Alt Field On or Off during engine start and shutdown

[bucko]

Just picked up my Mooney from Avionics Shop yesterday and was going over the set up of new equipment installed (G-5 HSI, vacuum system removed, etc).

Was sitting with the shop owner and noticed that the Alt Field switch was in the off position. I asked him about it, and he said he was taught to always start and shutdown the engine with Field Alt switched off, to avoid problems with voltage surge or fluctuations at those times.

I have been starting and stopping with the Field Alt switch energized.  There's nothing in the Airplane Manual or checklists regarding this.  Of course, the 1965 M-20E manual isn't very long.

Is this everyone's normal practice?

Happy to add it into my Start Up and Shut Down checklists, if necessary or a good idea.

Thoughts? 

Thanks in advance for constructive inputs.

Bucko Strehlow

 

 

[ArtVandelay]

Modern electronic voltage regulators don’t have this problem. Back in the “good old days” voltage regulators were mechanical. You won’t have a problem if you forget to shut off the avionics either for same reason, in addition there are voltage regulators built in, that’s why avionics are capable of 12 or 24 volts power.

Find out what kind of voltage regulator you have.

[DCarlton]

I was told once that if you start the engine with the ALT switch engaged, it reduces the power available to the starter.  I've always wondered if that was true.  I start my engine with the ALT disengaged but not based on any known or strong rationale.  Curious about the majority opinion too.  

[hais]

My POH does say alternator on before start. 

[DCarlton]

Just now, hais said:

My POH does say alternator on before start. 

Interesting.  This thread should be full of opposing opinions.  :> 

[ArtVandelay]

I was told once that if you start the engine with the ALT switch engaged, it reduces the power available to the starter.  I've always wondered if that was true.  I start my engine with the ALT disengaged but not based on any known or strong rationale.  Curious about the majority opinion too.  

For the same reasons above, also false.

Does anybody worry about starting their cars with alternator engaged?

[DCarlton]

1 minute ago, ArtVandelay said:

For the same reasons above, also false.

Does anybody worry about starting their cars with alternator engaged?

So the switch would be there just to check operation in the event of a concern or during run up?  Shut it off to check for a drop in voltage?  

[EricJ]

4 minutes ago, DCarlton said:

I was told once that if you start the engine with the ALT switch engaged, it reduces the power available to the starter.  I've always wondered if that was true.  I start my engine with the ALT disengaged but not based on any known or strong rationale.  Curious about the majority opinion too.  

A tiny bit.    With the ALT switch on there will be current going through the alternator field, which won't be going through the starter, but it isn't much.   It's not likely enough to make a difference.

FWIW, I think the main purpose of the ALT switch is to be able to reset the voltage regulator if it gets hung up or tripped.   It can also help a bit with debugging problems.   For these reasons it's useful compared to not having one, but when you don't have one you just use the field circuit breaker instead.

[ArtVandelay]

So the switch would be there just to check operation in the event of a concern or during run up?  Shut it off to check for a drop in voltage?  

Yes, as Eric points out.

Side note, the G3X (designed to come on with the master) also has a voltage conditioner (GAD 27) which instead of just dropping the voltage down to a safe voltage also will boost the voltage when voltage drops during startup.

[DCarlton]

5 minutes ago, EricJ said:

A tiny bit.    With the ALT switch on there will be current going through the alternator field, which won't be going through the starter, but it isn't much.   It's not likely enough to make a difference.

FWIW, I think the main purpose of the ALT switch is to be able to reset the voltage regulator if it gets hung up or tripped.   It can also help a bit with debugging problems.   For these reasons it's useful compared to not having one, but when you don't have one you just use the field circuit breaker instead.

Discussions like this reinforce how great MS is as a forum due to the folks willing share their opinions, experience and knowledge.  You'd never get this level of discussion and interaction on a Facebook site.  

[DCarlton]

17 minutes ago, hais said:

My POH does say alternator on before start. 

So now I'm wondering why this would be the case.  It almost seems elective based on the discussion so far.  Not seeing a strong rationale to do it either way yet other than a later model POH says do it.  

[Utah20Gflyer]

I leave my alt field switch on full time, right or wrong it’s one less thing to think about or screw up somehow. 

[ArtVandelay]

My POH does say alternator on before start. 

That’s probably just a safety thing…somewhere down the checklist after starting it probably says to check for proper voltage, I know the J has this.
Edit: one version has turn off alt field in prestart checklist and then turn on alt field in the start checklist….clearly the POHs weren’t edited for consistency.

[GeeBee]

The M20R checklist has the alt field OFF for start, on after start. When you think about it, makes sense. You don't start with your avionics on, except for G1000s which have absorbing capability. Why thrash your alternator and VR?

[Guest]

By starting with the switch off, you get to see that the alternator fail waring system and ammeter all agree that it is off.  Turning the switch on allows you to verify system output.  

[ArtVandelay]

By starting with the switch off, you get to see that the alternator fail waring system and ammeter all agree that it is off.  Turning the switch on allows you to verify system output.  

Wouldn’t the voltage reading verify the system’s charging correctly?

[Will.iam]

15 minutes ago, M20Doc said:

By starting with the switch off, you get to see that the alternator fail waring system and ammeter all agree that it is off.  Turning the switch on allows you to verify system output.  

This is my rational.  After start I turn on the engine driven alternator first which at idle will extinguish the low voltage light but will not increase my battery voltage reading until I bring the rpm's up to about 1300 once I see the voltage come up from 25volts to about 26 or so I then turn on my 2nd Alternator that is belt driven and the voltage jumps up to 28volts.  easy and quick to verify both systems are working.

 

[Ragsf15e]

Also, those of us with older planes don’t even have an alt switch.  The field is energized with the master on (theres a cb, but doubt anyone pulls it for start).  Our alternators, starters, and avionics seem to survive just fine.  I think the alt switch is only really useful for troubleshooting or resetting the vr.

[Scottknoll]

The M20R checklist has the alt field OFF for start, on after start. When you think about it, makes sense. You don't start with your avionics on, except for G1000s which have absorbing capability. Why thrash your alternator and VR?

Must be serial number specific. I believe this AFM (3600) is for 29-0001 through 29-0199 except 0183. Mine is on for start. Looks like the AFMs for 0183 and 29-0200 and up all have the Alt Field off for start. I wonder what changed?

[LANCECASPER]

18 hours ago, Scottknoll said:

Must be serial number specific. I believe this AFM (3600) is for 29-0001 through 29-0199 except 0183. Mine is on for start. Looks like the AFMs for 0183 and 29-0200 and up all have the Alt Field off for start. I wonder what changed?

Nothing has changed in the engine set-up on Ovations.  If they changed the procedure on the newer airplanes I would go with the latest information.

On my ‘08 M20TN it says Alt Field Switch OFF and that makes sense to me especially if you’re cranking away on a hot start.

[kortopates]

It really makes no difference; especially with modern avionics. 

What does make a difference is forgetting to turn on the alternator after starting and you have a bit of weak battery,  then when you do turn on alternator late your battery is drawing a much larger current than normal.

[M20F]

The field switch is left over from generators when the switch is open the generator is not engaged which allows the engine to start with less drag.  

[N201MKTurbo]

6 minutes ago, M20F said:

The field switch is left over from generators when the switch is open the generator is not engaged which allows the engine to start with less drag.  

A generator voltage regulator has three contacts, the voltage regulator, the current regulator and the circuit breaker. The circuit breaker contact disconnects the output of the generator from the system until it is turning fast enough to charge the battery. If it didn't do this, it would short the battery when it wasn't moving. So, it is already disconnected and not putting a load on the engine. Electronic generator regulators do the same thing. The only thing you will be doing by turning off the field switch is reduce the battery drain a bit just like with an alternator.

[larryb]

I turn off the field before shutdown and turn it on after startup. The reason I do it is to preserve as much battery power for the starter as possible. Before start the voltage regulator sees a lower voltage than it wants. Therefore it commands full field current in an attempt to bring up the voltage. It has no idea the engine is not running. This field current is around 5 amps. 10 amps if you have dual alternators. Since airplane batteries are very small, I'd like to save all possible current for the engine start and not waste it. 

[Pinecone]

As has been mentioned, many old aircraft did not have a Alt switch, only the master.

The rational I heard for starting with it On was it put less load on the coupling as the engine spun up, than engaging it with the engine running at 1000 - 1200 RPM, especially if the battery is low.

That said, my POH says Off for start.