A PIPER 30 AT 4000 FT LOST ALL ELECTRICAL SYS WHEN LOADING THE BATTERY BY ENG STARTER OR LNDG GEAR OP CAUSED BY CORRODED BATTERY TERMINALS.

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RECURRENCY BEING GIVEN TO A MULTI-COMMERCIAL INST RATED PLT. CLBED TO 4000 FT ABOVE THE BEACH TRAINING AREA (APPROX 10 NM S OF CHAWKTAW NOLF). PERFORMED AIR WORK AND THEN FAILED L ENG (PULLED MIXTURE LEVER). STUDENT EXECUTED BOLD FACE PROCS AND SECURED ENG UNEVENTFULLY. STARTED THE RESTART CHKLIST WHEN WE SELECTED THE L STARTER TO TURN THE ENG OVER; ALL ELECTRICAL SYS FAILED; RELEASED THE SWITCH AND SYS RETURNED. NO ROTATION ON ENG. INFORMED PNS THAT WE WERE SINGLE ENG AND BEGAN A SLOW DSCNT INTO THE MILTON PATTERN (FROM THE S). WINDS WERE FAVORING RWY 18. ENTERED AN EXTENDED R DOWNWIND FOR RWY 18 AT MILTON (2R4).UPON TURNING BASE TO FINAL; WE SELECTED GEAR DOWN -- ONCE AGAIN TOTAL ELECTRICAL FAILURE. TOLD STUDENT AND ASSISTED HIM IN EXECUTING A SINGLE ENG GAR (NO COMS) WITH THE INTENTION OF DOING A MANUAL GEAR EXTENSION THEN RETURNING TO LAND. WHEN PASSING THROUGH 600 FT MSL (APPROX 500 FT AGL) NOTED A VERY FAINT GEAR DOWN INDICATOR. SOON THEREAFTER; A BRIGHT AND STEADY GEAR DOWN INDICATION APPEARED AND THE ELECTRICAL SYS RETURNED. ACFT STABILIZED AT 700 FT MSL AND WE ELECTED TO LEAVE GEAR DOWN FOR FEAR IT WOULD FREEZE MIDWAY UP IF IT CAME UP AT ALL. I ELECTED TO ACCEPT A TAILWIND LNDG AND WE PERFORMED A SLOW TEARDROP TURN BACK TOWARD RWY 36. LNDG WAS UNEVENTFUL. UPON INSPECTION; CORRODED BATTERY TERMINALS WERE FOUND. TOLD STUDENT THAT HE PERFORMED EXCEPTIONALLY WELL UTILIZING CRM CONCEPTS (I WAS AN FE ON A 6-MAN HELI CREW FOR 11 YRS). I DISCOVERED AFTER SPEAKING WITH A MECH THAT AN ACFT'S ALTERNATORS; ONCE EXCITED; STILL RELY ON THE BATTERY TO CONTINUE EXCITATION; UNLIKE AN AUTOMOBILE THEY ARE NOT SELF-SUSTAINING ONCE EXCITED. SIMPLISTICALLY IT MAKES SENSE; BUT FROM A SAFETY STANDPOINT IT SEEMS LIKE THE BATTERY IS RELIED UPON TOO MUCH IN THE ELECTRICAL SYS.