Do I need a larger alternator?

[4RNB]

My plane might be near completion of new avionics and Quasar LED landing lights. On test flight, the 60 amp alternator did not generate sufficient power when the 13 amp landing lights were on in landing mode. 1974 C 172M. shop was doing some tests but one of the possible solutions is to up size the alternator. Have you heard of this being done?

I'm not seeing particularly large power requirements for anything else, especially continuous use.

Link is for one multi engine craft showing all of his power requirements: http://myplace.frontier.com/~tokosha/rv7/vaf/load.pdf

Thank you

 

[JOhnH]

What did the Quasar landing lights replace?
To my limited knowledge, 13 amps seems like an awful lot for LEDs.
(I could not read your attachment. Too much small print and to wide for my phone).

 

[GeorgeC]

Agree that 13 amps sounds high; 190W / 28V should be less than 7A.

 

[4RNB]

 

[4RNB]

What did the Quasar landing lights replace?
To my limited knowledge, 13 amps seems like an awful lot for LEDs.
(I could not read your attachment. Too much small print and to wide for my phone).

The attachment showed a max load of 40 amps for a multi engine aircraft.
Quasar wing tip lights are new, not replacement.

 

[tsts4]

Something else is going on. A 60A alternator shouldn't have any issues powering your 13+A load. I have an all-electric airplane and if I turn everything on and transmit simultaneously, my total load is around 20A and my 60A alternator has never had an issue.

 

[4RNB]

Something else is going on. A 60A alternator shouldn't have any issues powering your 13+A load.

Thanks, to be clear though, it is for the normal base load PLUS the 13 amp for landing lights.

 

[Bell206]

Have you heard of this being done?

Yes, but there are other checks to perform prior to upgrading the alternator as that can cause additional issues. Usually prior to adding/changing electrical equipment one does a quick check of the current system electrical load and can be done mathmatically. The target is for the full electrical load to be below 80% of the generating capability. However without the shops load calculation cant offer much more. Many things can affect how the system handles load but I know of only a few 12v 1974 172s that would need an alternator larger than 60amps unless you're running full glass, A/C, etc.

 

[tsts4]

So if 80% is the target max load, for a 60A alternator that's 48A. Even with full glass, A/P, etc, 48A seems to me to be plenty of capacity.

 

[Magman]

I agree with Bell’s comments.

AC43.13-1B details Electrical Load Analysis and methods of addressing the

situation. Some Service Manuals provide the Load each components has.

There are cheap inductive ammeters that can measure the Total Electrical Load

with no disassembly.

Form 337 usually has a statement regarding how the Load / Alternator Capacity

was addressed. Since there are options , it may be wise to read it.


I have encountered the problem with 20 Amp Generators but your

60 Amps is likely enough if it is functioning properly.

 

[Bell206]

So if 80% is the target max load, for a 60A alternator that's 48A. Even with full glass, A/P, etc, 48A seems to me to be plenty of capacity.

And in most cases it should be. However as stated in many other threads the most neglected system on these type aircraft is the electrical system with a number barely able to handle the factory loads due to voltage drops and other system losses. This was one of the biggest hurdles I had to cross when upgrading avionics or electrical equipment.

 

[EdFred]

Larger? No. New? Maybe.

 

[4RNB]

So if 80% is the target max load, for a 60A alternator that's 48A. Even with full glass, A/P, etc, 48A seems to me to be plenty of capacity.

Thanks, it is nearly full glass (g3x, dual G5s, G500AP, 750xi GPS) but per page 2-17 of the G3x installation manual those things really don't take much power, r emote radio b eing the worst at 7.5Amp

 

[4RNB]

Larger? No. New? Maybe.

Thanks. Kinda sucks, it was new in 2022

 

[4RNB]

Yes, but there are other checks to perform prior to upgrading the alternator as that can cause additional issues. Usually prior to adding/changing electrical equipment one does a quick check of the current system electrical load and can be done mathmatically. The target is for the full electrical load to be below 80% of the generating capability. However without the shops load calculation cant offer much more. Many things can affect how the system handles load but I know of only a few 12v 1974 172s that would need an alternator larger than 60amps unless you're running full glass, A/C, etc.

THe shop was supposed to do this yesterday, perhaps today? Looking for a ratio of alternator output to consumption.

 

[EdFred]

Thanks. Kinda sucks, it was new in 2022

Really need to have it tested to see how many amps it can actually generate.

 

[FORANE]

On test flight, the 60 amp alternator did not generate sufficient power when the 13 amp landing lights were on in landing mode. 1974 C 172M.
Thank you

Did the period of insufficient power occur both with engine at idle for landing and with higher RPM?
I have the same issue, but only during the last few moments on Final and landing when power is at idle.

 

[4RNB]

Did the period of insufficient power occur both with engine at idle for landing and with higher RPM?
I have the same issue, but only during the last few moments on Final and landing when power is at idle.

I asked, it happened in cruise flight, not in a low power setting.
And to explain, in case anyone is as ignorant as I was yesterday morning, the alternator requires a certain RPM to make full power, so low RPM stuff might require battery consumption.

 

[Bell206]

THe shop was supposed to do this yesterday, perhaps today? Looking for a ratio of alternator output to consumption.

Any ratios are very specific to the aircraft. Along with how the load test is performed. At the end of a proper load test you're total electrical consumption should add up to 80% or less. And this includes checking all the systems and their interaction with the normal average "running" load. Flap motors, battery charge requirements, landing lites, etc all figure into the equation otherwise you may have to limit which systems can be used at the same time. Regardless, I've seen electrical systems in such poor condition that the basic system "consumed" 40% of the available load simply from poor connections or an incomplete ground plane. See if you can get the load test results and ask how they performed it.

 

[Dan Thomas]

Thanks. Kinda sucks, it was new in 2022

Was it a new alternator or an overhaul/exchange?

First things first. Before the alternator is replaced, make sure the existing unit is deficient. Just throwing money at the problem can get expensive and frustrating.

Disconnect all the sparkplug leads. Disconnect the field wire at the alternator or regulator. In a 172M you can just pull the connector assembly off the regulator. The field wire in it is the F wire, on your far right as you're facing the firewall (pilot's side). Put an ohmmeter between that wire and ground at the firewall. You should see 3 to 5 ohms there. If it's higher, put the ground lead on the alternator case, on one of the ground studs. Check again. If it's 3 to 5 ohms, there's an alternator grounding problem. Not too common unless the engine ground strap is bad, and then you'd see poor cranking. If it's more that 3-5 ohms, turn the prop forward and see what the meter does. If it jumps around and varies a lot, the overhauler or factory put way too much grease in the alternator's rear bearing, and that grease gets extruded out when the rotor shaft goes in, and it ends up on the slip rings and brushes, the stuff that carries the field current. It makes a resistive sludge that limits the field current and therefore the alternator's output. The regulator controls the alternator's output by varying that field current, and when that grease messes things up, the alternator is severely handicapped. I have seen resistances of over 50 ohms. That cuts the field current to less than a tenth of what it needs for max output.

I have seen it many times. Kelly/Hartzell are terrible that way, and so are a lot of smaller shops. That rear bearing needs only the tiniest amount of grease, but the overhaulers tend to fill it up. What a mess.

 

[4RNB]

Was it a new alternator or an overhaul/exchange?

First things first. Before the alternator is replaced, make sure the existing unit is deficient. Just throwing money at the problem can get expensive and frustrating.

Disconnect all the sparkplug leads. Disconnect the field wire at the alternator or regulator. In a 172M you can just pull the connector assembly off the regulator. The field wire in it is the F wire, on your far right as you're facing the firewall (pilot's side). Put an ohmmeter between that wire and ground at the firewall. You should see 3 to 5 ohms there. If it's higher, put the ground lead on the alternator case, on one of the ground studs. Check again. If it's 3 to 5 ohms, there's an alternator grounding problem. Not too common unless the engine ground strap is bad, and then you'd see poor cranking. If it's more that 3-5 ohms, turn the prop forward and see what the meter does. If it jumps around and varies a lot, the overhauler or factory put way too much grease in the alternator's rear bearing, and that grease gets extruded out when the rotor shaft goes in, and it ends up on the slip rings and brushes, the stuff that carries the field current. It makes a resistive sludge that limits the field current and therefore the alternator's output. The regulator controls the alternator's output by varying that field current, and when that grease messes things up, the alternator is severely handicapped. I have seen resistances of over 50 ohms. That cuts the field current to less than a tenth of what it needs for max output.

I have seen it many times. Kelly/Hartzell are terrible that way, and so are a lot of smaller shops. That rear bearing needs only the tiniest amount of grease, but the overhaulers tend to fill it up. What a mess.

Thanks for feedback. I'll forward to my guy.

It was new from Airplains with the new engine. I flew for 10-20 hours to get my IR, then in Aug 2022 took it in for my avionics upgrade, components of which were paid for in December of 2021. Have not flown since August.

 

[mondtster]

I’m just going to mention this, not because I think it is your problem but it might be…

The alternator may be fine, along with the capacity of it. I have seen an increased number of circuit breakers failing as the aircraft fleet ages. The breaker may look fine and even potentially test fine and still not pass enough current to keep the battery charged and support all the electrical dependent devices that have been added. Problems like this can be hard to find, and usually results in the replacement of other charging system parts before stumbling on the real problem (your mention of a recent alternator swap may be an indicator).

Even if the problem is your circuit breaker I would strongly suggest doing a thorough inspection of the entire charging system. They are often neglected and also often only get fixed well enough to make them work again. In an electrically intensive aircraft that is unacceptable, at least in my opinion.

 

[Stewartb]

Landing lights are an occasional use item. The alternator doesn’t need to run them. The battery should do it nicely, then recharge when lights are off. That said, I can’t imagine where your plane is consuming so much power. You should have 13a available with everything else on.

 

[4RNB]

Landing lights are an occasional use item. The alternator doesn’t need to run them. The battery should do it nicely, then recharge when lights are off. That said, I can’t imagine where your plane is consuming so much power. You should have 13a available with everything else on.

Thanks, that was kind of my amateur assessment of things after looking at component needs and the spreadsheet in first post.

 

[4RNB]

Text from shop:

We got the GPS error message taken care of. We're still working on the charging system problem. At this point, it's not related to excessive load from the landing lights. It's behaving like it's an alternator problem itself. Which should be under warranty If we need one.

 

[Dan Thomas]

The OP's 172M typically had two 4509 lamps in the lower cowl. Those are 100-watt lamps, so they draw around 7.5 amps apiece at typical operating voltages.
He says that the new Quasars are in the wingtips and draw 6.8 amps each, for 13.6 amps total.

Now: Are the 4509s still in the cowl and were on when the alternator couldn't keep up? With all the glass stuff, a whole lot of small draws that do add up, and the new 13.6-amp light draw, he might be at the alternator's limit. Might. Still need to check that field resistance, but a loadmeter stuck in the alternator output line would tell the tale, along with the voltage. If the amperage cannot reach 60 while maintaining 13.5 volts or so, that alternator needs work, or the regulator is unable to supply full field current.

Those Ford DOFF10300 alternators on those airplanes can often provide full output at relatively low RPMs. They were adapted from cars, and cars spin their alternators, in cruise, a lot slower than an airplane engine does, so they had to be a really capable alternator. An alternator and engine's pulleys are sized to let the alternator reach its redline RPM when the engine reaches its redline. Cars typically cruise at around a third of their engine redline, while the aircraft engine cruises at around 90% of redline. So, in the airplane, that 172 alternator is able to generate a lot even at idle, which is usually a quarter of redline or so. And it's the reason why aircraft alternator brushes don't last like they do in your car, and why they need 500-hour inspections if you don't want to end up wandering around in the dark in IMC or stuck at some inconvenient spot far from home.

Much is made of the Plane Power alternators, but they also have field brushes that need regular inspections. They are not magic. It's right there in their installation manuals. 1000 hours or five years. Their ACU is a good upgrade, though.

 

[Stewartb]

If new is the path? Field approve a B&C alternator and LR regulator. Best alternator in aviation.

 

[Magman]

We put a “ Mirror Image” Landing/Taxi Light installation in the RIGHT wing of a

172. Thus there were 4 4509 lamps on at times. Also installed a flasher in the

circuit. Worked well but he did get a lot of “ Say again on type” at night from

towered airports.


Also installed was a Ford/ Cessna Alternator on the Conti engine.

I don’t recall what method we used to comply with the Load Analysis for the 337.


It seems new Alt Belts have a tendency to stretch and slip at times and might

need an adjustment.

Perhaps the issue here is as simple as a belt slipping?

 

[4RNB]

We put a “ Mirror Image” Landing/Taxi Light installation in the RIGHT wing of a

172. Thus there were 4 4509 lamps on at times. Also installed a flasher in the

circuit. Worked well but he did get a lot of “ Say again on type” at night from

towered airports.


Also installed was a Ford/ Cessna Alternator on the Conti engine.

I don’t recall what method we used to comply with the Load Analysis for the 337.


It seems new Alt Belts have a tendency to stretch and slip at times and might

need an adjustment.

Perhaps the issue here is as simple as a belt slipping?

good idea, wonder if my shop is tired of my emails yet...

 

[Magman]

Guy used to commute to his grass strip and return late.

Wife would light “ smudge pots” to aid in return.

fyi - Field Approval

 

[Stewartb]

I've seen a 3-wheeler on the approach end with the headlight down the runway.

 

[Dan Thomas]

Quite possible, though I've usually found belts too tight. Cessna manuals tell us what the tension should be, using a torque wrench:

 

[4RNB]

New alternator ordered, tests must have indicated it was not doing the job.

 

[Magman]

See NOTE for recheck on new belts.

 

[Dan Thomas]

And if it's a bearing grease problem, the new one might be no better. A mechanic can open an alternator and clean that up, but most won't. Service manuals have that info in them. I have had new/overhauled alternators fail at a few hours, and got tired of sending them back. I just opened them up and fixed them properly and put them back to work and they performed flawlessy until the next 500-hour inspection.

Sometimes you have to do what's necessary and correct the ongoing quality problems yourself. The overhaulers aren't much interested, it seems. There's a distinct lack of in-house troubleshooting of warranty return stuff to see why so much is being returned. One can file multiple SDRs on the same problem, and nothing changes.

 

[4RNB]

New alternator fixed issues.

Thanks for letting me discharge some frustration.

 

[WDD]

"Discharge" - Good one

 

[SoonerAviator]

New alternator fixed issues.

Thanks for letting me discharge some frustration.

You certainly got a little excited.

 

[WDD]

Shocking that the old alternator went bad so quickly.