Master Switch

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...

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?

I leave alt off during start, to prevent a power surge from going
through the alternator. Part of my checklist in several places is to
ensure that the switch is set on both before takeoff (it's easy to forget!)

Jose
--
Nothing takes longer than a shortcut.
for Email, make the obvious change in the address.

Jose 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?

I leave alt off during start, to prevent a power surge from going
through the alternator.

What would be the source of this surge? If it's such a problem, why don't
automobiles have alternator switches?

The only reason I could come up with having an alternator switch is to make
it easy to pull offline in case of a problem.

On Mon, 18 Jul 2005 12:57:42 GMT, "Lakeview Bill"
wrote in
::

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...



Of course, you should follow the starting procedure stated in the POH.
That usually has you turning on the alternator field winding after the
engine starts. Energizing the field winding causes about a 5 ampere
current draw. That five amps is better used for turning the starter
motor until the alternator is able to spin fast enough to produce
useable electrical output current.

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?

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

What does the POH say? The instructions (and posssibly some insight)
will be there.

As a rule, the ALT is left off until the engine is running to avoid
power spikes.

I was told the same thing from a UPS pilot friend of mine. While the
current draw to energize the alternator field is minimal it could make
a difference on a cold day(?) Just don't forget to hit the ALT switch
after engine start and verify the draw on the amp gauge.

What would be the source of this surge? If it's such a problem, why don't
automobiles have alternator switches?

I don't know. But there is an avionics master also, and it is off when
I start the engine for the same reason. Cars don't have that either.

Jose
--
Nothing takes longer than a shortcut.
for Email, make the obvious change in the address.

"Larry Dighera" wrote in message
...
On Mon, 18 Jul 2005 12:57:42 GMT, "Lakeview Bill"
wrote in
::

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...



Of course, you should follow the starting procedure stated in the POH.
That usually has you turning on the alternator field winding after the
engine starts. Energizing the field winding causes about a 5 ampere
current draw. That five amps is better used for turning the starter
motor until the alternator is able to spin fast enough to produce
useable electrical output current.


1. Does not the starter circuitry include a "contactor" solenoid that pulls
off the power to most everything else during the time that the starter is
engaged?

2. The generic 1976 C172 POH, Section 7 says "Normally, both sides of the
master switch should be used simultaneously". So that's how I start a 1976
172. I am sure another POH may recommend something else.

3. How does pulling the BAT half of the master during engine-running,
reconcile with the caution about pulling battery power on a boat/automobile?
( There ...boats/cars..., isolating the battery from the alternator while
running, is said to cause the alternator to blow diodes and fail. )

"Jose" wrote in message
...
What would be the source of this surge? If it's such a problem, why
don't automobiles have alternator switches?

I don't know. But there is an avionics master also, and it is off when I
start the engine for the same reason. Cars don't have that either.

Yeah, they do. it's just automatic. Same may go for the alternator.

What does the POH say? The instructions (and posssibly some insight)
will be there.

The instructions will be there, They may or may not be optimal. They
are unlikely to offer any insight.

As a rule, the ALT is left off until the engine is running to avoid
power spikes.

Correct procedure (field off until engine start), wrong reason. You
are not going to hurt your alternator field with a voltage spike
produced during starting - it's not at all delicate. Neither will the
alternator produce much in the way of a spike at starting RPM's. I've
heard that reason passed on from CFI to student for years, and it's
just an aviation OWT.

The real reason is far more prosaic - the alternator field coil draws
3-6 Amps (depending on the alternator) that are better used driving the
starter motor. Aviation batteries are sized to be just barely big
enough to start the engine to keep the weight down, and in terms of
quality and design they are the worst batteries available, far inferior
to any reasonable quality lawn tractor battery. As a result, available
starting power is often marginal and anything done to reduce it is a
bad idea.

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.

Michael