Starting Techniques

[BudG]

Howdy All,

Quick question. I'm taking my commercial in a C172. My instructor advised me on start up to just use the battery switch until after engine has caught. Then go ahead and turn on the alternator switch. I've always turned on both with the split master switch. Any pro's and con's to this techique?

Just so you don't think that I'm Hijacking this blog I normally fly MX7235 but with taking the commercial I don't want to abuse my "Bird" .

Thanks
CAVU2U
Bud

[AndrewK]

I've seen it and done it both ways as well.

From what I remember learning about it is using only the battery can make it easier to start the engine, especially if the battery is weak. This technique apparently reduces the resistance from the alternator (or something along those lines anyway) while cranking.
It is also a good way to check that the alternator is working because the ammeter clearly moves when you flip the switch on.

The disadvantage is that if the RPM is too high when you turn the alternator on it can send a spike through the system. Though I have heard as long as the RPM is low then it should be fine.

Would love to learn more about it as well from others.

[Andy Young]

Many airplanes don't have an alternator switch; it just comes on with the master. They seem to work fine. Also, never seen a car with an alternator switch, nor a motorcycle, and they all seem to work just fine. Generally, an alternator does not kick in and start generating current until about 1000 rpm, so it would follow that it's not adding any electro-mechanical resistance until then either. Thus, I doubt that it makes it any easier to start the engine with the alternator switch off.
So I see no advantage to leaving it off, other than being able to observe the ammeter needle jump when you turn it on, but you can get the same effect by turning on your landing lights once it's running. You can usually see the ammeter needle deflected to the right just after a start anyway, as a result of the battery being drawn down a bit by starting the engine. I do think you could get a spike by turning the alternator on after the engine is running, though as long as your avionics switch is off, it shouldn't hurt anything.
One last thought: There is a placard somewhere on my plane (or maybe it's a note in the POH) that says "Alternator not to be turned off except in an emergency" It might say specifically not to be turned off in flight; I can't remember. In any case, I don't see any advantage to waiting to turn it on until after starting.
Andy

[crbnunit]

The only thing leaving the alternator off would do is lighten the load on the battery as energizing the alternator field does draw a small amount of current. However, you can energize this field with a 9v battery so the draw shouldn't be enough to worry about. I would think the possibility of a voltage spike far outweighs the miniscule added current draw.

[BudG]

Thanks All,

I guess I'll stick with the same-o same-o for now. The Cessna that I performed the start has an avionics master, so the talking boxes were ok. I have an avionics master in my MX too. I like the soft touch of turning on the landing lights to get deflection in the amp meter.

CAVU2U

Bud

[cfiipilot]

Many airplanes don't have an alternator switch; it just comes on with the master. They seem to work fine. Also, never seen a car with an alternator switch, nor a motorcycle, and they all seem to work just fine. Generally, an alternator does not kick in and start generating current until about 1000 rpm, so it would follow that it's not adding any electro-mechanical resistance until then either. Thus, I doubt that it makes it any easier to start the engine with the alternator switch off.
So I see no advantage to leaving it off, other than being able to observe the ammeter needle jump when you turn it on, but you can get the same effect by turning on your landing lights once it's running. You can usually see the ammeter needle deflected to the right just after a start anyway, as a result of the battery being drawn down a bit by starting the engine. I do think you could get a spike by turning the alternator on after the engine is running, though as long as your avionics switch is off, it shouldn't hurt anything.
One last thought: There is a placard somewhere on my plane (or maybe it's a note in the POH) that says "Alternator not to be turned off except in an emergency" It might say specifically not to be turned off in flight; I can't remember. In any case, I don't see any advantage to waiting to turn it on until after starting.
Andy

Generally, an alternator does not kick in and start generating current until about 1000 rpm

I think you are thinking of a generator not alternator

[Andy Young]

Actually, I was thinking of alternators, though you're right that generators do that also. Less common with newer alternators, but they often do not start putting out current until about 1000 rpm. You don't tend to notice it on airplanes, as the RPMs usually jump over 1000 rpm right at startup until you reduce the throttle. Likewise, you don't see it on newer (fuel injected) cars, as they have electronic high-idle systems that get them above that threshold right away when they start cold. Find an old carbureted car (one with an alternator) and start it cold, without touching the gas pedal (thereby not activating the high-idle cam). If it starts, it will idle low, and the alternator light will quite possibly be on, until you blip the throttle.

Andy

[flyer]

In my Maule Operating Manual it says to turn on the Alternator after engine start, I think.


Dick