Master Switch

I take it back. I guess on some airplanes we ARE up to the basic
automotive level of technology.

I've never owned a Cessna, and the Pipers I've owned had no such
cutout.

Michael

"Icebound" wrote in message
...
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.

By 1981, though, the 172P POH says: "The primary bus is on anytime the
master switch is turned on, and is not affected by starter or external power
usage. Both bus bars are on anytime the master and avionics power switches
are turned on."

Perhaps Cessna decided that the electric-cutout switches created an
unnecessary risk of accidental loss of electricity.

--Gary

The alternator windings are energized by voltage from the battery, creating
a magnetic field that will begin delivering electricity as soon as the
engine starts. These windings are connected directly across the battery
through the BAT/ALT switch. When you leave the ALT side of the switch off,
these windings are no longer connected across the battery and do not present
a load to it. When the engine starts and you turn the ALT side on, you get
the electricity you need. My advice is to leave it OFF until needed.

Bob Gardner

"Lakeview Bill" wrote in message
.. .
On an aircraft with separate "battery" and "alternator" switches, should
you
turn on BAT and leave ALT off during engine start, or should you turn them
both on prior to start?

A little bit of the "why" on this would be nice also...

Lakeview Bill wrote:
On an aircraft with separate "battery" and "alternator" switches, should you
turn on BAT and leave ALT off during engine start, or should you turn them
both on prior to start?

With a 1995 Maule MX-7-160, both go on to start.

A little bit of the "why" on this would be nice also...

'Cause there's a little placard beside the switch that says to do it that way.

George Patterson
Why do men's hearts beat faster, knees get weak, throats become dry,
and they think irrationally when a woman wears leather clothing?
Because she smells like a new truck.