Master Switch

[Lakeview Bill]

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...

[Jose]

> 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.

[JohnH]

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.

[Larry Dighera]

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.

[Blanche]

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.

[[email protected]]

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.

[Jose]

> 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.

[Icebound]

"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. )

[Gig 601XL Builder]

"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.

[Michael]

> 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

[Maule Driver]

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.
>

[Stubby]

Michael wrote:

>>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.

But the _real_ reason is that when the field winding is energized
(current flowing), the alternator is bogging down the airplane engine.
Generating electricity requires lots of torque. And, the total amount
used to start the plane must be pumped back into the battery. That is
the "short term spike" that people confuse with some sort of static
discharge. Once the battery has been recharged, the torque required
tapers down to just enough to run the plane, but not the starter.

Turning off avionics master, the Pitot heater, and excess loads of that
nature also decrease the mechanical load during starting.

[nrp]

This'll add some more fuel to this fire -

There are three differences between car and airplane electrical systems
that apply here -

1) Only aircraft have a master switch relay - which may stutter if the
battery is low, and
2) Only aircraft have an alternator stator circuit breaker that if
tripped causes the alternator to disconnect abruptly.
3) Aircraft also have a circuit breaker and switch on the alternator
rotor.

Given number 1, if the avionics are on, the starter could briefly still
be connected to the avionics with no battery in the circuit allowing
the starter collapsing magnetic field to put a helluva positive voltage
spike into the avionics stack. That's why everyone is supposed to turn
off all electronics before engaging the starter.

Cessnas have a starter circuit activated drop out avionics relay, but
the avionics relay mechanical timing just might still allow the spike
to get to the avionics. Only if the battery stays in the circuit, does
the collapsing starter field pulse stay confined to arcing the starter
contactor.

Remember that there are no faster fuses than silicon cuircuits. I
would guess most 14 V electronic stuff could withstand say 20 V, but
certainly not 100 V

Number 2) causes the current in the alternator stator to collapse
putting a major (negative I think....) voltage spike into the
alternator regulator. If the regulator is solid state, it could be
wrecked. If it is mechanical, it could probably withstand this.

I don't think Number 3) presents a theoetical problem to the system.

You don't get voltage transients when connecting up inductors. You get
them in spades when you quickly interrupt the current flow to them.

I'll bet Jim Weir has some thoughts on this.

[Victor J. Osborne, Jr.]

Install a light so display when the alt system is not charging. Inexpensive
(as aviation goes) and bonus is the light comes on in flight when the alt
goes off line. Been there. Nice to know before the battery is exhausted.

--

Thx, {|;-)

Victor J. (Jim) Osborne, Jr.
> wrote in message
oups.com...
>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.
>

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...
>
>
>

[Michael]

> But the _real_ reason is that when the field winding is energized
> (current flowing), the alternator is bogging down the airplane engine.
> Generating electricity requires lots of torque.

Actually, I think you will find that at the RPM's involved in engine
start, negligible amounts of electricity are being generated so the
bogging down of the engine is negligible compared to the loss of power
to the starter.

Michael

[George Patterson]

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.

[Casey Wilson]

"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.

[George Patterson]

Victor J. Osborne, Jr. wrote:
> Install a light so display when the alt system is not charging.

Not only does the Maule have such a light, but the fuel gauges are on the field
coil line. Alternator goes off line, the fuel gauges read empty. Really gets
your attention!

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.

[Tauno Voipio]

Icebound wrote:

> 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. )

You should not do it, unless in an emergency situation.

The battery buffers the voltage spikes originating
in different electro-magnetic equipment on board. The
avionics are in danger if the spikes are not discharged
into the battery.

Pretty often the master switch halves are interlocked
so that it's not possible to keep the alternator on
without the battery.

The alternator diodes are not the only endangered
semiconductor components. There are many other
diodes, transistors and integrated circuits feeding
from the power bus.

HTH

--

Tauno Voipio (CFII, avionics engineer)
tauno voipio (at) iki fi

[Icebound]

"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.

[Michael]

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

[Dave Stadt]

"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.

[Gary Drescher]

"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