/I have a perplexing problem and hope some of you electrical
/type gurus can help. My C-150 Alternator CB (60 amp)
/trips under full load (radio, transponder,taxi and
/landing light, nav and beacon on) when engine rpm
/exceeds 1200 RPM and electric flaps are cycled.
This is a DESIGN problem endemic with ALL Cessnas.
It's a condition that happens too many times . . .
sometimes with unhappy consequences.
A 60-amp alternator, under some conditions, will put
out MORE than 60 amps. It's unusual for an airplane
to DRAW more than 60 amps but consider this scenario.
Master switch left on and you go out to fly on a cold
morning. You put jumper cables on and get the engine
started. Now, the alternator is COLD, battery discharged,
but you need to run lots of things on your way out to
the runway. Under this condition, the alternator
may indeed be putting out up to 70 amps because it's
cold and the airplane loads PLUS battery recharge loads
exceed 60 amps.
Now our happless friend is unaware of the fact that
his alternator b-lead breaker has opened and he
launches into the overcast only to have everything
go dark minutes later when he uses up what little
charge was put back into the battery during preflight.
Nothing is broke . . . everything is working just the
way it was designed to do. Yet a properly annointed
flight system has been designed into thousands of
airplanes with a potentially hazardous flaw. . . .
/CB will not trip when engine is not running, even when flaps
/are cycled and all electrics are on.
Yes, because the alternator is not putting out any power
when the engine is not running.
/I Have checked and cleaned all connections which is
/about the limit of my electrical expertise. Anything
/else is pure speculation on my part. My local
/maintenance shop tested CB to 45 amps and it held
/fine.
I'd hope so . . . it is after all a 60-amp breaker.
/I'm hesitant to spend bucks to replace CB if problem is
/voltage reg or alternator or ?. Bucks are tight and
/SUN-N-FUN calls. Would appreciate any help/suggestions.
First, how old is your battery? Is it fully charged
while all this is going on? Normally, the condition
you are experiencing can occur only if the battery
is ADDING to normal aircraft loads. It's also possible
that your alternator is a bit more robust than the
production average and/or the breaker has drifted down
slightly in its trip setting.
This doesn't represent much of a hazard if you
understand the phenomenon and avoid the situation
that precipitates the event . . . don't turn EVERYTHING
on at once. On the other hand, if this is a new
phenomenon then something has CHANGED. You should
track it down . . . shorted cell in battery? Voltage
regulator set too high? Have you checked the bus
votlage with engine running? Shouldn't be over 14.2 volts
in warm climes. Is the battery fully charged when this
happens (go fly around for an hour with as much stuff
OFF as you can . . . if the airplane is ammeter
equipped, see that battery recharge current has reduced
to a few amps and that the bus voltage is not too high.
You need to deduce the COMBINATION of things that's causing
the breaker to trip. Putting a clamp on ammeter on
the alternator b-lead while duplicating the trip
condition would be useful . . . you can SEE how much the
alternator is putting out when it happens. There's no
substitute for test equipment.
In amateur built airplanes, I recommend at least 70-amp
protection on a 60-amp alternator . . . remember, fuses
and breakers protect wires . . . not equipment. If the
60-amp breaker pops for reasons specific to it's job
in the airplane then the alternator is HURT BAD . . .
shorted diodes most likely. Having a 4AWG b-lead wire
protected with 70 amp fuse is very much in order.
This is another classic example of how little most
FBO staff know about troubleshooting electrical systems.
Protect yourselves guys . . . knowlege is a powerful
tool.
Moral of the Story: Don't jump-start any airplane and
launch into the blue until the battery is well on its
way to being fully charged. Exceptions: Day VFR, you
KNOW about the possiblity of nuisance trip, you REDUCE
loads as much as possible to keep the alternator loads
down until the battery is recharged, and you keep an
eye on the breaker. Unfortunately, while boring holes
in clouds, breakers are not on most pilot's scan lists.
Oh yes, ADD very active and attention getting low voltage
warning system to your airplane . . . I have schematics
and bill of materials for those interested.
Regards,
Bob . . .
AeroElectric Connection
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