Alternator on or off during engine start and stop?

CC268

CC268

Final Approach
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CC268
1965 Cherokee 140 - POH doesn't say anything about when to turn on the alternator. I've read about 8 pages over on the Vans forums about alternators in general. Seems like a mixed bag (although the most "scientific" posts seem to lead me to believe you should have the alternator switch on before start and after engine stop. Some people say just always leave the switch on and never even turn it off.

What do you experts say?
 
Don't think it matters much but I leave it off for engine start then verify amps after turning the alternator on. I'll typically switch it off before pulling the mixture on shutdown. I usually switch it on at low idle and have to move the throttle up to about 1,000 rpm to verify amps.

The important steps are to turn the avionics master on after energizing the alternator and to turn the avionics master off before de-energizing the alternator.
 
My flight school’s checklists universally say to start with only the BAT side ON, and to turn ON the ALT side only after the engine is running.

However, the "starting engine" procedures in the POHs for the three Cessna 172 models on the line say differently. Two refer to the Master Switch on the whole, and one literally says both alt and bat:

“Master Switch - ON” for the 172P (pg. 4-8)
“Master Switch - ON” for the 172R (pg. 4-12)
“ALT and BAT” for the 172S (pg.4-12)
 
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For my Sundowner it's master on, avionics and alternator off. Once started it's alt on then avionics.
 
The only reason I can think of to not turn the master/alt switches on/off at the same time is in the case of the alternator over-charging and needing to disable it before you can land to get it serviced. You don't turn your car's alternator no separately do you? Your Cherokee alternator is probably the exact same design that Chrysler used in their cars for a very long time, it's no different.

The practical purpose for separate battery/alt switches in aircraft like ours is to be able to troubleshoot electrical problems or stop something that's overheating from catching on fire in flight. It gives us an extra couple of troubleshooting options since we can't just pull over like a car can.

The big thing everyone always points at is the idea that a voltage spike could harm avionics. I personally don't believe that any spike that's likely to happen can harm modern avionics... maybe some kind of technology that hasn't been used for decades but anything remotely modern? If these radios are that delicate then what's the point of all this stuff getting certified and approved by the FAA at all? You're telling me a $3,000 radio or a $10,000 navigator with years of testing and compliance with all the FAA requirements doesn't have $2 worth of basic built-in protection from this sort of thing? Come on now. If anyone has seen this really happen from flipping a switch at the wrong let me know what radio that was so I don't buy one.
 
mscard88 said:
Go with what the POH states?
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This.

The 172/182 checklists and POHs I'm familiar with all positioned both switches together for starting and shut down.

When I was learning the Bonanza, the checklist had two steps for turning the alternator on - before and after start, but no explanation. Being new to the aircraft, I wasn't satisfied and so dug around in the POH until I found that if using a start cart, start with the alternator off, and the turn it on once engine is running (otherwise, just start with alt on). We clarified the checklist.

In general, it might be a little smoother on the electronics (those that are on during engine start) to start with the alternator on. This might provide gentler voltage increase as the alternator comes up to idle speed. If you switch it on with engine running, there's a sudden change in voltage - possibly a spike, too.

Another consideration is state of battery and expected ease of starting. When you turn on the alternator switch you have an additional load on the battery, and additional drag on the starter to rotate the alternator. Should never be a problem with a healthy engine.
 
It's important to realize what's actually happening here. The alternator isn't turned "on or off"... it runs whenever the engine runs as it's driven by prop shaft (usually a belt just behind the prop). The switch controls if any electricity from your alternator makes its way to the main electrical bus or not.

In this sense there's a concern that at start or shutdown you could have electrical fluxuatuons that could damage sensitive electronics. Simple electrical motors (like those in your turn coordinator) aren't that fussed. As such that's why you usually have an "avionics" master that turns radios and computers on after engine start and off before engine shutdown. Things that tend to be sensitive to electcial disruptions are usually placed on this additional "master" that sits past your main electrical bus.
 
If I recall correctly there are (were?) some (older?) aircraft with a split master with manuals advising only the battery to start. Just as there were aircraft with manuals advising starting on only one mag at a time with a separate starter button.

Beyond those examples, the POH of most (all?) piston aircraft simply say Master-On. Beyond that it's a matter of technique. The only good rationale I've heard for doing the split master start when not called for in the manual (other than, of course, "that's what I was taught to do") is that it allows for an alternator check right after engine start. Start on the battery, get it running, watch the ammeter move when the alternator comes on line.
 
rocketflyer84 said:
It's important to realize what's actually happening here. The alternator isn't turned "on or off"... it runs whenever the engine runs as it's driven by prop shaft (usually a belt just behind the prop). The switch controls if any electricity from your alternator makes its way to the main electrical bus or not.
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Yes, it's true that the alternator is always turning when the engine is turning. But the electrical situation is a little different. The alternator produces a variable amount of current. The amount is dependent on the current sent by the controller to the alternator field winding. More current to the field winding = more current being generated = more drag on engine. The controller monitors system voltage and adjusts accordingly. Turning the alternator master off turns the controller off, which cuts current to the field winding, which "shuts off" the alternator.

Now as I think about it, if the START position of the ignition switch bypasses the alternator controller, then the alternator is "off" (electrically) during starting. Hmm.
 
CC268 said:
1965 Cherokee 140 - POH doesn't say anything about when to turn on the alternator. I've read about 8 pages over on the Vans forums about alternators in general. Seems like a mixed bag (although the most "scientific" posts seem to lead me to believe you should have the alternator switch on before start and after engine stop. Some people say just always leave the switch on and never even turn it off.

What do you experts say?
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For those planes with a split master switch, (ALT/BAT), having both sections ON means that the battery load is the starter windings and the alternator windings; With BAT on and ALT off, only the starter load is placed on the battery. Best practice for those planes is to have only the BAT side on.

Bob
 
mscard88 said:
Go with what the POH states?
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My OP states there isn't anything in the POH specifically about when to turn it on which is odd

Maybe I can look online at other Cherokee POHs...

Anyways probabaly not a big deal either way. I should have just looked around more.
 
bobmrg said:
For those planes with a split master switch, (ALT/BAT), having both sections ON means that the battery load is the starter windings and the alternator windings; With BAT on and ALT off, only the starter load is placed on the battery. Best practice for those planes is to have only the BAT side on.

Bob
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Bob, what you said above is absolutely correct but irrelevant. The power required to start the engine is magnitudes more than that required to excite the alternator windings.

If your battery and starter systems are so marginal that starting or not starting hinges on an extra amp or two, you probably need a new battery or starter, elec cable maintenance, or some such.

Having said that, when a split switch is installed, I always turned the battery switch on after the engine was running. Why waste a few amps? But doing so is unneccesary. -Skip
 
midlifeflyer said:
Just as there were aircraft with manuals advising starting on only one mag at a time with a separate starter button.
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FWIW, this is less "advice" and more "only one mag has an impulse coupling and you only want to start on that mag." Some aircraft with keyed mags only have a single impulse coupling, but the keyed switch is designed to ground-out the non-impulse mag when turned to "start." When you have pushbutton start, you have to limit the mags yourself.
 
WillFly4Food said:
Now as I think about it, if the START position of the ignition switch bypasses the alternator controller, then the alternator is "off" (electrically) during starting. Hmm.
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It doesn't. The switch has nothing to do with the alternator. All it does is fire the starter's contactor.

In cars the whole accessory circuit, including the alternator, is disabled during the start. Airplane electrics are different. Designers try to limit the failure modes, and running alternator control though the ignition switch is just introducing another failure point.
 
bradg33 said:
FWIW, this is less "advice" and more "only one mag has an impulse coupling and you only want to start on that mag." Some aircraft with keyed mags only have a single impulse coupling, but the keyed switch is designed to ground-out the non-impulse mag when turned to "start." When you have pushbutton start, you have to limit the mags yourself.
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My use of the word "advises" was ill-advised.
 
Off. in fact, my alternator is actually a generator and also the starter. It won't do both at the same time.
 
mscard88 said:
Go with what the POH states?
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You haven't actually read a Cherokee 140 manual have you?

It is only slighty better than the Cruisemaster Manual which the entire landing section states something like "The Cruisemaster lands conventially and is easy to land. Other pilots will be impressed with your landings"

Brian
ASEL/CFIIG
 
brcase said:
You haven't actually read a Cherokee 140 manual have you?

It is only slighty better than the Cruisemaster Manual which the entire landing section states something like "The Cruisemaster lands conventially and is easy to land. Other pilots will be impressed with your landings"

Brian
ASEL/CFIIG
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I wish I still had it- I had manual for a 1950's vintage 182 that read that way.
 
Split Master here...
1. BAT ON, ALT OFF, No B+ is going to the alternator. Gives maximum power to starter.
2. After Engine Start, observe ammeter and voltmeter while raising flaps. Ammeter shows a serious discharge while flaps are in transit. Voltmeter shows battery voltage, ~12.8 vdc.
3. ALT ON - Ammeter centers or shows charging. Voltmeter jumps from 12.8 to 13.7 (typical on my plane).

I use this method because it easily shows if the alternator, ammeter, and voltmeter are working, and even though it's a small help (as Chip pointed out), I want every bit of available power cranking the engine.
 
One thing that hasn't been mentioned is voltage spikes. When you turn on capacitance circuit you get a spike and when you turn off inductor circuit you get a spike. These spikes can damage circuits and also cause sparks and cause a fire. Follow the POH.

Someone erroneously turned off a nearby hydrolectric generator all at once and the whole generator was cooked blue, total loss. Im not saying that will happen to your airplane, just that there may be a reason to turn it off and on when specified in the POH.
 
Most of the airplanes I currently fly only have a master, not separate battery/generator switches! Whatever shall I do?
 
midwestpa24 said:
Most of the airplanes I currently fly only have a master, not separate battery/generator switches! Whatever shall I do?
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Have fun flying. Those old airplanes started out with generators and many still use them. A generator is a different animal from the alternator. Turning the master on doesn't energize the field of a generator. The generator starts turning when the engine starts, and it generates a bit of power on its own due to residual magnetism in the field pole shoes, and that bit of output goes to the regulator and is then fed back to the generator's field. It basically powers itself up. There's a reverse-current cutout relay in a generator regulator that will not close and load the generator until the output voltgae is at least equal to the battery voltage, so there's little load on the engine and none on the electrical system during start. The disadvantage of the generator, besides having large commutator brushes that carry all the output and that wear out sooner because of that, is that the generator needs a lot more RPM to power your airplane, so at night you could get a dead battery taxiing around at 700 RPM with the taxi light on. And generators are far heavier than alternators.
 
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Long and short, from what I gathered from the thread I made that was linked to...


Though it isn't a significant difference, starting with the alternator OFF has some slight benefits, where as starting with it ON has between zero benefits to some slight disadvantages.
 
CC268 said:
1965 Cherokee 140 - POH doesn't say anything about when to turn on the alternator.
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This seems confusing. Looking around, I only see Cherokee 140s with one standard looking toggle switch labeled "Master."

@CC268, does your 1965 Cherokee 140 have a "split" master switch like a Cessna (ALT on one side and BAT on the other)? ... or otherwise have a separate alternator switch?
 
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My '67 140 has one master switch... it is either "Power" or "NoPower"... each radio has a switch, so they are either "on", or"off"... The Alternator field has a fuze, that screws in so I do not unscrew it other than to check that it is still good, during the annual inspection... it also has a Push to Start switch that is not the Mag switch.... Yes I know, OLD stuff.... but old stuff works well and having gray hair, I like old stuff...
 
register@teamandras.com said:
This seems confusing. Looking around, I only see Cherokee 140s with one standard looking toggle switch labeled "Master."

@CC268, does your 1965 Cherokee 140 have a "split" master switch like a Cessna (ALT on one side and BAT on the other)? ... or otherwise have a separate alternator switch?
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Actually since you brought it up...my Cherokee was essentially "rebuilt" in 2010...the panel was totally redone during that time to give it the standard six pack. Brand new alternator, engine, etc...the whole works. So when they redid the panel apparently they decided to do a separate alternator toggle switch next to the master.
 
Agreed, been beaten to death many a time. (but it sucks that the POH doesn't say)

And while you will always get pushback no matter how you do it, the correct procedure follows logic: do you need the alternator on for a start? No.

Similar to avionics master. Should you leave all avionics on so when you flip the master on to start, all radios light up? Logically: no.

The design engineers gave you switches for a reason. Use them wisely. :)
 
James331 said:
Long and short, from what I gathered from the thread I made that was linked to...

..where as starting with it ON has between zero benefits to some slight disadvantages.
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One benefit to starting with it on: you can't forget to turn it on after start.
 
Dan Thomas said:
One benefit to starting with it on: you can't forget to turn it on after start.
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If you need that level of dumbing down, you probably need to be in a group home.

ralph-eating-paste.gif
 
That's like saying you should never use a gust lock because you can forget to take it off before next flight.
 
cowman said:
You're telling me a $3,000 radio or a $10,000 navigator with years of testing and compliance with all the FAA requirements doesn't have $2 worth of basic built-in protection from this sort of thing? Come on now. If anyone has seen this really happen from flipping a switch at the wrong let me know what radio that was so I don't buy one.
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Bwaahhahhhahahahahah... you haven't spent any time in new product development land...
 
Dan Thomas said:
One benefit to starting with it on: you can't forget to turn it on after start.
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James331 said:
If you need that level of dumbing down, you probably need to be in a group home.
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Love your level of PC-ness, James! :) (really)
I was planning on replying something like "that is moot because you get the same benefit from following checklists and procedures". But you beat me to it and with a good zinger! PoA to the rescue! :D
 
My 1970 Cherokee 140 has a split switch and ammeter... no voltmeter.

After reading this thread, I'm inclined to start the plane on battery only.

My primer was disconnected a long time ago... so I prime with throttle... and the crank while priming thing seems like it'll serve me well in addition to this switch stuff.

Learn something new every day.
 
ircphoenix said:
My 1970 Cherokee 140 has a split switch and ammeter... no voltmeter.

After reading this thread, I'm inclined to start the plane on battery only.

My primer was disconnected a long time ago... so I prime with throttle... and the crank while priming thing seems like it'll serve me well in addition to this switch stuff.

Learn something new every day.
Click to expand...
A digital voltmeter from pepreillyzone plugged into the cigar lighter socket makes a fine non-tso'd installation to monitor a Piper's modern 1954 designed charging system.
 
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