Amp meter indicating a drop

[Dayron Nunez]

Hi guys, I'm flying an old 1957 C172 with a continental O300 not too fancy and not many things installed on it, I'm still getting familiar to this aircraft.

A couple days ago I flew at dusk for the first time with it, hence I used more current or power draw which ever way it should be called, only electrical equipment that was ON that I can remember was: Transponder, 1 Radio, Nav Light (red and blue fixed in wing, and white in tail) an panel light which only has a very shy and small yellow light, when I turned the taxi light on the AMP meter drop from the middle to about 1/3 and the red GEN light bulb came on, I tried different combinations with all those equipment but NAV light and Taxi always dropped until I was pretty much over 1700 RPM.

My questions are, is that normal for this kind of aircraft? I know when you are on the ground you usually get a drop or some negative number but I thought it was less, and also not getting back up until at least 1700RPM maybe is too much?

I would try to check whats the battery condition and alternator, but I never had any issues at daytime.

Second question would be how would you recommend to proceed if long taxi time is expected? I usually don't operate other than daytime, and when I do so is on a rented newer aircraft that have not give such issues.

Thanks for any advise.

 

[mjburian]

We had something similar in our 172N recently. Turned out to be a bad alternator that then took the voltage regulator with it. Not sure if your situation is the same, but I’d have an A&P take a look there to start.

 

[GRG55]

Doesn't sound unusual if the airplane still has a generator instead of an alternator.

A generator needs to turn at a higher RPM than an alternator before the output is enough to start charging/offsetting the drain from the battery. Generator output falls off very rapidly at lower RPM. However, 1700 RPM seems higher than it should be ; could be the commutator brushes are worn and it needs servicing.

 

[Dayron Nunez]

We had something similar in our 172N recently. Turned out to be a bad alternator that then took the voltage regulator with it. Not sure if your situation is the same, but I’d have an A&P take a look there to start.

Thanks, yes that's kind of what I thought either alternator or battery not holding enough or something like that, I'm going to get the annual done so I'll get that checked, but I would like to understand what's going on and if it is normal which doesn't seems like.

 

[Dayron Nunez]

Doesn't sound unusual if the airplane still has a generator instead of an alternator.

A generator needs to turn at a higher RPM than an alternator before the output is enough to start charging/offsetting the drain from the battery. Generator output falls off very rapidly at lower RPM. However, 1700 RPM seems higher than it should be ; could be the commutator brushes are worn and it needs servicing.

Thanks, to be honest this is the first aircraft that I take care off you know more than just renting and flying as a private pilot, and I have learn a few things maintenance related and want to learn more, but I haven't checked about the generator ever before, I actually thought it still had an alternator but you are right it should be just a Gen then, and that's why it has a red Gen bulb in the panel. If I'm not wrong.

 

[Dayron Nunez]

The Gen indicator, It looks something like this, but is not exactly that one

 

[Skip Miller]

Nav Light (red and blue fixed in wing, and white in tail)

red and green? -Skip

 

[Dan Thomas]

Those old airplanes typically had 25 or 35 amp generators in them, and they had to be turning pretty fast to make that. A 4509 landing/taxi lamp is a 100 watt affair, meaning it draws around 7 or 8 amps. If the other stuff is drawing most of the generator's limited low-RPM capacity, turning on the taxi light could easily overwhelm the generator. Old radios draw more than newer units, too.

There are good reasons why 60-amp alternators became common in the 1960s. One of those reasons is that they will generate lots of amperage even at idle.

 

[iflyvfr]

The old generators become more of an issue at night, when you have lights on inside and out and when it's time to land at low RPMs you suddenly want landing lights too. I converted my O300 172 to an alternator, and power issues were a thing of the past. I really disliked the limitations of a generator.