Comanche Quirk - How to solve it?

Chrisgoesflying

Chrisgoesflying

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Chrisgoesflying
The Piper Comanche has a weird electric set up. The electric fuel pumps (on their own switch), nav lights and interior lights (on the same switch) and turn & bank coordinator (on its own switch) all go through the same breaker. When I have the nav lights and turn & bank coordinator on and then turn the fuel pumps on, the breaker pops after about 30 seconds. It doesn't happen with the fuel pump and turn & bank coordinator on. It also doesn't happen with the nav lights and turn & bank coordinator on. What could cause this? My mechanic is investigating the issue. So far, he looked at all the lights and the wires going to the lights and didn't see anything obvious. Next is looking at the wiring to and on the fuel pumps. He said he'll take things apart if nothing obvious is jumping at him. If he has to take things apart to investigate, we'll probably isolate the fuel pumps on their own breaker going forward as they're critical. Do any of you have an idea what is causing this? Other Comanche owners, do you have this issue? Did you change the electric setup from the original where those three come off the same breaker? I never noticed this happening until I flew past sunset once and had to turn on the nav lights. During the day, I leave them off since they also turn on the interior lights (again, nav lights and interior lights are on the same switch), which in turn dims my gear indicator lights to a point where they're very hard to see.
 
unsafervguy said:
It’s no quirk. There’s to much load on the breaker when everything is on. Have your a@p put the fuel pump on its own breaker.
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I agree. The Comanche electrical system schematic I have from when I flew a 1959 250 shows the Aux Fuel Pump on it's own breaker, whether on a generator or alternator system. (or am I misreading the schematic - electrical systems are not my strong suit.)

1719675677069.png
 
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Just out of curiosity, how many amps is the breaker rating?
 
You need to fix that. No way I’d fly a plane with those 3 things on one breaker. Turn/bank and fuel pumps need to both be isolated from everything else. Combining nav and interior lights, care less about that.
 
The breaker is to protect the wire, nothing else. So, if the breaker is small enough to protect the smallest wire, it’s gonna pop when all that crap is on. You can’t make the breaker bigger and still protect the smallest wire, so you need more breakers. But that’s a secondary issue to having your interior lights able to crap out your turn and bank. (Sharing a breaker)
 
Is this a new setup or has it been this way for years and the breaker just started tripping?
If this was working before, get an amp meter and check the load vs the breaker rating.
If the load is less than the breaker rating the breaker may need replacing, they sometimes they can go bad.
If the breaker should be tripping at the load something has changer and you may need to split the load and add a breaker.
 
Salty said:
You need to fix that. No way I’d fly a plane with those 3 things on one breaker. Turn/bank and fuel pumps need to both be isolated from everything else. Combining nav and interior lights, care less about that.
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How could you fly at night with the outside nav and interior lights on at the same time? Maybe I am missing something?
I want my cockpit dark inside at night.
 
Gary Ward said:
How could you fly at night with the outside nav and interior lights on at the same time? Maybe I am missing something?
I want my cockpit dark inside at night.
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What does the breaker they are on have to do with your question?
 
Chrisgoesflying said:
we'll probably isolate the fuel pumps on their own breaker going forward as they're critical.
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Do that regardless. You really don't want a shorted light to kill your fuel pump or T&B indicator.

Is that setup really original? I'm surprised someone would design an electrical system that way, and also surprised it could get certified. Of course, back when our antique planes were designed FMECA probably hadn't yet worked its way into civil aviation.
 
Gary Ward said:
How could you fly at night with the outside nav and interior lights on at the same time? Maybe I am missing something?
I want my cockpit dark inside at night.
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Piper quirk. Nav lights are on a dimmer switch for interior lights. Turn it past the "click", nav lights are on. Rotate the switch more and cabin lights come up slowly. Easy to have nav lights on and no cabin illumination. Actually, might be the instrument/panel lights on that switch, but same concept either way.
 
Salty said:
breaker <> switch
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Wanna bet? :) My plane is full of these:
www.aircraftspruce.com

Tyco Circuit Breaker W31 Series | Aircraft Spruce

Tyco Circuit Breaker W31 Series These bi-metallic single pole breakers weigh less than 2 oz. and are especially suitable for homebuilt aircraft. Used as original equipment in Beech, Cessna and Piper aircraft.
www.aircraftspruce.com www.aircraftspruce.com

Fuel pump, strobes, landing light, etc., all have combo breakers/switches.
 
Half Fast said:
Wanna bet? :) My plane is full of these:
www.aircraftspruce.com

Tyco Circuit Breaker W31 Series | Aircraft Spruce

Tyco Circuit Breaker W31 Series These bi-metallic single pole breakers weigh less than 2 oz. and are especially suitable for homebuilt aircraft. Used as original equipment in Beech, Cessna and Piper aircraft.
www.aircraftspruce.com www.aircraftspruce.com

Fuel pump, strobes, landing light, etc., all have combo breakers/switches.
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What I’m saying is that he is not talking about them being on one switch, only one breaker. He specifically says each has own switch.
 
Yep, that’s what it shows, but I sure wouldn’t have designed it that way. Guess nobody considered failure modes back then.
 
I believe that the problem is an aged breaker. They are thermal devices, and any corrosion raises the heat generated internally, and lowers trip amperes.

Replacing the breaker should correct the tripping. This is a common problem on older planes.

The wiring diagram shows the original connections are as your plane is functioning, so that is not a problem.
 
Don't just guess, do a proper troubleshooting procedure. You need another drawing, the one that shows the Q2A wiring, fuel pump switch, the pump, and the F1A wiring to the T&B indicator, which may be on yet another drawing. That'll show the entire circuit, which should be inspected for faulty connections or shorts caused by deteriorated insulation.

The proper path of diagnosis after physically inspecting the wiring should begin with operating the fuel pump and determining its amperage draw, then comparing that with the manufacturer's specifications. If it's anomalous, determine why that is.

Next the total circuit amperage draw should be determined with the pump, T&B, nav, and interior lights energized, in the time period before it trips. The load should be less than 80% of the breaker rating, in this instance 8 amps. If that's the case, then the breaker is defective.

Moving the pump to a dedicated circuit breaker or replacing the breaker would do nothing to determine why the breaker is tripping under a load which is as designed in the schematic. The pump is a critical component, something isn't right, and it needs to be investigated.

I should add your mechanic must have a clamp on ammeter capable of determining DC current and know how to use it. Some don't.
 
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Those schematics show the ground circuit. Where does power come in? Maybe I just overlooked it. And do you have fuel pump lights?
 
Mikey52 said:
Those schematics show the ground circuit. Where does power come in? Maybe I just overlooked it. And do you have fuel pump lights?
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Looks to be coming from the main bus bar through a 10 amp breaker.

I stand by what I said initially. No way I’d keep that config. I’d want 3 breakers for that equipment.
 
I agree there's a decent chance an aging breaker tripping at a lower current could be your problem.
Easy troubleshooting, with the breaker open, use a cheap multimeter on the 20A setting and put the probes across the breaker contacts. Current measured should be 80% or less of the breaker rating.

Also, put the fuel pump and turn coordinator on their own breakers. That is a dumb and unsafe design that wouldn't stand to modern electrical regulations. Not sure what Piper was thinking.
 
Mikey52 said:
Those schematics show the ground circuit. Where does power come in? Maybe I just overlooked it. And do you have fuel pump lights?
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Ten amp breaker, top right corner. Both drawings are similar, they apply to two different serial number ranges.
 
AV8R_87 said:
That is a dumb and unsafe design that wouldn't stand to modern electrical regulations. Not sure what Piper was thinking.
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It's really not that uncommon in these old planes. The electrical designs are terrible by aerospace standards and are more akin to 1960s automotive wiring. No thought was given to designing a proper grounding system, to correct shielding techniques, to circuit loop area, to fault modes and graceful degradation, etc.

For the simple avionics of the day, it worked okay, but when we put in modern equipment that's more susceptible to noise, and in some cases generates more noise, we have problems.

In this particular case, though, I suspect it's just a worn breaker. Not at all uncommon. It would be worthwhile to check the breaker connections before replacing it, though, as a poor or corroded connection could generate heat that's causing the breaker to trip prematurely.

Even if that corrects the problem, I'd still get these circuits put on their own breakers at the first opportunity.
 
I definitely want to put the fuel pump on its own breaker. But even then, I'd want to know what's causing the breaker to trip in the current configuration when it shouldn't be happening since this is how it's been designed. I also would want a separate switch for instrument lights and nav lights as right now, they're on one switch. Since I want to install a tail or sky beacon, I would have to keep the nav lights on at all times but I don't want the instrument lights on at all times since it dims my gear indicator lights to a point where I can barely see them.

Speaking of tail or skybeacons: The Comanche has two rear position lights. Installing a tail beacon in the upper rear light position would interfere with the elevator. I read somewhere that installing it on the lower rear position light (where it doesn't interfere with the elevator) isn't desirable as the tailbeacon needs unobstructed view upwards for GPS signal. Any thoughts on this?

Skybeacon also comes with a challenge as I have those wing tips that house the nav lights inside a glass housing. The fin on the beacon is a little too long for the housing. Has anyone been able to install the skybeacon and simply cut a little hole in the glass housing to let the fin stick out?
 
Chrisgoesflying said:
Speaking of tail or skybeacons: The Comanche has two rear position lights. Installing a tail beacon in the upper rear light position would interfere with the elevator. I read somewhere that installing it on the lower rear position light (where it doesn't interfere with the elevator) isn't desirable as the tailbeacon needs unobstructed view upwards for GPS signal. Any thoughts on this?

Skybeacon also comes with a challenge as I have those wing tips that house the nav lights inside a glass housing. The fin on the beacon is a little too long for the housing. Has anyone been able to install the skybeacon and simply cut a little hole in the glass housing to let the fin stick out?
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Perhaps a GDL 82 would be a better solution.
 
Half Fast said:
It's really not that uncommon in these old planes. The electrical designs are terrible by aerospace standards and are more akin to 1960s automotive wiring. No thought was given to designing a proper grounding system, to correct shielding techniques, to circuit loop area, to fault modes and graceful degradation, etc.
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Whatever their mindset was, they corrected it rather quickly. By the time the PA-28 went into production a few years later, the fuel pump, turn coordinator and nav lights had separate circuit breakers.
 
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