Alternator Failure (sort of exciting night)

jsstevens

jsstevens

Final Approach
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jsstevens
I managed after many tries to complete the two two hour cross counties required by 61.129 yesterday.

We flew from KSFB (Sanford) to KSSI (St. Simon's Island) leaving at 5:45 and arriving at 7:45 (we did do a little sight seeing over St. Simons). We flew up at 6,500 feet which put us above the (scattered) clouds. Super smooth, nice flight.

After dinner (I was shocked at how many places close at 7 or 8 PM-no Southern Soul BBQ for us. :( ) We preflighted, loaded up and got ready to leave. All was fine, and we departed 22 headed to KCRG (Craig in Jacksonville) to stay over land, then from there direct to KSFB. Engaged the auto-pilot, climbed up to 4,500 feet and just cruised along in the dark. Again, super smooth and lovely cloudless night.

About 20 miles south of KCRG, the engine monitor alarmed on battery voltage (24.2V). We checked the ammeter and sure enough it was discharging. I cycled the alternator switch, no help. So, we shed load (COM2, G-500, auto-pilot, landing light and beacon-dimmed everything down) so I was now hand flying (no big deal) on the two GI-275s. CFI called Daytona approach to let them know in case we went dark. They gave us a squawk and monitored us. After shedding the loads the battery alarm went away, but the ammeter told the tale-we were still losing current.

We discussed diverting, but we were 30-40 minutes from home plate and the closest diversion were still going to be 20 plus minutes. So we continued. We kept the RPMs up because we were getting some from the alternator, just not enough. SInce it was smooth, I just pushed over and did our descent at about 130 kias.

We were cleared for the Monroe arrival (VFR procedure for the class C at KSFB) and headed in. Voltage was down to 22.7 or so at this point. Since the flaps are a sizable electric motor, I opted for a no flap landing and the airport is well lighted so I opted for a no landing light landing. We asked for and got runway 9L (9,000+ foot runway) so I had plenty of room. The only real challenge I had on final was I had been flying at 130 knots for several minutes so 80 knots, while way to fast for final felt slow. But I conquered that and did a relatively nice no-flap landing.

Voltage at shutdown was 22.3.

It was a training flight and it was educational. :) And my 61.129 requirements are now met.

John
 
Please let us know what the mechanics find. Everyone needs to learn. Alternators are not immortal, and they need regular inspections to make sure their brushes are still within wear limits. A large percentage seem to be getting ignored and are run to failure, and this night excitement is the result.
 
Dan Thomas said:
Please let us know what the mechanics find. Everyone needs to learn. Alternators are not immortal, and they need regular inspections to make sure their brushes are still within wear limits. A large percentage seem to be getting ignored and are run to failure, and this night excitement is the result.
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Will do. This plane had the same symptom a couple of weeks ago and was "fixed" but I don't know what they found nor what they fixed.

And brushes might fit the symptoms because it was producing some. At least as near as I could tell from an ammeter that only moved about a needle width from 0.
 
MauleSkinner said:
Did the ammeter move when you cycled the alternator switch?
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It was very hard to tell. Ammeter is on right side of panel and it was only moving a needle width anyway. Definitely discharging the whole time. CFI in right seat said we were getting some benefit by keeping RPMs up.
 
A partial ground short (like from worn insulation) on the alternator output might be a possibility.
 
Half Fast said:
“Partial” short, that is, a significant load but greater than zero ohms.
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That would create heat that would burn through whatever the resistance was. In any case, the ammeter's indication would not stay constant for very long at all.

The voltage would fall if there was a partial short, causing the regulator to feed max current to the field and making the alternator produce its max current. 60 or 70 amps can make a lot of heat. And smoke. As it was, the regulator would, if it was OK, be trying to feed max current to the field if the voltage was down to 22 volts. If the brushes are bad the current can't get there. If someone had the alternator apart and put too much grease in the rear bearing, that grease extrudes out when the rotor shaft goes in and it ends up flying around and getting on the brushes and slip rings, creating a resistive sludge that limits the field current and killing the output. Maybe that's what happened here. Hartzell/Kelly alternators have had this problem for decades already. No amount of SDRs that I filed seemed to have any effect.
 
My guess from the symptoms described is one or more blown diodes.
 
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