...

It is the avionics master is off during engine start to keep spikes
from getting to the avionics. I suspect that's the cause of the OWT.
In airplanes lacking an avionics master, optimal procedure is to shut
down the avionics prior to engine shutdown and turn them on only after
engine start. In cars this is accomplished by means of an automatic
contactor, but in the aviation world we're not up to that level of
technology.


I am reading from the generic POH for 1976 Cessna 172, section 7, Airplane
and Systems Descriptions, Electrical System:

quote:
Both sides of the bus are on at all times except when either an external
power source is connected, or the *starter switch is turned on*; then a
power contactor is automatically activated to open the circuit to the
electronic bus. Isolating the electronic circuits in this manner prevents
harmful transient voltages from damaging the transistors in the electronic
equipment.
:unquote.