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
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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.

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

"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.

"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.

My '97 Chev. Lumina has an avionics cut-out switch for starting. Come to
think of it, so does my '96 GMC Sonoma pickup.

"JohnH" wrote in message
...
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?


Many do but it is automatic. They don't power the buss for a second or two
after start to allow the alternator circuit to stabilize.

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.

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.

It's a great little nugget of knowledge to have if you've let your
battery age to the point where it just barely starts you up. You can
get one or two more starts by knowing to keep it off during the crank.
Worked for me at a time of great need!

wrote:
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.