Alternator circuit breaker tripping - electrical question
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Alternator circuit breaker tripping - electrical question
After landing (a C150) at the weekend I noticed the low voltage light illuminated - a quick check revealed the alternator circuit breaker had tripped. It had not tripped during the flight.
Whilst taxying in (radios, transponder, beacon all still on) I pushed the circuit breaker back in, but it immediately popped out again - not good, I thought.
I decided to do some fault finding while on the ground - I cycled the radio and transponder, breaker wouldn't stay in. However, after I turned the beacon off, and pushed the circuit breaker in again, it remained in. I finished taxying, and decided to turn the beacon on again to replicate the fault so I could present a decent prognosis to the maintainer. However, this time the circuit breaker remained in. Typical.
The only theory I have is there is an intermittent fault in the beacon wiring (would this plausibly cause the alternator circuit breaker to pop?), which seems to occur whilst the aircraft is subject to shock/movement (chafed wire perhaps?). Is this a reasonable theory? Secondly, as our maintainer isn't on-site, is the aircraft safe to fly to them as long as we don't use the beacon?
Thanks all
Whilst taxying in (radios, transponder, beacon all still on) I pushed the circuit breaker back in, but it immediately popped out again - not good, I thought.
I decided to do some fault finding while on the ground - I cycled the radio and transponder, breaker wouldn't stay in. However, after I turned the beacon off, and pushed the circuit breaker in again, it remained in. I finished taxying, and decided to turn the beacon on again to replicate the fault so I could present a decent prognosis to the maintainer. However, this time the circuit breaker remained in. Typical.
The only theory I have is there is an intermittent fault in the beacon wiring (would this plausibly cause the alternator circuit breaker to pop?), which seems to occur whilst the aircraft is subject to shock/movement (chafed wire perhaps?). Is this a reasonable theory? Secondly, as our maintainer isn't on-site, is the aircraft safe to fly to them as long as we don't use the beacon?
Thanks all
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The C 150 has two alternator CB's, with the second one being hidden behind the panel. It is the field breaker, which the 172 has as a 2 or 5 amp on the panel (so you know it opened). That's probably not your problem, but just so you consider that. The problem is that when that field breaker opens, you have no alternator, and only checking the ammeter will tell you. It sounds like your alternator is working - too well!
The beacon could certainly be an element of your problem, but not solely. The 60 amps produced by the alternator will set the beacon wiring aglow, if it were your only problem. More likely that the sum of a malfunctioning beacon load, and other loads together are exceeding the alternator capacity. This is not something to experiment with, even on the ground, 60 amps going to the wrong place in a 150 is bad news.
I won't say the plane is safe to fly with the beacon turned off, but that's because I'm being internet cautious. I could bring myself to do it, if I could isolate that, and other suspect circuits. If its a 1975 or earlier, is has fuses, I'd consider removing any which are not required for that flight - (were I to make the flight). If also been known to fly with the master off, when I suspected an electrical fault. Just on for flap use, and radio if you must. But that's just me...
Explain to the maintenance staff what you have described here, you've done your pilot reporting job well, don't worsen the problem attempting troubleshooting on your own. That said, it is entirely fair that you should learn what's to be learned about this, so ask for an informal report later on what was found, or better, ask if you can lurk out of the way as the fault checking is performed.
A concept not well understood is worth review at this point - not that it's causal, but you're in the territory....
The 60 amp output of the alternator is current, the 14 volts is voltage. Volts times amps equals watts, which is power. Power is also expressed as torque times RPM. As RPM increases, torque decreases for the same power.
So, the alternator which consumes some torque while producing X power at cruise engine RPM, consumes a lot more torque if that same X power is being produced at idle RPM. There is a drive coupling in the O-200 for the alternator (as it is not belt drive, where the belt would otherwise be the sacrificial drive), so go easy with a lot of electrical load when the engine is at low power, turn off whatever big consumers you can as early as it as safe to do so. It'll prevent premature failure of that coupling.
The beacon could certainly be an element of your problem, but not solely. The 60 amps produced by the alternator will set the beacon wiring aglow, if it were your only problem. More likely that the sum of a malfunctioning beacon load, and other loads together are exceeding the alternator capacity. This is not something to experiment with, even on the ground, 60 amps going to the wrong place in a 150 is bad news.
I won't say the plane is safe to fly with the beacon turned off, but that's because I'm being internet cautious. I could bring myself to do it, if I could isolate that, and other suspect circuits. If its a 1975 or earlier, is has fuses, I'd consider removing any which are not required for that flight - (were I to make the flight). If also been known to fly with the master off, when I suspected an electrical fault. Just on for flap use, and radio if you must. But that's just me...
Explain to the maintenance staff what you have described here, you've done your pilot reporting job well, don't worsen the problem attempting troubleshooting on your own. That said, it is entirely fair that you should learn what's to be learned about this, so ask for an informal report later on what was found, or better, ask if you can lurk out of the way as the fault checking is performed.
A concept not well understood is worth review at this point - not that it's causal, but you're in the territory....
The 60 amp output of the alternator is current, the 14 volts is voltage. Volts times amps equals watts, which is power. Power is also expressed as torque times RPM. As RPM increases, torque decreases for the same power.
So, the alternator which consumes some torque while producing X power at cruise engine RPM, consumes a lot more torque if that same X power is being produced at idle RPM. There is a drive coupling in the O-200 for the alternator (as it is not belt drive, where the belt would otherwise be the sacrificial drive), so go easy with a lot of electrical load when the engine is at low power, turn off whatever big consumers you can as early as it as safe to do so. It'll prevent premature failure of that coupling.
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The '77 150M probably has the cheap circuit breakers (which you cannot pull) rather than fuses, so you cannot disable a circuit. This should be factored into a decision to fly with a suspected defect.
